NMPC physicists take part in ongoing research and publishing opportunities, disseminating their work in full peer-reviewed publications and book chapters as well as conference abstracts.

 

Peer-reviewed publications and book chapters

 

Hitting Moving Targets in Cancer Treatment.

Fagerstrom JM, Bry VN, Colbert CM and Windsor C

Front. Young Minds. 12:1349460. doi: 10.3389/frym.2024.1349460 23, 4 (2024).

In this article, we will explore the way radiation can be used to treat cancer. Radiation for cancer therapy consists of high-energy particles or light that can damage living cells, including cancer cells. Radiation beams can be generated using a special machine, called a linear accelerator, and they are precisely aimed at a patient’s cancer. When cancer is located near the patient’s lungs, the cancer moves around as the patient breathes in and out. Hitting the cancer with the radiation beam can be hard when the cancer does not stay still. It is a little like trying to hit a moving target in a video game. In that case, there are some tricks that patients and radiation therapy workers can use to make sure that the radiation beam hits the cancer and misses healthy organs.

 

AAPM medical physics practice guideline 13.a: HDR brachytherapy, part A

Susan L. Richardson, Ivan M. Buzurovic, Gil’ad N. Cohen, Wesley S. Culberson, Claire Dempsey, Bruce Libby, Christopher S. Melhus, Robin A. Miller, Daniel J. Scanderbeg, Samantha J. Simiele

Journal of Applied Clinical Medical Physics (2023).

The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education, and professional practice of medical physics. The AAPM has more than 8000 members and is the principal organization of medical physicists in the United States.

The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines (MPPGs) will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner.

Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized.

 

AAPM Medical physics practice guideline 15.A: Peer review in clinical physics

Per H. Halvorsen, Alan H. Baydush, Courtney R. Buckey, Navneeth Hariharan, Mary A. Keenan, Jeffrey P. Limmer, Kate E. Lofton, Robin A. Miller, Jeffrey M. Moirano, Joseph Och, Douglas E. Pfeiffer

Journal of Applied Clinical Medical Physics (2023).

The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education, and professional practice of medical physics. The AAPM has more than 8000 members and is the principal organization of medical physicists in the United States.

The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner.

Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized.

 

Overview of medical physics education and research programs in a non-academic environment

Jessica M. Fagerstrom,¹ Thomas A. D. Brown,² Darryl G. L. Kaurin,³ Saikanth Mahendra,³ M. Miron Zaini^3
1University of Washington, Seattle, WA; ²Maine Medical Center, Portland, ME; ³Northwest Medical Physics Center, Lynnwood, WA

Journal of Applied Clinical Medical Physics (2023).

Northwest Medical Physics Center (NMPC) is a nonprofit organization that provides clinical physics support to over 35 radiation therapy facilities concentrated in the Pacific Northwest. Although clinical service is the primary function of NMPC, the diverse array of clinical sites and physics expertise has allowed for the establishment of structured education and research programs, which are complementary to the organization’s clinical mission. Three clinical training programs have been developed at NMPC: a therapy medical physics residency program accredited by the Commission on Accreditation of Medical Physics Education Programs (CAMPEP), an Applied Physics Technologist (APT) program, and a summer undergraduate internship program. A partnership has also been established with a major radiation oncology clinical vendor for the purposes of validating and testing new clinical devices across multiple facilities. These programs are managed by a dedicated education and research team at NMPC, made up of four qualified medical physicists (QMPs). The education and research work has made a significant contribution to the organization’s clinical mission, and it has provided new training opportunities for early-career physicists across many different clinical environments. Education and research can be incorporated into nonacademic clinical environments, improving the quality of patient care, and increasing the number and type of training opportunities available for medical physicists.

 

Equity, diversity, and inclusion topics at a medical physics residency journal club

Jessica M. Fagerstrom,¹ Cheyann Windsor,² Daniel Zaks^2
1University of Washington, Seattle, WA; ²Northwest Medical Physics Center, Lynnwood, WA

Journal of Applied Clinical Medical Physics (2023).

A journal club program was initiated in a clinically focused, geographically distributed medical physics therapy residency program. This program currently supports two residents at different clinical sites, who regularly present at the new journal club. For one of the sessions, residents were assigned to present on topics related to the broad themes of equity, diversity, and inclusion (EDI) in the context of medical physics, radiation oncology, or medical oncology. As in other journal club sessions, residents were responsible for choosing their respective articles within required criteria and with approval from the program director. The session was executed in late 2022, with both residents leading and facilitating discussion for the residents, the residency program director, and all residency faculty members. This education case report will include the learning objectives for the journal club session, a description of the content covered in the session, discussion regarding the session’s alignment with the original learning objectives, and ideas for program directors intending to include evidence-based EDI topics in journal clubs.

 

Therapeutic radiation beam output and energy variation across clinics, technologies, and time

Mehran Miron Zaini,¹ Jessica M. Fagerstrom,¹ Edward I. Marshall,¹ Kathryn M. Hedrick,¹ Daniel Zaks,¹ Hung Tran,¹ Trevor M. Fitzgerald^1
1Northwest Medical Physics Center, Lynnwood, WA 

Journal of Applied Clinical Medical Physics (2023).

Over the past several decades, a medical physics service group covering 35 clinical sites has provided routine monthly output and energy quality assurance for over 75 linear accelerators. Based on the geographical spread of these clinics and the large number of physicists involved in data acquisition, a systematic calibration procedure was established to ensure uniformity. A consistent measurement geometry and data collection technique is used across all machines for every calendar month, using a standardized set of acrylic slabs. Charge readings in acrylic phantoms are linked to AAPM’s TG-51 formalism via a parameter denoted k_acrylic, used to convert raw charge readings to machine output values. Statistical analyses of energy ratios and k_acrylic values are presented. Employing the k_acrylic concept with a uniform measurement geometry of similar acrylic blocks was found to be a reproducible and simple way of referencing a calibration completed in water under reference conditions and comparing to other machines, with the ability to alert physicists of outliers.

 

Determination of commissioning criteria for multileaf-collimator, stereotactic radiosurgery treatments on Varian TrueBeam and Edge machines using a novel anthropomorphic phantom

Thomas A. D. Brown,¹ Jessica M. Fagerstrom,¹ Caleb Beck,¹ Connor Holloway,¹ Krista Burton,¹ Darryl G. L. Kaurin,¹ Saikanth Mahendra,¹ Marcus Luckstead,¹ Kayla Kielar,² James Kerns^2
1Northwest Medical Physics Center, Lynnwood, WA; ²Varian Medical Systems, Palo Alto, CA

Journal of Applied Clinical Medical Physics, 23, 4 (2022).

An anthropomorphic phantom has been developed by Varian Medical Systems for commissioning multileaf-collimator (MLC), stereotactic radiosurgery (SRS) treatments on Varian TrueBeam and Edge linear accelerators. Northwest Medical Physics Center (NMPC) has collected end-to-end data on these machines, at six independent clinical sites, to establish baseline dosimetric and geometric commissioning criteria for SRS measurements with this phantom. The Varian phantom is designed to accommodate four interchangeable target cassettes, each designed for a specific quality assurance function. End-to-end measurements utilized the phantom to verify the coincidence of treatment isocenter with a hidden target in a Winston-Lutz cassette after localization using cone-beam computed tomography (CBCT). Dose delivery to single target (2 cm) and single-isocenter, multitarget (2 and 1 cm) geometries was verified using ionization chamber and EBT3 film cassettes. A nominal dose of 16 Gy was prescribed for each plan using a site’s standard beam geometry for SRS cases. Measurements were performed with three Millennium and three high-definition MLC machines at beam energies of 6-MV and 10-MV flattening-filter-free energies. Each clinical site followed a standardized procedure for phantom simulation, treatment planning, quality assurance, and treatment delivery. All treatment planning and delivery was performed using ARIA oncology information system and Eclipse treatment planning software. The isocenter measurements and irradiated film were analyzed using DoseLab quality assurance software; gamma criteria of 3%/1 mm, 3%/0.5 mm, and 2%/1 mm were applied for film analysis. Based on the data acquired in this work, the recommended commissioning criteria for end-to-end SRS measurements with the Varian phantom are as follows: coincidence of treatment isocenter and CBCT-aligned hidden target < 1 mm, agreement of measured chamber dose with calculated dose ≤ 5%, and film gamma passing > 90% for gamma criteria of 3%/1 mm after DoseLab auto-registration shifts ≤ 1 mm in any direction.

 

Commissioning a multileaf collimator virtual cone for the stereotactic radiosurgery of trigeminal neuralgia

Thomas A. D. Brown,¹ Rex G. Ayers,¹ Richard A. Popple^2
1Northwest Medical Physics Center, Lynnwood, WA; ²University of Alabama at Birmingham, Birmingham, AL

Journal of Applied Clinical Medical Physics, 23, 4 (2022).

A multileaf collimator (MLC), virtual-cone treatment technique has been commissioned for trigeminal neuralgia (TGN) at Tri-Cities Cancer Center (TCCC). This novel technique was initially developed at the University of Alabama in Birmingham (UAB); it is designed to produce a spherical dose profile similar to a fixed, 5-mm conical collimator distribution. Treatment is delivered with a 10-MV flattening-filter-free (FFF) beam using a high-definition MLC on a Varian Edge linear accelerator. Absolute dose output and profile measurements were performed in a 20 × 20 × 14 cm³ solid-water phantom using an Exradin W2 scintillation detector and Gafchromic EBT3 film. Dose output constancy for the virtual cone was evaluated over 6 months using an Exradin A11 parallel plate chamber. The photo-neutron dose generated by these treatments was assessed at distances of 50 and 100 cm from isocenter using a Ludlum Model 30–7 Series Neutron Meter. TGN treatments at TCCC have been previously delivered at 6-MV FFF using a 5-mm stereotactic cone. To assess the dosimetric impact of using a virtual cone, eight patients previously treated for TGN with a 5-mm cone were re-planned using a virtual cone. Seven patients have now been treated for TGN using a virtual cone at TCCC. Patient-specific quality assurance was performed for each patient using Gafchromic EBT-XD film inside a Standard Imaging Stereotactic Dose Verification Phantom. The commissioning results demonstrate that the virtual-cone dosimetry, first described at UAB, is reproducible on a second Edge linear accelerator at an independent clinical site. The virtual cone is a credible alternative to a physical, stereotactic cone for the treatment of TGN at TCCC.

 

Resilience: Learning and healing during COVID-19

Jessica M. Fagerstrom¹

¹Northwest Medical Physics Center, Lynnwood, WA 

Practical Radiation Oncology, 11, 6 (2021).

A community education art project was undertaken as a partnership between Northwest Medical Physics Center and student artists at Franklin High School in Seattle, Washington, to celebrate community resilience in the face of challenges during the coronavirus disease 2019 (COVID-19) pandemic. Franklin students used discarded radiation therapy masks as a unique outlet to explore resilience in all forms and to express hope in the face of a difficult year. The collection, titled “Resilience: Learning and Healing During COVID-19,” was available free to the public during the summer of 2021 at Columbia City Gallery. Photographs of example artwork are included.

 

Fury Road: Medical physics education using film

Jessica M. Fagerstrom,¹ Edward I. Marshall,¹ Matthew J. Nyflot,² Jessica R. Miller³

¹Northwest Medical Physics Center, Lynnwood, WA; ²University of Washington School of Medicine, Seattle, WA; ³University of Wisconsin-Madison School of Medicine and Public Health, Madison, WI

The Physics Teacher, 59, 3 (2021).

As part of a public education outreach effort, an introduction to the health effects of ionizing radiation and the field of medical physics was developed by a clinical medical physicist. The presentation was delivered to a broad public audience in 2019 (prior to COVID-19 safety concerns) through a community outreach science literacy program that pairs popular films with educational material. The program is a collaboration between a local science center and a community movie theater, and includes content developed by a keynote guest scientist and the screening of a popular film. The film chosen for the medical physics curriculum was “Mad Max: Fury Road” (Warner Bros. Pictures, Burbank, CA), which takes place in a fictional post-apocalyptic environment with widespread radiation contamination. The lecture introduced the audience to concepts of ionizing radiation, DNA damage from radiation, risk models, historical significance, ionizing radiation in the film, and ionizing radiation in our world (including therapeutic medical applications). A panel of clinical medical physicists answered audience questions following the film screening. Event attendance was high, with ticketed and lecturer seats occupying 98% of the theater’s seating capacity. Informal feedback from audience members indicated attendees had an increased interest of the field of medical physics following the program. The described education and outreach format may be used by educators interested in similar opportunities in their own communities, and also may be adapted as an entertaining and accessible installment of a physics club, seminar, or colloquium event in more formal educational settings. The electronic lecture slides are available for download as supplementary materials.

 

A Gottingen minipig model of radiation-induced coagulopathy

Karla D. Thrall,¹ Saikanth Mahendra,² Keven Jackson¹

¹Altasciences Preclinical Seattle, Everett, WA; ²Northwest Medical Physics Center, Lynnwood, WA

International Journal of Radiation Biology, 96, 12 (2020).

Purpose:  Total body irradiation of the Gottingen minipig results in a characteristic hematopoietic response, including anemia, neutropenia, lymphocytopenia, and thrombocytopenia. Currently, there are no well-characterized large or small animal models for radiation-induced thrombocytopenia. The study described here using the Gottingen minipig was focused on understanding which aspects of the coagulation cascade leads to radiation-induced coagulopathy. In this study, multiple clinical pathology parameters were determined prior to and for 45-days following total body irradiation using a 6 MV photon linear accelerator.

Materials and methods:  Following irradiation, frequent analyses of conventional hematology and coagulation parameters provided time-course information on the onset and recovery of thrombocytopenia. In addition, thromboelastography (TEG) was utilized to monitor coagulation dysfunction, namely clotting time, clot formation time, clot strength, and fibrinolysis. Coagulation factor activity levels were measured for factors II, V, VII, VIII, IX, X, XI, XII, XIII, Protein C, fibrin monomers, antiplasmin and D-dimer using a Siemen’s coagulation analyzer to provide time course information of changes in activity post irradiation exposure.

Results:  These analyses revealed that in total body irradiated minipigs, TEG tracings demonstrate long R (time to initial clot formation) and K (time to achieve a certain clot strength) times, and low alpha-angle (rate of clot formation) and MA (overall stability of the clot) during onset of thrombocytopenia (typically post irradiation day 10–15). Low alpha-angle and MA directly correlated with decreased platelet counts. A long R time is suggestive of a deficiency in clotting factors and was compared to measured activity levels of individual coagulation factors. The data indicates that coagulation factors are significantly changed early after irradiation exposure prior to thrombocytopenia and factors VIII, XI, XII and XIII are markedly altered during the critical point of thrombocytopenia.

Conclusion:  These data support the continued use of multiple approaches to evaluate the coagulation cascade in order to provide the most meaningful interpretation of the hematopoietic changes that occur post irradiation.

 

Simple phantom fabrication for MRI‐based HDR brachytherapy applicator commissioning

Jessica M. Fagerstrom,^1,2 Sukhjit Kaur¹

¹Northwest Medical Physics Center, Lynnwood, WA; ²Kaiser Permanente, Seattle, WA

Journal of Applied Clinical Medical Physics, 21, 11 (2020).

A new high dose rate (HDR) brachytherapy program was initiated in a community hospital setting, with the goal of using magnetic resonance (MR) images with the implant in place during the planning process. Physics acceptance testing and commissioning was completed for key program components, including multiple applicators. To image new applicators for MRI‐based planning prior to use with patients, agar gel doped with copper sulfate was created using simple, MR‐safe household materials as a practical and inexpensive alternative to custom‐machined precision phantoms. Applicators in‐phantom were scanned in a 1.5 T MRI scanner using the same sequences developed for the brachytherapy program, then rigidly registered to high‐resolution computed tomography (CT) images to assess distortion, artifact, and geometric displacement. To date, Varian tandem and ring sets, segmented cylinders, cervical probes, endometrial applicators; and third‐party plastic needles, tandems, and vaginal guides have been imaged in phantom and are available for use clinically.

 

Computed tomography, sinograms, and image reconstruction in the classroom

Jessica M. Fagerstrom^1
1Northwest Medical Physics Center, Lynnwood, WA

Physics Education, 55, 3 (2020).

As part of an undergraduate introductory survey course on medical physics, x-ray computed tomography (CT) was used to illustrate fundamental principles and mathematics in imaging science. A qualitative description of sinograms was presented to students through a hands-on activity involving simple classroom materials, then the basics of tomographic image reconstruction were presented. Modern applications of CT imaging, including for the diagnosis and treatment of disease, were used to emphasize the utility of medical physics and medical imaging. A simple, qualitative description of convolution, including a very elementary presentation on Fourier transforms and inverse transforms, was included to offer a basic introduction to some of the mathematical tools used in medical imaging physics. Electronic media and materials for the lesson plan are available upon request.

 

Introducing health and medical physics to young learners in preschool to fifth grade

Jessica M. Fagerstrom^1
1Northwest Medical Physics Center, Lynnwood, WA

Health Physics, 118, 1 (2020).

A hands-on learning activity was developed to introduce young learners to concepts and careers in health and medical physics. Inexpensive materials were used to create a work station with learning tools that were designed to be approachable and accessible for this audience. Visitors to a local independent, nonprofit science museum may interact with the activity work station to learn basic information regarding radiation in everyday life and to hear about careers in radiation sciences. Approximately 60 volunteer hours have been contributed associated with the activity. Interested physicists may adapt the lesson plan as a simple and straightforward way to participate in public education efforts in their own communities. A detailed lesson plan, equipment list, and electronic media are available upon request.

 

A comparative dose-response relationship between sexes for mortality and morbidity of radiation-induced lung injury in the rhesus macaque

Karla D. Thrall,¹ S. Mahendra,² M. K. Jackson,¹ William Jackson III,³ Ann M. Farese,⁴ Thomas J. MacVittie⁴

¹SNBL USA, Ltd, Everett, WA; ²Northwest Medical Physics Center, Lynnwood, WA; ³Statistician, Rockville, MD; ⁴University of Maryland, School of Medicine, Department of Radiation Oncology, Baltimore, MD

Health Physics, 116, 3 (2019).

Radiation-induced lung injury is a characteristic, dose- and time-dependent sequela of potentially lethal, delayed effects of acute radiation exposure. Understanding of these delayed effects to include development of medical countermeasures requires well-characterized and validated animal models that mimic the human response to acute radiation and adhere to the criteria of the US Food and Drug Administration Animal Rule. The objective herein was to establish a nonhuman primate model of whole-thorax lung irradiation in female rhesus macaques. Definition of the dose-response relationship to include key signs of morbidity and mortality in the female macaque served to independently validate the recent model performed with male macaques and importantly, to establish the lack of sex and institutional bias across the dose-response relationship for radiation-induced lung injury. The study design was similar to that described previously, with the exception that female rhesus macaques were utilized. In brief, a computed tomography scan was conducted prior to irradiation and used for treatment planning. Animals in 5 cohorts (n = 8 per cohort) were exposed to a single 6-MV photon exposure focused on the lung as determined by the computed tomography scan and treatment planning at a dose of 9.5, 10, 10.5, 11, or 11.5 Gy. Subject-based supportive care, including administration of dexamethasone, was based on trigger-to-treat criteria. Clearly defined euthanasia criteria were used to determine a moribund condition over the 180-day study duration post-whole-thorax lung irradiation. Percent mortality per radiation dose was 12.5% at 9.5 Gy, 25% at 10 Gy, 62.5% at 10.5 Gy, 87.5% at 11 Gy, and 100% at 11.5 Gy. The resulting probit plot for the whole-thorax lung irradiation model estimated an LD50/180 of 10.28 Gy, which was not significantly different from the published estimate of 10.27 Gy for the male rhesus. The key parameters of morbidity and mortality support the conclusion that there is an absence of a sex influence on the radiation dose-response relationship for whole-thorax lung irradiation in the rhesus macaque. This work also provides a significant interlaboratory validation of the previously published model.

 

A hands‐on introduction to medical physics and radiation therapy for middle school students

Jessica M. Fagerstrom,¹ Wendy Gao,² Gene E. Robertson¹

¹Northwest Medical Physics Center, Lynnwood, WA; ²Tacoma Valley Radiation Oncology Centers, Puyallup, WA

Journal of Applied Clinical Medical Physics, 20, 4 (2019).

Lesson plans were developed to present concepts of medical physics and radiation therapy to a middle school audience. These workshop learning units relied on hands‐on participation and collaboration within student groups to acquaint students with computed tomography simulation and treatment planning processes. These lesson plans were delivered at two different educational outreach programs targeted at student groups that have traditionally been underrepresented in science, technology, engineering, and mathematics (STEM) fields. The lesson plans are scheduled to be delivered at a third program in the future. The activities were used to introduce occupations in medical physics and radiation therapy as possible career opportunities for students, and to generate enthusiasm for continuing STEM education. Lesson plans are available upon request for educators interested in exploring medical physics educational outreach activities in their communities.

 

Dosimetric characterization of a rigid, surface-contour-specific thermoplastic bolus material

Jessica M. Fagerstrom^1,2

¹Northwest Medical Physics Center, Lynnwood, WA; ²Kaiser Permanente, Seattle, WA

Medical Dosimetry, 44, 4 (2019).

A dosimetric analysis of a commercially available thermoplastic sheet bolus, Klarity EZ Bolus^TM, was completed. Attenuation characteristics were evaluated using different configurations of a rectilinear water-mimicking plastic phantom irradiated by a high-energy linear accelerator using three photon energies, five electron energies. These results were compared with data obtained during the linear accelerator commissioning process to determine depths of water that attenuated beams similarly. CT scans of the flat, unmolded sheet bolus, as well as of the bolus molded to a cylindrical phantom, were analyzed. The product was found to form a durable and rigid, contour-specific bolus with a water-equivalent thickness of approximately 6 mm for a single sheet, and 11 mm for two sheets in tandem.

 

Redefining and reinvigorating the role of physics in clinical medicine: A Report from the AAPM Medical Physics 3.0 Ad Hoc Committee

Ehsan Samei,¹ Todd Pawlicki,² Daniel Bourland,³ Erika Chin,⁴ Shiva Das,⁵ Mary Fox,⁶ D. Jay Freedman,⁷ Nicholas Hangiandreou,⁸ David Jordan,⁹ Melissa Martin,¹⁰ Robin Miller,¹¹ William Pavlicek,¹² Daniel Pavord,¹³ Lisa Schober,¹⁴ Bruce Thomadsen,¹⁵ Brendan Whelan¹⁶

¹Duke University, Durham, NC; ²University of California, San Diego, CA; ³Wake Forest University, Winston-Salem, NC; ⁴British Columbia Cancer Agency, Vancouver Island Centre, Victoria, BC, Canada; ⁵University of North Carolina, Chapel Hill, NC; ⁶Minneapolis Radiation Oncology, Minneapolis, MN; ⁷Riverside Cancer Care Center, Newport News, VA; ⁸Mayo Clinic, Rochester, MN; ⁹University Hospitals Cleveland Medical Center, Cleveland, OH; ¹⁰Therapy Physics, Inc., Signal Hill, CA; ¹¹Northwest Medical Physics Center, Lynnwood, WA; ¹²Mayo Clinic, Scottsdale, AZ; ¹³Health Quest, Poughkeepsie, NY; ¹⁴American Association of Physicists in Medicine, Alexandria, VA; ¹⁵University of Wisconsin, Madison, WI; ¹⁶University of Sydney, Australia

Medical Physics, 45, 9 (2018).

Derived from 2 yr of deliberations and community engagement, Medical Physics 3.0 (MP3.0) is an effort commissioned by the American Association of Physicists in Medicine (AAPM) to devise a framework of strategies by which medical physicists can maintain and improve their integral roles in, and contributions to, health care and its innovation under conditions of rapid change and uncertainty. Toward that goal, MP3.0 advocates a broadened and refreshed model of sustainable excellence by which medical physicists can and should contribute to health care. The overarching conviction of MP3.0 is that every healthcare facility can benefit from medical physics and every patient’s care can be improved by a medical physicist. This large and expansive challenge necessitates a range of strategies specific to each area of medical physics: clinical practice, research, product development, and education. The present paper offers a summary of the Phase 1 deliberations of the MP3.0 initiative pertaining to strategic directions of the discipline primarily but not exclusively oriented toward the clinical practice of medical physics in the United States.

 

Standardizing nomenclatures in radiation oncology

Charles S. Mayo (Chair),¹ Jean M. Moran (Vice Chair),¹ Walter Bosch,² Ying Xiao,³ Todd McNutt,⁴ Richard Popple,⁵ Jeff Michalski,² Mary Feng,⁶ Lawrence B. Marks,⁷ Clifton D. Fuller,⁸ Ellen Yorke,⁹ Jatinder Palta,¹⁰ Peter E. Gabriel,³ Andrea Molineu,⁸ Martha M. Matuszak,¹ Elizabeth Covington,¹¹ Kathryn Masi,¹² Susan L. Richardson,¹³ Timothy Ritter,¹⁴ Tomasz Morgas,¹⁵ Stella Flampouri,¹⁶ Lakshmi Santanam,² Joseph A. Moore,⁴ Thomas G. Purdie,¹⁷ Robert Miller,¹⁸ Coen Hurkmans,¹⁹ Judy Adams,²⁰ Qing-Rong Jackie Wu,²¹ Colleen J. Fox,²² Ramon Alfredo Siochi,²³ Norman L. Brown,²⁴ Wilko Verbakel,²⁵ Yves Archambault,¹⁵ Steven J. Chmura,²⁶ Don G. Eagle,²⁷ Thomas J. Fitzgerald,²⁸ Andre L. Dekker,²⁹ Theodore Hong,²⁰ Rishabh Kapoor,¹⁰ Beth Lansing,³⁰ Shruti Jolly,¹ Mary E. Napolitano,³¹ James Percy,³⁰ Mark S. Rose,³² Salim Siddiqui,³³ Christof Schadt,³⁴ William E. Simon,³² William L. Straube,² Sara T. St. James,³⁵ Kenneth Ulin,²⁸ Sue S. Yom,⁶ Toruun Yock²⁰

¹University of Michigan, Ann Arbor, MI; ²Washington University, St. Louis, MO; ³University of Pennsylvania, Philadelphia PA; ⁴Johns Hopkins University, Baltimore, MD; ⁵University of Alabama at Birmingham, Birmingham, AL; ⁶University of California San Francisco, San Francisco, CA; ⁷University of North Carolina, Chapel Hill, NC; ⁸MD Anderson Cancer Center, Houston, TX; ⁹Memorial Sloan Kettering Cancer Center, New York, NY; ¹⁰Virginia Commonwealth University, Richmond VA; ¹¹Birmingham, Alabama; ¹²Karmanos Cancer Center, Detroit, MI; ¹³Swedish Medical Center, Seattle, WA; ¹⁴Hunter Holmes McGuire VA Medical Center, Richmond, VA; ¹⁵Varian Medical Systems, Palo Alto, CA; ¹⁶University of Florida, Jacksonville, FL; ¹⁷The Princess Margaret Cancer Center, Toronto, ON, Canada; ¹⁸Mayo Clinic, Jacksonville, FL; ¹⁹Catharina Hospital, Eindhoven, The Netherlands; ²⁰Massachusetts General Hopsital, Boston, MA; ²¹Duke University, Durham, NC; ²²Dartmouth-Hitchcock Medical Center, Lebanon, NH; ²³West Virginia University, Morgantown, WV; ²⁴Baptist Medical Center, Jacksonville, FL; ²⁵VY University Medical Center, Amsterdam, Netherlands; ²⁶University of Chicago, Chicago, IL; ²⁷Northwest Medical Physics Center, Lynnwood, WA; ²⁸University of Massachusetts, Worcester, MA; ²⁹Maastricht University Medical Center, Maastricht, Netherlands; ³⁰Elekta Corporation, St. Louis, MO; ³¹Consultant, Suwanee, Georgia; ³²Sun Nuclear, Melbourne, FL; ³³Henry Ford Health System, Detroit, MI; ³⁴Brain Lab, Chicago, IL; ³⁵University of Washington, Seattle, WA

Report of AAPM Task Group No. 263 (2018).

The radiation oncology community can benefit from standardized nomenclatures applied to targets, normal tissue structures, and treatment planning concepts and metrics. Such conformity enhances safety and quality efforts within and between clinics for routine ongoing practice, and it enables data pooling for outcomes research, registries, and clinical trials. Standardization is a vital precursor to the development of scalable uses of scripting for quality assurance and treatment plan evaluation. Increased clarity and consistency through standardizing nomenclatures in these areas would provide broad benefits. The charge of AAPM Task Group 263 is to provide nomenclature guidelines in radiation oncology for use in clinical trials, data-pooling initiatives, population-based studies, and routine clinical care by standardizing: 1. structure names across image processing and treatment planning system platforms; 2. nomenclature for dosimetric data (e.g., dose/volume histogram [DVH]-based metrics); 3. templates for clinical trial groups and users of an initial subset of software platforms to facilitate adoption of the standards; and 4. formalism for nomenclature schema which can accommodate the addition of other structures defined in the future.

 

Prototype modulated orthovoltage stereotactic radiosurgery cones

Jessica M. Fagerstrom,¹ Larry A. DeWerd,² Benjamin Palmer,² Wesley S. Culberson^2
1Northwest Medical Physics Center, Lynnwood, WA; ²Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, WI

Radiation Measurements, 119, 33 (2018).

This work sought create a set of prototype modulated orthovoltage stereotactic radiosurgery (SRS) cones, and to perform a dosimetric characterization of the prototypes. Four radiosurgical cone collimators (with cone diameters of 5 mm, 6 mm, 8 mm, and 10 mm) were built for use with an orthovoltage unit, along with epoxy-infiltrated bonded tungsten filters designed to shape the resulting dose distributions. Dosimetry measurements were performed using radiochromic film in a water phantom for both filtered and unfiltered cones at depths of 2.5 cm, 5.0 cm, and 7.5 cm. Films were scanned using a flatbed scanner and the beam profiles were analyzed. Extracted beam profiles validated the ability to optimize dosimetric results based on desired dose distributions, where for this work the goal distributions were rectangular functions. Radiochromic film measurements of dose distributions in water confirmed that the prototypes were able to achieve distributions approaching rectangular functions at depth, as determined by penumbra and flatness statistics. A prototype set of novel, modulated orthovoltage SRS filtered cones was successfully constructed, and a full dosimetric characterization was completed in water. In all configurations, filtered, optimized cones were able to achieve distributions more closely approaching the goal distributions compared to open cones.

 

Experimental investigation of GafChromic® EBT3 intrinsic energy dependence with kilovoltage x-rays, ¹³⁷Cs, and ⁶⁰Co

Cliff G. Hammer,¹ Benjamin Saul Rosen,² Jessica M. Fagerstrom,³ Wesley S. Culberson,¹ Larry A. DeWerd^1
1Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, WI; ²Department of Radiation Oncology, University of Michigan, Ann Arbor, MI; ³Northwest Medical Physics Center, Lynnwood, WA

Medical Physics, 45, 1 (2018).

Purpose: To determine experimentally the intrinsic energy response, k_bq, of EBT3 GafChromic radiochromic film with kilovoltage x-rays, ¹³⁷Cs, and ⁶⁰Co in therapeutic and diagnostic dose ranges through direct measurement with an accompanying mathematical approach to describe the physical processes involved.

Methods: The EBT3 film was irradiated with known doses using ⁶⁰Co, ¹³⁷Cs, and 13 NIST-matched kilovoltage x-ray beams. Seven dose levels, ranging from 57 to 7002 mGy, were chosen for this work. Monte Carlo methods were used to convert air-kerma rates to dose rates to the film active layer for each energy. A total of 738 film dosimeters, each measuring (1.2 × 1.2) cm², were cut from three film sheets out of the same lot of the latest version of EBT3 film, to allow for multiple dosimeters to be irradiated by each target dose and beam quality as well as unirradiated dosimeters to be used as controls. Net change in optical density in excess of the unirradiated controls was measured using the UWMRRC Laser Densitometry System (LDS). The dosimeter intrinsic energy response, k_bq, for each dose level was determined relative to ⁶⁰Co, as the ratio of dosimeter response to each beam quality relative to the absorbed dose to the film active volume at the same dose level. A simplified, single-hit mathematical model was used to derive a single-free-parameter, β, which is a proportionality constant that is dependent on beam quality and describes the microdosimetric interactions within the active layer of film. The response of β for each beam quality relative to ⁶⁰Co was also determined.

Results:  k_bq was determined for a wide range of doses and energies. The results show a unique variation of k_bq as a function of energy, and agree well with results from other investigations. There was no measurable dose dependence for k_bq within the 500-7002 mGy range outside of the expanded measurement uncertainty of 3.65% (k = 2). For doses less than 500 mGy, the signal-to-noise ratio was too low to determine k_bq accurately. The single-free-parameter, β, fit calculations derived from the single-hit model show a correlation with k_bq that suggests that β, at least in part, characterizes the microdosimetric interactions that determine k_bq .

Conclusion: For the beam qualities investigated, a single energy-dependent k_bq correction can be used for doses between 500 and 7002 mGy. Using the single-hit model with the single-free-parameter fit to solve for β shows promise in the determination of the intrinsic energy response of film, with β being the mathematical analog of the measured k_bq.

 

Proposed changes to the American Association of Physicists in Medicine governance

Todd Pawlicki,¹ Rex G. Ayers,² Kristy K. Brock,³ Jessica B. Clements,⁴ Bruce H. Curran,⁵ James T. Dobbins,⁶ Ehsan Samei,⁶ Eva Adams,⁷ Melissa Carol Martin,⁸ Lisa Schober^9
1University of California, San Diego, CA; ²Northwest Medical Physics Center, Lynnwood, WA; ³University of Texas, MD Anderson Cancer Center, Houston, TX; ⁴Kaiser Permanente, Los Angeles, CA; ⁵Virginia Commonwealth University Medical Center, Richmond, VA; ⁶Duke University, Durham, NC; ⁷TG-281; ⁸Therapy Physics, Inc., Signal Hill, CA; ⁹AAPM, Columbia, MD

Journal of Applied Clinical Medical Physics, 18, 4 (2017).

This issue’s invited Editorial is provided by Todd Pawlicki, AAPM Secretary and Chair of Task Group No. 281, Governance Assessment Communications Plan. This is an opinion article and as such does not necessarily reflect the views and position of the JACMP EIC leadership team nor those of the JACMP Board of Editors. It is being provided to inform the medical physics community of the work andsentiments of TG-281 and to assist the AAPM membership in its decision respecting the upcoming vote on this topic. –Michael Mills, JACMP EIC

 

AAPM Medical Physics Practice Guideline 8.a.: Linear accelerator performance tests

Koren Smith,¹ Peter Balter,² John Duhon,³ Gerald A. White Jr.,⁴ David L. Vassy Jr.,⁵ Robin A. Miller,⁶ Christopher F. Serago,⁷ Lynne A. Fairobent^8
1Mary Bird Perkins Cancer Center, Baton Rouge, LA; ²MD Anderson Cancer Center, Houston, TX; ³e+ OncoLogics, Lafayette, LA; ⁴Colorado Associates in Medical Physics, Colorado Springs, CO; ⁵Spartanurg Regional Healthcare System, Spartanburg, SC; ⁶Northwest Medical Physics Center, Lynnwood, WA; ⁷Mayo Clinic, Jacksonville, FL; ⁸AAPM Headquarters Staff, Alexandria, VA

Journal of Applied Clinical Medical Physics, 18, 23 (2017).

Purpose: The purpose of this guideline is to provide a list of critical performance tests in order to assist the Qualified Medical Physicist (QMP) in establishing and maintaining a safe and effective quality assurance (QA) program. The performance tests on a linear accelerator (linac) should be selected to fit the clinical patterns of use of the accelerator and care should be given to perform tests which are relevant to detecting errors related to the specific use of the accelerator.

Methods: A risk assessment was performed on tests from current task group reports on linac QA to highlight those tests that are most effective at maintaining safety and quality for the patient. Recommendations are made on the acquisition of reference or baseline data, the establishment of machine isocenter on a routine basis, basing performance tests on clinical use of the linac, working with vendors to establish QA tests and performing tests after maintenance.

Results: The recommended tests proposed in this guideline were chosen based on the results from the risk analysis and the consensus of the guideline’s committee. The tests are grouped together by class of test (e.g., dosimetry, mechanical, etc.) and clinical parameter tested. Implementation notes are included for each test so that the QMP can understand the overall goal of each test.

Conclusion: This guideline will assist the QMP in developing a comprehensive QA program for linacs in the external beam radiation therapy setting. The committee sought to prioritize tests by their implication on quality and patient safety. The QMP is ultimately responsible for implementing appropriate tests. In the spirit of the report from American Association of Physicists in Medicine Task Group 100, individual institutions are encouraged to analyze the risks involved in their own clinical practice and determine which performance tests are relevant in their own radiotherapy clinics.

 

“Brachytherapy Physics” in Comprehensive Biomedical Physics

B. Thomadsen,¹ R. Miller^2
1University of Wisconsin-Madison, Madison, WI; ²Northwest Medical Physics Center, Lynnwood, WA

Book chapter. Anders Brahme, editor-in-chief, Vol 9, Amsterdam: Elsevier, 315-381 (2014).

Brachytherapy is the treatment of disease using small radioactive sources placed in or near the tissues to be treated.    This chapter explores some of the most common sources used for brachytherapy as well as their therapeutic applications.  The delivered dose rate is one means utilized to categorize brachytherapy treatments into low, medium or high dose-rate delivery.  Further characterization is based on how long the sources are implanted, either as a temporary or a permanent implant.  Several clinical examples are reviewed.  Calculation methods, optimization and quality management are also discussed.

 

VMAT testing for an Elekta accelerator

Darryl G.L. Kaurin,^1,2 Larry E. Sweeney,^1,2 Edward I. Marshall,^1,2 Saikanth Mahendra^1,2
1Northwest Medical Physics Center, Lynnwood, WA; ²Seattle Cancer Care Alliance, Radiation Oncology, Seattle, WA

Journal of Applied Clinical Medical Physics, 13, 2 (2012).

Volumetric-modulated arc therapy (VMAT) has been shown to be able to deliver plans equivalent to intensity-modulated radiation therapy (IMRT) in a fraction of the treatment time. This improvement is important for patient immobilization/localization compliance due to comfort and treatment duration, as well as patient throughput. Previous authors have suggested commissioning methods for this modality. Here, we extend the methods reported for the Varian RapidArc system (which tested individual system components) to the Elekta linear accelerator, using custom files built using the Elekta iComCAT software. We also extend the method reported for VMAT commissioning of the Elekta accelerator by verifying maximum values of parameters (gantry speed, multileaf collimator (MLC) speed, and backup jaw speed), investigating: 1) beam profiles as a function of dose rate during an arc, 2) over/under dosing due to MLC reversals, and 3) over/under dosing at changing dose rate junctions. Equations for construction of the iComCAT files are given. Results indicate that the beam profile for lower dose rates varies less than 3% from that of the maximum dose rate, with no difference during an arc. The gantry, MLC, and backup jaw maximum speed are internally consistent. The monitor unit chamber is stable over the MUs and gantry movement conditions expected. MLC movement and position during VMAT delivery are within IMRT tolerances. Dose rate, gantry speed, and MLC speed are accurately controlled. Over/under dosing at junctions of MLC reversals or dose rate changes are within clinical acceptability.

 

Meeting abstracts

 

Careers in Medical Physics

Krista Burton¹

¹Northwest Medical Physics Center, Lynnwood, WA 

American Association of Physics Teachers Annual Winter Meeting, Portland, OR (2023).
* Invited presentation *

Working in the field of medical physics can take many different forms, with careers including regulatory, industry, clinical, research, and/or teaching responsibilities. Medical physicists work in a variety of specializations, including health physics, imaging science, nuclear medicine, and therapy radiation physics. Depending on their specialty, a day in the life of a medical physicist may involve calculating the thickness of concrete shielding required for a proton therapy vault, modeling the radiobiological effects of various radioisotopes, using machine learning to improve computer-aided diagnosis of disease, or acquiring linear accelerator data with a high-precision water tank. The American Association of Physicists in Medicine (AAPM), an AAPT affiliated organization, offers resources for educators interested in discussing a real-world career field for students who hope to help patients while pursuing a career in physics.

 

Medical Physics as A Tool for Classroom Engagement

Jessica M. Fagerstrom¹

¹Northwest Medical Physics Center, Lynnwood, WA 

American Association of Physics Teachers Annual Winter Meeting, Portland, OR (2023).
* Invited presentation *

Physics education research has indicated that students who perceive curricula as relating to real life applications are more engaged with learning materials, and medical physics may provide an absorbing anchoring context. Medical physics is an evolving and expanding field in which concepts in physics and engineering are applied to challenges in medicine. Medical physicists perform critical job functions in healthcare every day, but many physics students complete their entire course of study without learning about the existence of the field. Medical physics, as an applied science, offers an abundance of real-world examples for wide ranging concepts in physics courses. For example, an introductory unit on basic concepts in nuclear physics could be brought to life by a discussion of radioisotopes and positron emission tomography (PET) imaging. Students learning about electromagnetic radiation may bolster their engagement by learning about radiation therapy used to ablate lung tumors.

 

Medical physics as an anchor for physics learning

Jessica M. Fagerstrom¹

¹Northwest Medical Physics Center, Lynnwood, WA 

IUPAP International Conference on Physics Education 2022, Virtual (2022).

Medical physics is a diverse and dynamic field in which principles in physics are used in a healthcare setting. Scientists in medical physics often hold graduate degrees after focused studying and research on specific subtopics within medical physics, but topics in medical physics may be explored in more general introductory physics courses at the university and secondary school levels. Individual lesson plans, units, or even full survey courses covering medical physics topics may be taught to introduce students to topics including medical imaging, nuclear medicine, radiation therapy, health physics, and biophysics.

Previous work has shown that curricula that students perceive as applicable to the “real world” and/or connected to content from other coursework is considered by students as engaging and interesting (Geller, Turpen, & Crouch, 2018). Medical physicists describe finding their field rewarding, often reporting high levels of career satisfaction (Chen et al., 2015). However, students interested in physics early in their studies may not be aware of the existence of this field, or of the possibility to pursue medical physics as a career (Buckley, 2016). Integrating topics in medical physics into the curricula may then serve two purposes: first, it may stimulate learning as a relevant and connected topic to students’ prior knowledge and interests; and second, it may inspire students to consider pursuing physics as a serious topic of study in preparation for a possible career opportunity.

Examples of medical physics topics that may be used in the physics classroom could include examining the concept of radioactive decay to explore basic statistical processes and distributions; reviewing magnetic resonance imaging (MRI) physics to discuss introductory topics in quantum mechanics and electricity and magnetism; using radiation protection and radiation shielding design to discuss practical applications of the inverse square law; and reviewing medical ultrasound to explore oscillations and waves. Physics educators are encouraged to consider integrating some examples from medical physics into their curricula to spur student engagement and to acquaint students to one example of a rewarding career opportunity in physics.

 

Therapeutic radiation beam output and energy variations analysis across clinics, people, technology, and time

M. M. Zaini,¹ E. I. Marshall¹, K. M. Hedrick,¹ J. M. Fagerstrom,¹ D. Zaks,¹ T. M. Fitzgerald¹

¹Northwest Medical Physics Center, Lynnwood, WA 

IUPESM World Congress on Medical Physics and Biomedical Engineering, Singapore (2022).

Our group of about 50 clinical physicists has been engaged in measuring various beam characteristics of more than 50 therapeutic linear accelerators in more than 30 clinics manufactured by the main two radiation oncology machine manufacturers in North America over many decades. For the past decade, we have been using the same measurement setup for all our machines every month. These linac radiation output measurements employ a set of acrylic slabs, Farmer ion chambers, electrometers, and temp/pressure measuring gadgets. Such charge readings in acrylic phantoms are precisely linked to AAPM’s TG-51 formalism. A unique parameter, called, kacrylic, has been used for converting the raw readings of such radiation measurements to the actual machine output calibration values. Statistical analysis of this parameter, radiation output, and beam energy of our therapeutic linacs over time and among machine types by various physicists is presented.

The number of Varian and Elekta linacs included in this work are 42 and 9, respectively. The two linac types showed similar coefficient of variations (COV) for kacrylic for all radiation beams across different linacs at different clinics. For Varian machines, COV of kacrylic varied from 0.1% to 1.6% for 16 photon and electrons beams and that for Elekta machines ranged from 0.1% to 1.5% for 11 radiation beams. However, COV of kacrylic for single photons beams on individual linacs across time ranged from 0% to 1.8% for Varian machines, whereas that for Elekta linacs varied from 0% to 0.8%. This variation for individual electron beams of each linac spanned the range of 0% to 2.0% and 0% to 1.1% for Varian and Elekta linacs, respectively. Histograms of radiation beam energies and outputs for all the machines throughout the network are depicted. The impact of slight deviations in the measurement apparatus as well as observational disparities among different physicists are discussed.

 

A novel anthropomorphic head phantom for the commissioning of MLC-based stereotactic radiosurgery on a linear accelerator

T. Brown,¹ C. Beck,¹ H. Holloway,¹ J. Kerns,² J. Fagerstrom,¹ D. Kaurin,¹ K. Kielar^2
1Northwest Medical Physics Center, Lynnwood, WA; ²Varian Medical Systems, Palo Alto

ESTRO Annual Meeting, Madrid, Spain (2021).

Purpose/Objective: A new anthropomorphic head phantom has been designed by Varian Medical Systems for the purposes of commissioning MLC-based SRS treatments on TrueBeam and EDGE linear accelerators. The validation of the phantom prototype and development of an end-to-end testing procedure was performed by Northwest Medical Physics Center, a non-profit, clinical physics consulting group, at two independent community cancer centers with active SRS programs.

Material/Methods: The initial phantom prototype was designed to accommodate four interchangeable target cassettes for CT-MRI fusion verification, Winston-Lutz and hidden target tests, an ion chamber, and two perpendicular EBT3 film segments. A 2-cm, contrast-enhanced target located at the center of the ion chamber and film cassettes allowed for single-target SRS verification. Phantom testing and end-to-end procedure development were performed using a Varian TrueBeam and EDGE linear accelerators equipped with Millennium and high-definition MLC, respectively. Treatment plans at 6FFF and 10FFF were designed and tested using a nominal prescription dose of 18 Gy. End-to-end testing comprised of phantom simulation, isocenter size determination, and treatment delivery using a pinpoint ion chamber and EBT3 film.

Results: Initial prototype testing resulted in design changes for a revised, final design, including a change in the Winston-Lutz central marker material, addition of a second (1 cm) target to the film cassette, inclusion of fiducial points to the EBT3 film for registration with DICOM dose planes, and an adjustment to the external dimensions of the film cassette. Pinpoint ion chamber measurements showed agreement with the planned dose to within 3% for all plans tested. DoseLab software was used to perform a relative gamma analysis of the film dose planes compared to extracted data from the treatment planning system. Film irradiations showed gamma-passing results >90% for 3% and 1 mm, using auto-registration shifts ≤1 mm in any direction.

Conclusion: Initial testing of a new anthropomorphic SRS head phantom indicates that the system is robust for verifying end-to-end MLC-based SRS treatments. Ongoing work is now concerned with collecting end-to-end data, using the revised phantom design, from additional clinical sites for the purposes of establishing acceptance criteria for end-to-end measurements.

 

A multi-clinic validation of an end-to-end procedure for MLC-based stereotactic radiosurgery with a novel phantom

T. Brown,¹ J. Fagerstrom,¹ C. Beck,¹ C. Holloway,¹ J. Kerns,² D. Kaurin,¹ K. Kielar^2
1Northwest Medical Physics Center, Lynnwood, WA; ²Varian Medical Systems, Palo Alto

AAPM Annual Scientific Meeting, Virtual (2021).

Purpose: An end-to-end procedure for a new anthropomorphic phantom was developed for MLC-based SRS commissioning on a linear accelerator. This procedure was developed and tested by Northwest Medical Physics Center at three independent clinical sites with Varian Edge and TrueBeam accelerators, following local standards for MLC-based SRS planning.

Methods: The end-to-end procedure uses a novel anthropomorphic SRS phantom recently developed by Varian Medical Systems. This phantom was used to verify isocenter coincidence, targeting accuracy, treatment modulation and absolute dosimtery for dose delivery to a single target and single-isocenter, multitarget geometries. Testing was performed at three clinical sites in an all-Varian environment consisting of ARIA, Eclipse treatment planning, and TrueBeam/Edge machines with Millennium and HD-MLCs. CT simulation and treatment delivery was performed using Qfix Encompass and Brainlab mask systems. Treatment plans were developed for RapidArc and dynamic conformal arcs at 6FFF and 10FFF. Consistent planning criteria was applied across all three clinical sites. A nominal dose of 16 Gy was prescribed for each plan using a sites’ standard beam geometry for SRS cases.

Results: Pinpoint ion chamber readings showed agreement with the planned dose to within 3% for treatment delivery to a 2-cm target. DoseLab software was used to perform relative gamma analysis of the film dose planes compared to extracted data from the treatment planning system. Most single and multitarget treatment plans showed gamma passing rates >90% for 3% and 1 mm after auto-registration shifts ≤ 1 mm in any direction.

Conclusion: Validation of the end-to-end procedure at three independent clinical sites indicates that it is acceptable for the commissioning of MLC-based SRS treatments. Data is now being acquired at additional clinical sites for the purposes of establishing acceptance criteria for end-to-end measurements performed on TrueBeam and Edge machines.

 

Computational evaluation of different dose engines and in-vivo measurements of a VMAT chest wall irradiation case with brass bolus

M. M. Zaini,^1,2 S. S. Honeywell,² M. A. Whiton,² A. P. Mazza,^1,2 R. Poortvliet,² C. Kahnke^2
1Northwest Medical Physics Center, Lynnwood, WA; ²Skagit Valley Hospital, Mount Vernon, WA

AAPM/COMP Annual Scientific Meeting, Virtual (2020).

Purpose: To evaluate two TPS algorithms and their agreement with in-vivo measurements of a brass bolus VMAT treatment case.

Methods: VMAT treatment was prescribed for a young post-mastectomy right breast patient with curative intent. Surgery had resulted in a large uneven skin surface, yet full skin dose was desired. Brass mesh bolus was employed to escalate the skin dose. In-vivo measurements using MOSFETs in conjunction with treatment plan-based evaluations guided the radiation delivery setup schemes. Both convolution-superposition and Monte Carlo techniques were used in this comparison. Modelling the brass bolus in RayStation had to be conducted differently for the two photon algorithms because of their underlying physics.

Results: In-vivo measurements using a buildup cap on the MOSFETs, which is a good surrogate of dmax dose of the underlying tissue, revealed that the prescribed dose was delivered accurately to within the precision range of such measurements. However, MOSFET measurements with no buildup cap indicated that employing no bolus on the treatment field resulted in about 45% underdosage of skin. Two layers of brass bolus provided adequate skin dose values. One layer enhanced the skin dose by 20% and two layers by about 35% from the no bolus case. Medical judgments determined the duration of employing the bolus to reach the desired skin dose, while avoiding skin toxicity. Appending VMAT treatment with electron beams was also considered based on clinical evaluations.

Conclusion: It is possible to achieve clinically acceptable skin dose levels in VMAT treatment of chest wall patients using brass bolus. Combination of different treatment techniques can help reach this goal. Modelling brass bolus using the convolution-superposition algorithm was not as adherent to measurements as that achieved by employing careful Monte Carlo computational systems.

 

Benchmark performance measurements of a novel biology-guided radiotherapy (BgRT) machine using TG-148 and TG-142

D. Zaks,¹ R. Bassalow,² O. Volotskova,³ M. Narayanan,¹ C. Huntzinger,¹ S. Shirvani,¹ S. Mazin,¹ G. Kuduvalli^1
1RefleXion Medical, Hayward, CA; ²Northwest Medical Physics Center, Silverdale, WA; ³Sutter Health, Berkeley, CA

AAPM/COMP Annual Scientific Meeting, Virtual (2020).

Purpose: To benchmark the mechanical performance of the RefleXion™ X1 biology-guided radiotherapy (BgRT) machine, which employs a novel architecture with linac, kVCT and PET subsystems in the same treatment gantry, using TG-148 and TG-142 methodologies.

Methods: Measurements were executed with adherence to AAPM TG-148 section V.B. methodology, with modification allowed for meaningful differences in machine geometry. Six tests were executed using Red Virtual Water Phantoms (RVWP) from Standard Imaging Inc and XRQA2 film from Ashland Gafchromic. Phantoms were scanned using an EPSON 12000XL-PH flatbed scanner and analyzed using an RITG148+ QA module. The X alignment of MLC to source test was executed using MV EPID images and an in-house Python script to determine the out of focus parameter. The Y jaw alignment to source test was executed using an Exradin A17 ion chamber, using smaller jaw sweep steps due to a smaller range of jaw sweep motion. Finally, a “star shot” test was delivered to XRQA2 film and solid water phantoms to evaluate isocenter size per AAPM TG-142.

Results: All measured parameters passed the evaluated criteria established in TG-148 and TG-142, including Y-jaw twist (0.04° measured vs. 0.5°), Y-jaw divergence (0.48 mm measured vs 0.5 mm), Y-jaw alignment (0.14 mm baseline), MLC twist (0.04° measured vs. 0.5°), MLC Offset (0.39 mm measured vs 1.5 mm), MLC x-alignment (0.21% measured vs 2%), treatment field centering (0.1 mm measured vs 0.5 mm), synchronicity (0.083 mm per 5 cm measured vs 1 mm per 5 cm), and radiation isocenter radius (0.56 mm measured vs 1.0 mm)

Conclusion: The experimental results demonstrated that the novel architecture of the RefleXion X1 machine was able to meet the expected mechanical performance requirements outlined in AAPM TG-148 and TG-142 reports.

 

Characterization of the IMRT and SBRT performance of a novel biology-guided radiotherapy (BgRT) machine using ArcCHECK

D. Zaks,¹ M. Narayanan,¹ R. Bassalow,² O. Volotskova,³ Y. Voroneknko,¹ D. Pal,⁴ D. Rigie,¹ J. Burns,¹ A. Purwar,¹ P. Olcott,¹ G. Kuduvalli^1
1RefleXion Medical, Hayward, CA; ²Northwest Medical Physics Center, Silverdale, WA; ³Sutter Health, Berkeley, CA; ⁴GE Healthcare, Menlo Park, CA

AAPM/COMP Annual Scientific Meeting, Virtual (2020).

Purpose: To characterize the IMRT and SBRT performance of the RefleXion™ X1 biology-guided radiotherapy (BgRT) machine using the ArcCHECK (AC) dosimetry device.

Methods: Plans were first generated on the accompanying X1 TPS using CT simulation images of custom inserts designed for use with the AC. The custom inserts represented targets or OARs positioned at specific locations and included a 22 mm diameter sphere and a c-shape target with 26mm axial length with 255° partial annulus with major and minor radii of 27 mm and 12 mm, respectively. The background material in the insert was either a homogenous water medium or a water filled heterogeneous medium with a Styrofoam mesh simulating lung tissue surrounding the target.

A variety of combinations of homogenous and heterogenous inserts and custom target/OARs were used for the experiments, as follows: homogenous insert with a spherical target, homogenous insert with two targets: spherical target + C-shaped target, homogenous insert with C-shaped target, and heterogenous insert with a spherical target.

After dose delivery, the AC software was used to compare the diode dose readings with the DICOM RTDose output of the X1 TPS to determine the accuracy of dose delivery using the Gamma Index, as demonstrated by the Gamma Passing rate (GP).

Results: The following is a representative sample of 5 test cases and the corresponding GP result; 1) C-shape, homogenous insert, 2Gy, GP = 93.4%, 2) C-shape, homogenous insert, 10Gy, GP = 91.0% 3) Sphere, homogenous insert, 10Gy, GP = 99.4% 4) Sphere, heterogenous insert, 10Gy, GP = 96.9% and 5) C-shape + Sphere, homogenous insert, 10Gy, GP = 98.0%.

Conclusion: All IMRT and SBRT test cases met or exceeded the Gamma Index passing rate of at least 90% of the measurement points met or exceeded the 3%/3 mm criteria.

 

Performance validation of a novel biology-guided radiotherapy (BgRT) TPS following the IAEA-TECDOC-1540 methodology

D. Zaks,¹ R. Bassalow,² S. Maganti,¹ Y. Voroneknko,¹ D. Rigie,¹ O. Volotskova,³ P. Olcott,¹ G. Kuduvalli^1
1RefleXion Medical, Hayward, CA; ²Northwest Medical Physics Center, Silverdale, WA; ³Sutter Health, Berkeley, CA; ⁴GE Healthcare, Menlo Park, CA

AAPM/COMP Annual Scientific Meeting, Virtual (2020).

Purpose: To validate the performance of the RefleXion™ X1 biology-guided radiotherapy (BgRT) TPS following the testing methodology outlined in IAEA-TECDOC-1540: Specification and Acceptance Testing of Radiotherapy Treatment Planning Systems.

Methods: Static field plans were developed for each of the tests on the X1 TPS and the resulting RTDOSE DICOM files were exported for comparison to measurements. Dose measurements were made utilizing a commercially available water phantom and solid slab materials simulating water, bone and lung. All ion chambers were calibrated by an ADCL or ADCL-equivalent laboratories.

Results: Depth Doses for field sizes of 10×1, 10×2, 40×1 and 40×2 (cm2) and depths of 10 and 20cm all passed within a 2% tolerance. X/Y profiles for field sizes of 10×1, 10×2, 40×1 and 40×2 (cm2) at 85 cm SSD, 1.5 cm depth all passed within a tolerance of 2%/2mm DD/DTA gamma criteria. Profile widths for 5×1, 5×2 10×1 and 10×2 (cm2) field sizes passed within a 2% (IEC-Y) and 1mm (IEC-X) criteria. Water phantom measurements were also conducted to test the correspondence between calculations and measurements of the following parameters; profile penumbras, build up region, centrally closed MLC leaves, beam symmetry, absolute dose output, field output factor and beam quality.

Variable SSD was tested utilizing solid water and field sizes of 5×1, 5×2, 10×1 and 10×2 (cm2) at SSD=70 cm and depths of 2, 5, 8.5, 13.5 cm. Variable CT and mass densities were tested utilizing two different configurations of solid material slabs simulating embedded lung and bone, and field sizes of 5×1, 5×2, 10×1 and 10×2 (cm2). All measurements passed within the 3% tolerance.

Conclusion: Tests assessing dose calculation accuracy between the X1 TPS and physical measurements of the 6MV photon beam in a variety of test conditions demonstrated compliance to tolerances drawn from well-accepted industry standards.

 

Validation of ArcCHECK for use with a novel ring gantry-based biology-guided radiotherapy (BgRT) machine

D. Zaks,¹ M. Narayanan,¹ R. Bassalow,² O. Volotskova,³ C. Huntzinger,¹ S. Shirvani,¹ S. Mazin,¹ G. Kuduvalli^1
1RefleXion Medical, Hayward, CA; ²Northwest Medical Physics Center, Silverdale, WA; ³Sutter Health, Berkeley, CA; ⁴GE Healthcare, Menlo Park, CA

AAPM/COMP Annual Scientific Meeting, Virtual (2020).

Purpose: To validate the ArcCheck (AC) dosimetry device as a dosimetry tool to measure the dose delivered by a the RefleXion™ X1 biology-guided radiotherapy (BgRT) machine.

Methods: Seven IMRT and SBRT plans were created in the X1 TPS. The plans included a centered 8×5 cm cylinder, a centered 25 mm ball, a centered 40 mm ball, an off axis 25 mm ball, an off axis 40 mm ball, 2 symmetrically offset 25 mm balls, and an off-axis C-shape. The seven plans were delivered to the assembly of ArcCheck, multi-plug, ion chamber, and film. The AC multi-plug is designed to hold ion chambers and film within the AC cavity. For each of the delivered plans, the data was recorded by AC, calibrated A14SL ion chambers and EBT3 Gafchromic film. The point doses and dose distribution extracted from the film were compared to the dose predicted by the treatment plan. The overall passing rate on the AC absolute surface dose was calculated by comparison to the calculated DICOM RTDose object from the X1 TPS. The overall passing rate on the 2D relative film dose was compared with a 2D plane of the 3D DICOM RTDose object, as evaluated in MATLAB. Both passing rates were calculated at a 3%/3mm gamma.

Results: The three on-axis test objects (8×5 cylinder, 25mm ball, 40mm ball) had passing rates of 99.5%, 98.8% and 99.7% with film and 97.4%, 100%, and 92.3% with AC. The four off-axis test objects (25mm ball, 40mm ball, 2 balls, C shape) had passing rates of 93%, 95.6%, 93.3% and 92.3% respectively.

Conclusion: All 7 test cases passed the test criteria, validating the use of AC as a dosimetry tool for use with the novel ring-based architecture of the X1 biology-guided radiotherapy machine.

 

Community hospital experience commissioning a new MR-based HDR program

Jessica M. Fagerstrom,^1,2 Jeffrey Marotta,² Justin Cantley^1,2
1Northwest Medical Physics Center, Lynnwood, WA; ²Kaiser Permanente, Seattle, WA

AAPM/COMP Annual Scientific Meeting, Virtual (2020).

Purpose: A community hospital developed an MRI-based high dose rate (HDR) brachytherapy program. The program launched treating locally advanced cervical and non-operable endometrial cancer, intending to add other sites in the future. To initiate the program, physics acceptance testing and commissioning were performed.

Methods: A Varian Bravos afterloader was purchased for use with BrachyVision treatment planning system (TPS), and Varian and third-party applicators. TPS validation of the TG-43 algorithm was completed by following the vendor-provided installation product acceptance procedures, and by comparing published values to calculated points and volumes. Secondary dose check software was reviewed following similar procedures. Interrupt testing determined an internal policy for minimum planned dwell time.

The facility’s MR safety team reviewed all applicators and approved their use. Inventory and visual inspection were completed, and autoradiographs were acquired of all applicators. Radiographs were acquired including marker wires indicating source positions. A length assessment device was used to establish channel lengths. Vendor-provided digital solid models were validated for solid applicators, where applicable. Sterilization procedures were established with central processing.

Applicators were set in all-plastic containers filled with agar gel doped with copper sulfate using food-grade agar powder, distilled water, and CuSO4 solution, with applicators held in place using custom-cut foam blocks. The phantoms were scanned in both MRI and CT using the sequences intended for patient procedures.

Results: Recommendations for applicator use were made based on commissioning measurements. Varian MR-compatible vaginal segmented cylinders, rigid guide tubes, mould probes, and a cervix probe set; and third-party vaginal guides, plastic needles, and polymer tandems were commissioned and approved for clinical use.

Conclusion: Acceptance testing and commissioning for an MR-based HDR program was completed in a community hospital setting. AAPM Task Groups Reports 236, 284, and 303, all currently under development, will offer future guidance on this process.

 

Dose agreement dependence on chamber orientation in Quality Assurance (QA) for stereotactic radiosurgery

Christopher Anderson,^1,2 Jose Garcia-Cobian,² Bing Fang^2
1Pacific Lutheran University, Tacoma, WA; ²Northwest Medical Physics Center, Lynnwood, WA

Radiosurgery Society (RSS) Annual Scientific Meeting, Virtual (2020).

Purpose: In CyberKnife® (Accuray – Sunnyvale, CA) radiosurgery plans created to treat brain lesions, orienting the ion chamber across the couch instead of longitudinally has been observed to sometimes yield closer agreement between the dose predicted by the planning system (Precision, Accuray – Sunnyvale, CA) and the dose measured during QA. Methods were developed to test the hypothesis that dose agreement in QA conducted with a certain orientation of the A16 Standard Imaging (Middleton, WI) chamber is related to the proportion of measured radiation incident posterior to and delivered by beams directly intersecting the chamber.

Methods: Individual beams were delivered to a chamber from various angles and the resulting depth-corrected dose calculated from electrometer readings were compared. For seventeen SRS treatment plans, patient specific QA was conducted with the chamber oriented along and across the couch, and measurements were converted to dose using a transfer factor calculated at the time of TG-51. Next, dose agreement resulting from both orientations were compared. For each treatment plan, the proportion of planned monitor units delivered by beams that intersected the chamber thimble was recorded. In six SRS treatment plans, a new isocentric beam set was generated to control for directly intersecting dose. QA was conducted for these plans with the chamber oriented along and across the couch (for a total of twelve plans) and the depth-corrected dose from each beam was recorded. Additionally, in four plans, the angles of beams that delivered monitor units were analyzed.

Results: Data showed that the A16 ion chamber has little angular dependence, except when radiation is delivered from a point 45° from a direction posterior to it. Irradiation from such a point produces a result lower by 1.1% than when delivered from a perpendicular direction. Orienting the chamber across the couch during QA improved dose agreement in four out of seventeen non-isocentric plans, suggesting that there many situations in which this orientation is not optimal. In plans in which QA dose agreement improved with the chamber crosswise, the proportion of monitor units directly intersecting the chamber appeared to have a stronger correlation with improved dose agreement in QA conducted with the chamber along the couch than in QA conducted with the chamber across the couch, though the significance of these results could be further evaluated by using larger samples. In the isocentric plans, correlation coefficients calculated for a proposed relation between the distribution of irradiation angles and dose agreement suggest that agreement is improved by delivering a smaller proportion of monitor units from behind the chamber and a larger proportion from the front and side. In isocentric QA conducted with the chamber oriented across the couch, beams originating posteriorly to the chamber were slightly overrepresented in the group of beams delivering the lowest dose, after corrections were made for SAD and phantom depth.

Conclusion: The results suggest that dose agreement in QA plans with a given chamber orientation might be predicted by incidence angle and proportion of beams that intersect the chamber.

 

Prostate intra-fraction motion monitoring using an integrated linear accelerator kV imaging system

R. Alex Hsi,¹ Adam Schoen^1,2
1Evergreen Health Cancer Care, Kirkland, WA; ²Northwest Medical Physics Center, Lynnwood, WA

ASTRO Annual Scientific Meeting, Chicago, IL (2019).

Purpose: To assess the outcomes of intra-fraction prostate motion monitoring using an integrated linear accelerator based KV imaging system.

Methods: Between June 2017 and December 2018, 1516 radiation treatment fractions were delivered to 35 prostate cancer patients. Each patient had 3 gold fiducial markers placed in the prostate prior to initiation of therapy and underwent IMRT treatment planning using an arc therapy technique. A commercially available integrated kV imaging system (capable of 2D and 3D cone beam imaging) with localization software was used for each treatment. Prior to initiation of treatment, a cone beam CT was obtained to align the fiducial markers and assess bladder and rectal filling. Once treatment was initiated, a planar kV image was obtained every 40 degrees in the treatment arc (approximately every 5-7 seconds) and analyzed for fiducial marker position. If the fiducial marker position varied by greater than 3.5mm, the treatment was stopped with less than 1 MU delivered. Fiducial marker position was reassessed with another kV image within 10 seconds and if the markers were still outside of the 3.5mm threshold, the patient was repositioned and treatment resumed.

Results: All treatment fractions were delivered successfully without imaging system failure. The average time for a treatment fraction was 9.4 minutes ± 2.7 minutes (6.8 – 28.8 minutes). 672 of the treatments (44.3%) had at least one beam interruption due to fiducial marker variation over 3.5 mm from anticipated location. 171 (11.3%) treatments required at least 2 treatment interruptions, 74 (4.9%) treatments required at least 3 treatment interruptions and 35 (2.3%) treatments required 4 or more interruptions. Patient repositioning was required in 365 (24.1%) treatments.

Conclusion: An integrated linear accelerator based kV imaging system can be used to successfully and efficiently to monitor intra-fraction prostate motion.

 

A small community cancer center’s experience with providing radiation therapy with a single Varian Halcyon linear accelerator

Jennifer Fisher,¹ Darryl Kaurin^1
1Northwest Medical Physics Center, Lynnwood, WA 

AAPM Southwest Chapter Meeting, Little Rock, AK (2019).

Invited presentation at the annual American Association of Physicists in Medicine Southwest Chapter Meeting.

 

Profile- and output-determined P_rp output corrections for a Varian TrueBeam and Elekta Versa HD

D. G. Kaurin,¹ A. Eagle,¹ A. Hart,¹ C. Holloway,¹ G. J. Courlas,¹ P. Stevens,¹ O. Gopan,¹ A. Schoen^1
1Northwest Medical Physics Center, Lynnwood, WA

ASTRO Annual Scientific Meeting, Chicago, IL (2018).

Purpose: For calibration of flattening filter-free (FFF) linac beams, one needs to either use a chamber small enough to avoid volume averaging that occurs with the conventional Farmer-type chamber, or apply a gradient correction (P_rp) to the Farmer chamber. Continued use of the Farmer chamber is attractive due to the desirable characteristics of the chamber. We measured P_rp using two methods, the recommended profile gradient method, and an output method with a small chamber having negligible volume averaging, for both a Varian TrueBeam and Elekta Versa 6MV and 10MV FFF beams for 90 cm source-to-surface distance (SSD), 100 cm source-to-axis (SAD) calibrations.

Methods: For profile-determined P_rp, profiles were measured along the calibration plane (100 cm SAD, 90 cm SSD, 10 cm depth) using a diode having a sensitive diameter of 0.8mm. The profiles were integrated numerically along both the length and width of both chamber types, with the deviation from unity giving the magnitude of P_rp. The Farmer chamber was 23 mm in length, 3.05 mm in radius, and had a volume of 0.6cc. The smaller chamber was 6.5 mm in length, 2.75 mm in radius, and a volume of 0.125cc. For the output-determined P_rp, beam outputs were measured for 6MV conventional flattening filter (cFF), 6FFF, 10cFF, and 10FFF beams using the TG51 protocol for six Farmer and six 0.125cc chambers. Use of six chambers was thought to improve statistics. The 0.125cc chamber was not reference grade, and required cross calibration with the Farmer chamber using cFF beams. To validate the lack of a gradient for the cFF beams, profiles of the cFF beams were measured as well, with a corresponding profile-determined P_rp. A ratio of the FFF beam output between Farmer and 0.125cc chambers gives the output-determined P_rp.

Results:  Values of profile-determined P_rp for the TrueBeam were 1.003 and 1.007 for 6FFF and 10FFF beams, respectively. Values of profile-determined P_rp for the Versa were 1.005 and 1.007 for 6FFF and 10FFF beams, respectively. Surprisingly, the Versa 10cFF beam used was not “flat”, having a central axis divot, giving a P_rp 0.995. This beam was not included in the cross-calibration of the 0.125cc chambers. Values of the output-determined P_rp were within 0.1% of the profile-determined P_rp for the TrueBeam, and within 0.4% for the Versa. Repeated Versa measurements gave similar results. One reference for the 0.125cc chamber used here indicates an anomalous dose-per-pulse issue for cFF beams, which may be exacerbated for FFF beams. Values of P_rp from other linacs available to us and from the literature indicate good agreement (within 0.1%, one 0.2% outlier) for the TrueBeam and profile-determined P_rp for the Versa.

Conclusion: Profile- and output-determined P_rp values for the TrueBeam are consistent. A P_rp correction for the Versa 10cFF may be warranted, but appears linac or energy dependent. The profile-determined P_rp appears to be more reproducible.

 

On-board cone beam computed tomography as radiotherapy simulation for emergent cases

G. Johnson,¹ J. F. Raymond,¹ R. Moore,² H. Nottingham,² M. Quiles,² D. G. Kaurin^1
1Northwest Medical Physics Center, Lynnwood, WA; ²Providence RadiantCare Radiation Oncology, Lacey, WA

ASTRO Annual Scientific Meeting, San Antonio, TX (2018).

Purpose: To investigate dosimetric clinical appropriateness of on-board kV cone beam computed tomography patient simulation (CBCT-Sim) for palliative treatment plans at a remote satellite clinic having no conventional computed tomographic simulation (CT-Sim) when travel isn’t a viable option. Our current approach is a separation calculation using two opposing fields on a water phantom. A CBCT of the patient would improve the plan by utilizing the patient’s actual geometry and we expect to see dose points in the CBCT-Sim plan image set less than 3% different than a conventional CT, and the separation calculation to be within 5% of the conventional CT-Sim plan.

Methods: Patients treated for the four most likely emergent or immobile disease sites (brain, thoracic spine, lumbar spine, and pelvis) with an original CT-Sim and at least one CBCT during course of treatment were investigated retrospectively. Three brain patients, three T-spine, three L-spine, and two Pelvis patients were used. The patients’ images were copied into a new, anonymous patient. A new CT-density curve was created in the treatment planning software for the CBCT based on CT-density phantom data.

A standard plan for each given site was planned on the original CT image set, then copied on both a heterogenous and homogenous CBCT image set. The homogenous CBCT image was density overridden to water. A rectangular water phantom was also created for a separation calculation plan, which is our standard practice for these cases. For the brain cases, the CT and heterogenous CBCT plans were both contoured with critical structures.

For all cases, seven to eleven points of interest in all directions (isocenter, and fractional separation distances in 3-dimentional space) were used to compare both CBCT-Sim plans and the separation calculation to the standard CT plan.

Results:  For all cases, the average dose difference per case compared to the standard CT-Sim plan was less than 3% for either CBCT-Sim plan and less than 4% for the separation calculation plan; however, the separation calculations showed large deviations, up to 7.2%, while the CBCTs showed a maximum of 4.8%. Between all cases, the average dose difference was lowest compared to CT-Sim plans for the heterogenous CBCT plan with 1.2±0.7%, while the homogenous CBCT plan difference was 1.4±0.7% and the separation calculation was 1.7±1.1%. The most dosimetric benefit for the heterogenous CBCT-Sim plans was seen for the brain plans, followed by the T-spine plans, with negligible benefit for pelvis and L-spine plans.

Conclusion: Dosimetric accuracy is improved with a CBCT-Sim compared to our standard separation-based plan, due to the use of actual patient geometry and density.

 

Optically Stimulated Luminescence Dosimeter (OSLD) linear vs. nonlinear calibration optimization

K. Wake,¹ S. Mahendra^1
1Northwest Medical Physics Center, Lynnwood, WA

AAPM Annual Scientific Meeting, Nashville, TN (2018).

Purpose: To explore observed dose discrepancies between photon and electron irradiated OSLDs and determine the optimum procedure for creating and using linear and nonlinear microSTARii calibration files based on specific dose range and energy source of clinical relevance.

Methods: Commercial dosimeters (Landauer screened nanoDots) were irradiated according to manufacturer’s recommended dose range (5 – 1300 cGy) at 6 MV to create clinic specific calibration sets of OSLDs. A quality control (QC) set was created with photon (6MV) and electron (6MeV/12 MeV) beams for purposes of testing calibrations at typical therapeutic dose range. Scatter conditions, depth, and doses for typical treatments were approximated. Calibration files (linear and nonlinear) were created in microSTARii software using 6 MV irradiated nanoDots and tested with QC dosimeters.

Results:  Electron irradiated OSLDs consistently read 7% lower than actual exposure in clinically relevant dose range of 200 – 350 cGy. Photon irradiated OSLDs, however, are well within the expected +/- 5%. On average, linear calibration read photon exposed OSLDs between 4% and 9% above exposed dose from 250 – 350 cGy. Nonlinear calibration read the same OSLDs within +/-2%. The same procedure was used to read electron irradiated OSLDs. A correction factor of 1.07 was multiplied by the readings to adjust for observed -7% error in electrons. After adjustment, linear calibration read between 7 and 12% higher than exposure. Nonlinear read the same OSLDs within +/- 1%.

Conclusion: While it is reported that OSL behaves linearly up to 300 cGy, we observed that nonlinear calibration is potentially more accurate as low as 200 cGy. Therefore, tests should be performed to determine usability of each type of calibration according to dose. When using OSLD for electrons, clinics should ascertain consistent error percentage and consider using a correction factor, the factor being +7% in our clinic.

 

Stereotactic detector comparisons for small field relative dosimetry

G. Johnson,¹ C. Malmer,¹ B. Owen,¹  J. Garci-Cobian,¹ S. Small,¹ D. King,² D. Siergiej^3
1Northwest Medical Physics Center, Lynnwood, WA; ²Madigan Army Medical Center, Joint Base Lewis-McChord, WA; ³University of New Mexico, Albuquerque, NM

AAPM Annual Scientific Meeting, Nashville, TN (2018).

Purpose: To investigate different detectors for relative dosimetry of small fields using a traditional linear accelerator and following the International Atomic Energy Agency (IAEA) Code of Practice (COP) Report No. 483 published in 2017.

Methods: Four detectors were investigated including the Sun Nuclear Edge, IBA Dosimetry Razor, Standard Imaging Exradin A16, and the PTW microDiamond. An IBA Stealth Chamber was used as the reference detector for profiles and depth curves. The measurements were made on a linac with photon energies of 6MV, 6MV SRS, and 10MV. A three-dimensional water phantom was used to take measurements of square field sizes down to 0.5 cm x 0.5 cm, including PDD curves at SSD 100cm, as well as profiles and output factors at a depth of 10cm, 90cm SSD. All output factors were daisy chained at 4cm to an IBA CC13 chamber. Correction factors were applied as stated in the COP.

Results:  The difference from average for the uncorrected and corrected output factors were -0.87% and 0.48% for the Edge, 0.60% and -1.01% for the Razor, -0.43% and 0.30% for the microDiamond, and 0.94% and 0.52% for the A16, respectively. From the PDD curves, the difference from average for the d20/d10 ratio was 0.94% for the Edge, 0.26% for the Razor, 0.29% for the microDiamond, and 0.93% for the A16. From the profiles, the difference from average FWHM was 0.76% for the Edge, 0.78% for the Razor, 0.51% for the microDiamond, and 0.98% for the A16.

Conclusion: All the data was found to be within 1.01% different from average. Corrected output factors converged for all detectors, except the Razor. This study shows that any of the four detectors can be used to measure small field relative dosimetry within clinically acceptable deviation for stereotactic applications.

 

The effect of jaw hysteresis on half-beam fields

G. Johnson,¹ E. Marshall^1
1Northwest Medical Physics Center, Lynnwood, WA

AAPM Annual Scientific Meeting, Nashville, TN (2018).

Purpose: To develop a jaw calibration technique and independent jaw test tolerances, accounting for fluctuation of position for a Varian C-series linac.

Methods: The jaws were tested to determine how much their position fluctuates in three different scenarios. Motion direction was tested by opening or closing the jaw to the prescribed position at gantry 0° and again at gantry 270° with the jaw aligned in the direction of gravity. Positional dependence examined the jaw position at several points across its range of motion. Ten images were taken, using the EPID, of each jaw with a thick lead block opposite of the tested jaw. This was repeated at each tested position and the fifty percent intensity line was used to define the field edge. The resulting average and maximum deviations were used to set an abutment dose tolerance. An independent jaw test, with asymmetric fields matched at CAX, was evaluated before and after calibration to determine the abutment dose. The jaw’s position was calibrated by correlating the potentiometer readout with known distances from the dose difference and needed adjustment values were based off prior penumbra measurements.

Results:  The jaws with the two largest deviations for a 5cm distance moving at a combination of in and out to the location were X1 and Y2, with maximum deviations of 0.3mm and 0.5mm, respectively. Increasing the field size increased the uncertainty of the jaws by an average of 0.28mm. With the gantry at 270o, the jaws showed an average standard deviation of 0.06mm. The jaw calibration procedure changed the abutment dose variation from 17% to 8%.

Conclusion: The jaw hysteresis shows an average deviation of 0.12mm and a maximum of 0.50mm. The abutment dose varies at 2% per 0.1mm per jaw. A tolerance of ±10% is acceptable after calibration and shows better clinical results.

 

Lead shield on wheels for fetus protection

D. McFadden,¹ P. Jewell,² N. McIlmoil,² K. Wake^2
1Northwest Medical Physics Center, Lynnwood, WA; ²Olympic Medical Cancer Center, Sequim, WA

AAPM Annual Scientific Meeting, Nashville, TN (2018).

Purpose: To shield the fetus from radiation exposure during an IMRT treatment to the brain, per TG 36 and TG 158.

Methods: A mobile shield on wheels was designed to straddle the treatment couch and hold up to 5 HVLs of lead. The cart, based off of a design by Dr. Podgorsak at Roswell Park Cancer Institute, was made of aluminum and fitted with approximately 2 HVLs (2.5 cm) of lead sheets. Lead shielding covered four sides: top, right, left, and front. The front piece was custom fit around the patient curvature 10 cm superior to the fundus. Before starting treatment, various treatment plans were delivered on an in-house water equivalent phantom to determine the desired shielding quantity and planning technique. Several dose reduction techniques were explored per TG 158. Once the shield was assembled, measurements were made with and without shielding using a farmer chamber at the approximate location of the fundus. On the first day of treatment, OSLD measurements were taken at the fundus, umbilicus, and pubis under 0.5 cm bolus per TG 36.

Results:  Ion chamber measurements were made pretreatment on an in-house water equivalent phantom with and without the shielding in place. The chamber was placed 50 cm from the isocenter at a depth of dmax. The measurement showed a 45% reduction in dose with the shielding in place. Patient measurements were taken with OSLDs at the fundus, umbilicus, and pubis per TG 36 and measured a dose for the entire course of treatment of 1.5 cGy, 0.9 cGy, and 0.9 cGy respectively.

Conclusion: By using treatment planning techniques described in TG 158 and a mobile shield on wheels described in TG 36, dose reduction to the fetus was achieved while maintaining a quality treatment plan.

 

Annealing and decay characteristics of OSLDs

Brianna Owen,¹ Saikanth Mahendra,¹ Bryan Jackson^1
1Northwest Medical Physics Center, Lynnwood, WA

AAPM Spring Clinical Meeting, Las Vegas, NV (2018).

Purpose: The purpose of this work is to characterize annealing and decay characteristics of OSLD’s using the NMPC annealer and ambient clinical light. This required commissioning a new MicroSTAR ii system.

Methods: To commission the MicroSTAR ii system, calibration OSLD’s spanning the whole clinical range were irradiated, and QC OSLD’s for a subset were also irradiated. Calibration files were verified by reading the dose of QC OSLD’s. To test decay by annealing, OSLD’s with various residual doses were initially read with the MicroSTAR ii reader prior to annealing. The active portions of multiple OSLD’s were exposed and placed into the annealer, and the dose was read periodically. The NMPC annealer has an aluminum case and uses blue LED’s to anneal the OSLD’s. To test decay by ambient clinic light, six OSLD’s were also left in different places with different amounts of ambient clinic light to characterize the decay that took place.

Results:  With the annealer, OSLD’s with more dose required more annealing time to reach a background dose of 0.1 cGy. The dose on OSLD’s with their active portions open to the light decayed faster than those that had less ambient light exposure.

Conclusion: Our recommendation is that OSLD’s below 200 cGy need to be annealed for 45 minutes, between 200 cGy and 500 cGy they need to be annealed 60 minutes, and between 500 cGy and 1000 cGy they need to be annealed 90 minutes. Also, it is important not to leave OSLD’s with their active portion left open to the light.

 

Stereotactic radiotherapy for benign diseases: beam characterization and treatment quality assurance

Thomas Brown^1,2
1Northwest Medical Physics Center, Lynnwood, WA; ²Tri-Cities Cancer Center, Kennewick, WA

Columbia Chapter Meeting, Health Physics Society

Professional Symposium presented at the Health Physics Society (2017).

 

Commissioning experience of a superficial kV system for guinea pig cutaneous irradiation

Saikanth Mahendra,¹ Adam Schoen,¹ Karla D. Thrall^2
1Northwest Medical Physics Center, Lynnwood, WA; ²SNBL USA, Ltd., Everett, WA

Radiation Research Society (RRS) Annual Meeting, Cancun, Mexico (2017).

Cutaneous radiation injury (CRI) studies in guinea pigs are considered a good small animal model to assess the human dermal response to low-penetrating acute irradiation exposures for prospective medical countermeasures (MCMs). The characteristics of superficial Xstrahl 100 kV irradiator were utilized for development of dose-response models and key dosimetric parameters are presented. In order to mimic the depth of penetration and dose fall off under the skin similar to a pure beta source, we selected a 60kV beam potential, a beam current of 6mA and a custom 0.15mm inherent filter setting. The irradiation area was achieved by a 4x4cm square applicator placed in contact with the skin. All measurements were performed in accordance with the AAPM Task Group 61 (TG-61) protocol. A soft X-ray parallel plate chamber was utilized in air and in a Soft X-ray slab phantom to determine different beam characteristics. The Half Value Layer (HVL) of the beam was measured in air and compared using three different purities of Aluminum (Al) filters. Al purity of 99.0%, 99.99% and 99.999% yielded 0.163, 0.171, 0.161 mm of Al as HVL’s respectively. The Percent Depth Dose (PDD) at 50% was found to be 1.884mm and less than 30% at 4mm in water equivalent plastic slabs. A steep dose fall off was desired to limit the dose to deeper tissues to decrease any normal tissue complications. The absolute beam output was determined to be 6.532 Gy/min at surface, 4.290 Gy/min at 1mm depth and 3.165 Gy/min at 2mm depth. Beam consistency and constancy check measurements were performed with minimal coefficient of variation (0.1%). GafChromic EBT-3 film was used to determine dose homogeneity across the irradiated area and was found to be well within the required +/- 5%.

 

The radiation dose response relationship for whole thorax lung irradiation in the female rhesus macaque

Karla D. Thrall,¹, Saikanth Mahendra,² Thomas J MacVittie^3
1SNBL USA, Ltd., Everett, WA; ²Northwest Medical Physics Center, Lynnwood, WA; ³Department Of Radiation Oncology, University of Maryland, School of Medicine, Baltimore, MD

Radiation Research Society (RRS) Annual Meeting, Cancun, Mexico (2017).

Concerns over nuclear and radiological threats have prompted the need to improve methods to protect the general population from the health hazards associated with exposure to ionizing radiation. The development of effective medical countermeasures requires efficacy studies be conducted in an appropriate animal model (“the Animal Rule” 21 CFR 314.600 for drugs). However, the reliance on non-human efficacy data places an enormous importance on appropriately developed and validated animal models. A whole thorax lung irradiation (WTLI) model has been developed for the male rhesus macaque (Garofalo et al. 2014). The objective of this study was to develop a lethality dose response profile for WTLI specific to this institution, an endeavor necessary to validate the model prior to conducting efficacy studies under the criteria of the Animal Rule. The study design was similar to that described previously, with the exception that female, rather than male, rhesus macaques were utilized. In brief, a computed tomography (CT) scan was conducted prior to irradiation and used for treatment planning. Animals were exposed to a single 6 MV photon exposure focused to the lung as determined by the CT scan and treatment planning at a dose of 9.5, 10, 10.5, 11 or 11.5 Gy. Supportive care, including administration of dexamethasone, was based on trigger-to-treat criteria and clearly defined euthanasia criteria were used to determine moribund condition over 180 days post irradiation. Percent mortality per radiation dose was 12.5% at 9.5 Gy exposure, 25% at 10 Gy exposure, 62.5% at 10.5 Gy exposure, 87.5% at 11 Gy exposure, and 100% at 11.5 Gy exposure. The resulting probit plot for the WTLI model estimated a 180 day LD50 of 10.28 Gy, which compared well with the previously published estimate of 10.27 Gy for the male rhesus. Additionally, WTLI allows longitudinal definition of combined organ injury to both lung and heart. These data suggested the absence of a gender influence on the radiation dose response for WTLI in the rhesus and provided an inter-laboratory validation of the previously reported model.

 

Radiation physics and dosimetry of 6 MeV linear accelerator beam for swine cutaneous irradiation

Saikanth Mahendra,¹ Karla D. Thrall^2
1Northwest Medical Physics Center, Lynnwood, WA; ²SNBL USA, Ltd., Everett, WA

Radiation Research Society (RRS) Annual Meeting, Puako, HI (2016).

The characteristics of a linear accelerator generated 6MeV radiation beam are well suited for development of radiation-induced skin injury models. Based on literature, swine has been widely accepted as the closest skin model compared to humans. To study the evolution of cutaneous radiation injuries, the lowest electron energy – 6MeV, available in our Varian 21EX linear accelerator was preferred, taking the below parameters into consideration – high surface dose deposition within the first few millimeters, uniformity of dose distribution across the irradiation site and practical range (Rp) of the beam. The dorsal skin surface of three different porcine breeds – Yorkshire, White Sinclair and Göttingen were exposed to a single fraction of 25, 35, 45, and 60Gy to two 4-cm circular fields using a nominal dose rate of 4Gy/min. Ultrasound measurements of swine skin thickness enabled us to employ a 0.8cm superflab bolus material to ensure approximately 95% dose homogeneity from the swine skin surface to a depth of 0.7cm. The depth of maximum dose (dmax) was found to occur around 0.5cm below the swine skin surface. A steep dose fall off beyond the dose deposition depth of 0.8cm was desired to limit the dose to deeper tissues to decrease any normal tissue complications. Relative doses of less than 50% at 1.5cm, 15% at 2cm and less than 5% at 2.2cm below the swine skin surface were observed with virtual water phantom measurements which mimic tissue. The percentage depth-dose dosimetry was characterized using a small-volume parallel-plate ionization chamber which serves as an ideal choice in measuring doses near to the skin surface. The output factors and uniformity profiles were measured using Electron diodes for desired applicators. A 20×20 applicator cone was used along with 3.2mm Lead (Pb) surface cutouts to minimize the radiation penumbral effects on the skin ensuring a more defined irradiation boundary. The variability of beam uniformity – both flatness and symmetry across the 4cm circular irradiated field was observed to be less than 1%. Optically Stimulated Luminescence (OSL) dosimeters were placed on the dorsal surface to evaluate dose delivery and below the lead cutouts to evaluate shielding efficacy and average measured differences were less than 2% and 0.25% respectively.

 

A cross-breed comparison of cutaneous radiation injury in swine

Karla D. Thrall,¹ Ronald Manning,¹ Saikanth Mahendra,² George De Los Santos,¹ Koichiro Fukuzaki,¹ Ryoichi Nagata^3
1SNBL USA, Ltd., Everett, WA; ²Northwest Medical Physics Center, Lynnwood, WA; ³Shin Nippon Biomedical Laboratories, Ltd., Kagoshima, Japan

Radiation Research Society (RRS) Annual Meeting, Weston, FL (2015).

Development of medical countermeasures to treat radiation-induced injuries requires an appropriate animal model. For skin burn injuries, the pig is widely accepted as the best model, and the literature contains numerous examples of the use of pigs in the study of thermal and radiation burns. However, no single breed of pig has emerged as a preferred model. In the present study, the evolution of radiation-induced skin injury was evaluated in three porcine breeds. Age matched female Yorkshire, white Sinclair, and Göttingen minipigs were exposed to a single fraction of 25, 35, 45, and 60 Gy with 6-MeV electrons to a 4×4-cm field on the dorsal skin surface of the animal (4 fields per animal, 1 dose per field, and 4 doses per animal). Ultrasound was used to estimate differences in skin thickness at each irradiation field and skin was overlaid with tissue equivalent bolus material to ensure that greater than 90% of the prescribed dose was limited to a depth of 2 cm. Furthermore, bolus material was varied appropriately to ensure uniform depth of dose delivery across all irradiation locations for all animals, regardless of breed. Following irradiation exposure, animals were observed and skin was numerically scored for erythema and desquamation separately over a sufficient period of time to allow full progression of injury. Skin scores were used to compare qualitative observations of gross changes, including erythema, dry desquamation and moist desquamation. A comparison of skin scores indicate clear differences between breeds in both degree of injury, as evidenced by higher skin scores, as well as in the time course of onset and progression. Based on skin scores per delivered dose, the Göttingen minipig appears to be more radioresistant than either the Yorkshire or Sinclair pigs. In summary, the study reported here provided a unique side-by-side comparison of radiation-induced skin injury over a range of doses across three commonly utilized research breeds.

 

Low energy therapeutic x-ray calibration methods

M. M. Zaini,¹ A. R. Schoen,¹ L. M. Arvan,¹ E. I. Marshall,¹ G. E. Robertson,¹ C. G Beck,¹ A. L. Eagle,¹ L. E. Sweeney^1
1Northwest Medical Physics Center, Lynnwood, WA

World Congress on Medical Physics and Biomedical Engineering, Toronto, Canada (2015).

NMPC also chaired this scientific session.

The application of low energy X-rays has recently increased in the dermatology workspace, with the generating tube potential of these radiation beams spanning from 10 to 70kV. Absolute calibration of these low energy radiation units from three different manufacturers has been conducted. Various low-energy calibration protocols were considered for calibrating the low energy beams of the X-ray tubes from three different manufacturers. These protocols included the American Association of Physicists in Medicine (AAPM) Task Group 61 Report (for 40-300 kV beams), the UK Code of Practice from the Institution of Physics and Engineering in Medicine and Biology (IPEMB) for very low-energy X-rays (8-50 kV), International Atomic Energy Agency (IAEA) Technical Reports Series No. 398 (for X-ray up to 100 kV), and Report 10 of the Netherlands Commission on Radiation Dosimetry for low energy X-ray (50-100 kV). Review and comparison of the methodology of these protocols, pertaining to the energy range of Grenz-rays to superficial X-rays, are conducted.

 

Superficial therapy: More than just skin deep

E. Marshall,¹ M. Zaini,¹ A. Schoen,¹ L. Arvan,¹ G. Robertson^1
1Northwest Medical Physics Center, Lynnwood, WA

NWAAPM Spring 2015 meeting, Young Investigator Presentation. Portland, Oregon (2015).

 

Calypso® and laser-based localization systems comparison for left-sided breast cancer patients using deep inspiration breath hold

S. Robertson,¹ D. Kaurin,^1,2 J. Kim,^2,3 L. Fang,^2,3 L. Sweeney,^1,2 K. Halloway,² A. Tran^2,3
1Northwest Medical Physics Center, Lynnwood, WA; ²Seattle Cancer Care Alliance, Seattle, WA; ³University of Washington Medical Center, Seattle, WA

AAPM Annual Scientific Meeting, Austin, TX (2014).

Purpose: Our institution uses a manual laser-based system for primary localization and verification during radiation treatment of left-sided breast cancer patients using deep inspiration breath hold (DIBH). This primary system was compared with sternum-placed Calypso(R) beacons (Varian Medical Systems, CA). Only intact breast patients are considered for this analysis.