University of Wisconsin–Madison
Mark Geurts, MS

Mark Geurts, MS

Associate Researcher

Department of Human Oncology

I am a board-certified medical physicist in the Department of Human Oncology and funambulate my time among clinical service, research and teaching. I primarily serve the satellite clinics and enjoy the agility that small, dedicated clinical teams can achieve when working together to improve patient care.

My home base is the UW Health East Clinic, where I manage a Varian TrueBeam® system. This clinic initiates many advanced treatment techniques, such as Real-Time Position Management™ (RPM) or fiducial guided intra-fraction motion management and intensity modulated pulsed reduced dose rate radiotherapy. Once matured and refined, we transfer these innovations to our other centers. I also oversee physics support at the Aspirus UW Cancer Center in Wisconsin Rapids, co-lead the department quality assurance committee and manage our IMRT validation and QA  program.

One of the most exciting and rewarding facets of working in cancer care is the speed at which the field continues to advance. Having started my career at TomoTherapy Inc., working with one of the newest innovations at the time, I naturally progressed to stay on the leading edge of cancer research and development by joining the team at the University of Wisconsin–Madison. I am an engineer by training and therefore focus my research on introducing new ideas and technology into clinical use, such as a new treatment or quality assurance technique. For more information on current projects see my research page.

I believe service is a core responsibility for every clinician and scientist. I am actively involved in professional organizations and academic governance as well as teaching and mentoring. I support the global effort of improving cancer care through publications, presentations and open-source software.


MS, University of Wisconsin–Madison, Medical Physics (2007)

BS, University of Wisconsin–Madison, Nuclear Engineering (2006)

Academic Appointments

Adjunct Professor, Health Professions, University of Wisconsin–La Crosse (2015–pres.)

Associate Researcher, Human Oncology (2014–pres.)

Assistant Researcher, Human Oncology (2012 – 2014)

Boards, Advisory Committees and Professional Organizations

American Association of Medical Physicists, TomoTherapy QA Update Task Group Member, 2017–pres.

Academic Staff Assembly District 333 Alternate, 2017–pres.

American Association of Medical Physicists, Working Group for Veterinary Radiation Member, 2016–pres.

American Association of Medical Physicists, Task Group 244 Member, 2013–2014

American Association of Medical Physicists, Survey Validation Subcommittee Member, 2008–2010

American Association of Medical Physicists, Placement Services Subcommittee Guest Member, 2008–2009

Research Focus

IMRT QA Systems, Satellite Clinic Support, RayStation® Treatment Planning, RPM-Guided DIBH Radiotherapy, Intra-Fraction Fiducial Tracking, TomoTherapy®, ViewRay®

Quality engineering best practices can be used to increase patient safety and plan quality in radiation therapy.

I focus much of my research on the application of quality engineering best practices to increase patient safety and plan quality in radiation therapy. Although this commonly is applied to small process improvement and technology adoption projects around the clinic, below are some larger projects that I am working on.

Development of Open Source Tools for Radiotherapy Research

In addition to being a medical physicist, I consult as a software developer with the primary focus of Web-based scientific applications (PHP, Python, AJAX, SQL). I leverage this experience to build new tools for my own and others’ research in radiation therapy (primarily in MATLAB®). I maintain and distribute all code through GitHub with the belief that sharing these tools with the community as open-source software provides the best opportunity for this work to be leveraged to improve cancer care. As Linus Torvalds once said, “To do something well, you have to get a lot of people involved.”


One example of the tools currently available to researchers is a suite of applications to perform ViewRay®  system machine quality assurance. As an early adopter of ViewRay, my colleagues and I devised new methods for quality assurance, including designing new equipment, procedures and software tools. This software is now used by numerous ViewRay treatment system users around the world to support their clinical and research programs.

Biological Effects of Sublethal Repair during Treatment Delivery

The goal of this project is to investigate how sublethal DNA damage repair during radiotherapy affects patient outcomes. Although sublethal damage repair during radiotherapy has been known for almost 80 years, its study to present has been limited to in vitro studies, simple models and point measurements. Unfortunately, modern intensity modulated techniques deliver extremely complex spatially and temporally variant dose rates across the tumor and nearby critical structures, obfuscating the true impact of sublethal damage repair.


The first component of this project includes development and validation of tools to calculate and measure changes in dose rate during IMRT and model their biological impact to patients. The plot below is a demonstration of the application of the Lea-Catcheside factor to a bi-exponential linear-quadratic repair model, where effects of fraction dose and duration are compared for early and late tissues. Models such as this are being used to calculate the BED as a function of time and spatial position.


Next, modalities and planning techniques for different treatments are being studied and compared. One such technique is pulsed reduced dose rate radiotherapy, where 3D conformal treatments have already been demonstrated to take advantage of sublethal repair to spare normal tissue during re-irradiation. These results are being compared to different IMRT delivery techniques.


Related publications and abstracts:

  • Geurts, M. (2017), TrueBeam low dose rate investigation for pulsed reduced dose rate IMRT. IJMPCERO, 6: 139-149. doi: 10.4236/ijmpcero.2017.62013
  • Geurts, M. (2016), TU-H-BRC-04: Feasibility of Using TomoDirect for Pulsed Reduced Dose-Rate Radiotherapy. Phys., 43: 3766. doi:10.1118/1.4957611

Improving Treatment Plan Quality using Data Analytics

This project explores the application of data analytics to the opportunities of “big data” research into radiotherapy treatment planning and delivery. We treat more than 100 patients per day, collecting images and computing dose volumes that accumulate in the hundreds of terabytes. The illustration below highlights the many sources of data generated during modern cancer care. Leveraging my own software and database experience, I am working to develop tools to scan the various data systems, study patient and plan characteristics and identify trends in plan quality. By studying the gestalt, we can identify how each individual patient is unique and better personalize our care.


Clinical Impact of Treatment Delivery Errors in Radiotherapy

The goals of this work are to produce clear guidelines on uncertainty impacts, to support clinical decisions on robust treatment methods, to aid in the optimization of verification approaches and to produce an optimized error detection framework for clinical implementation. This project is funded by the Cancer Council NSW in collaboration with the University of Sydney. Delivery errors are being studied in a range of advanced radiotherapy techniques for head and neck cancers, prostate cancers and SBRT treatments of lung and liver cancers. For each cancer type and treatment method, random and systematic sources and magnitudes of likely delivery errors are modeled and evaluated in terms of their clinical impact on treatment.


As an example, the plots below illustrate the dosimetric impact of different TomoTherapy® delivery errors on the target and critical structure doses for a sample head and neck plan. By quantifying the level of impact, effect-driven quality assurance recommendations can be established.

plots below illustrate the dosimetric impact of different TomoTherapy® delivery errors on the target and critical structure doses for a sample head and neck plan. By quantifying the level of impact, effect-driven quality assurance recommendations can be established


Related publications and abstracts:

  • Deshpande, S., Xing, A., Metcalfe, P.E., Holloway, L., Vial, P., Geurts, M. (2017), Clinical implementation of an exit detector-based dose reconstruction tool for helical tomotherapy delivery quality assurance. Phys. In press. doi: 10.1002/mp.12484
  • Deshpande, S., Geurts, M., Vial, P., Metcalfe, P.E., Holloway, L.C. (2017), Sensitivity evaluation of two commercial dosimeters in detecting helical tomotherapy treatment delivery errors. Physica Medica 37: 68–74. doi: 10.1016/j.ejmp.2017.04.011
  • Deshpande, S., Geurts, M., Vial, P., Metcalfe, P., Lee, M., Holloway, L. (2017), Clinical significance of treatment delivery errors for helical tomotherapy nasopharyngeal plans – a dosimetric simulation study. Physica Medica 33: 159–169. doi: 10.1016/j.ejmp.2017.01.006
  • Deshpande, S., George, A., Xing, A., Holloway, L., Metcalfe, P., Vial, P., Geurts, M. (2013), PO-0779: Sensitivity of three commercial dosimeters to delivery errors for pre-treatment QA in helical tomotherapy. Radiotherapy and Oncology, 106: S297. doi: 10.1016/S0167-8140(15)33085-1
  1. De Costa, A.M., Bassetti, M., Mittauer, K., Ko, H.C., Geurts, M., Hill, P., Bayouth, P., Rosenberg, S., Harari, P. (2017), Rapid access palliative radiation workflow using MRI-guided single-session simulation, online adaptation, and treatment. To be presented at the ASTRO Annual Meeting in San Diego, CA.
  2. Geurts, M. (2017), A modern approach to pre-treatment chart review using Mobius3D. Presented at the 59th AAPM Annual Meeting in Denver, CO.
  3. Geurts, M. (2017), ScandiDos Delta4: Outcomes and usage in a multi-location clinic. Presented at the 59th AAPM Annual Meeting in Denver, CO.
  4. Besemer, A., Geurts, M., Shepard, A., Smilowitz, J. (2017), Is patient-specific IMRT QA needed for TomoTherapy 3D Conformal Radiation Therapy (3D-CRT) treatment plans? Presented at the 59th AAPM Annual Meeting in Denver, CO.
  5. Geurts, M. (2016), Expanding SBRT with TomoTherapy: from pets to satellites. Presented at the ASTRO Annual Meeting in Boston, MA.
  6. Geurts, M. (2016), Plan for success: TG248 practice guidelines. Presented at the ASTRO Annual Meeting in Boston, MA.
  7. Wojcieszynski, A., Hill, P., Rosenberg, S., Hullett, C., Mittauer, K., Geurts, M., Labby, Z., Bayouth, J., Anderson, B. (2016), Prospective results of real-time MRI-Guided Lumpectomy Cavity Boost Treatment. Presented at the ASTRO Annual Meeting in Boston, MA. doi: 10.1016/j.ijrobp.2016.06.159
  8. Chen, I., Mittauer, K., Henke, L.E., Acharya, S., Lu, Y., Chen, C.C., Rosenberg, S., Geurts, M., Wojcieszynski, A., Parikh, P., Bassetti, M., Kashani, R. (2016), Quantification of interfractional gastrointestinal tract motion for pancreatic cancer radiation therapy. Presented at the ASTRO Annual Meeting in Boston, MA. doi: 10.1016/j.ijrobp.2016.06.954
  9. Rosenberg, S., Wojcieszynski, A., Hullett, C., Geurts, M., Bayouth, J. Harari, P., Bassetti, M. (2016), Real-time MRI-guided radiotherapy for gastroesophageal junction/gastric cancers. Presented at the ASTRO Annual Meeting in Boston, MA. doi: 10.1016/j.ijrobp.2016.06.157
  10. Geurts, M. (2016), Feasibility of using TomoDirect for pulsed reduced dose-rate radiotherapy. Presented at the 58th AAPM Annual Meeting in Washington, D.C. doi:10.1118/1.4957611
  11. Hill, P., Geurts, M., Mittauer, K., Bayouth, J. (2016), MRI-guided single-session simulation, online adaptation, and treatment. Presented at the 58th AAPM Annual Meeting in Washington, D.C. doi: 10.1118/1.4956018
  12. Smilowitz, J., Dunkerley, D., Geurts, M., Hill, P., Yadav, P. (2016), Long term dosimetric stability of 6 TomoTherapy systems. Presented at the 58th AAPM Annual Meeting in Washington, D.C. doi: 10.1118/1.4955718
  13. Mittauer, K., Geurts, M., Toya, R., Bassetti, M., Harari, P., Paliwal, B., Bayouth, J. (2016), Implications for online adaptive and non-adaptive radiotherapy of gastic and gastroesophageal junction cancers using MRI-guided radiotherapy. Presented at the 58th AAPM Annual Meeting in Washington, D.C. doi: 10.1118/1.4958168
  14. Mittauer, K., Rosenberg, S., Geurts, M., Bassetti, M., Chen, I., Henke, L., Olsen, J., Kashani, R., Wojcieszynski, A., Harari, P., Labby, Z., Hill, P., Paliwal, P., Parikh, P., Bayouth, J. (2016), Indications for online adaptive radiotherapy based on dosimetric consequences of interfractional pancreas-to-duodenal motion in MRI-guided pancreatic radiotherapy. Presented at the 58th AAPM Annual Meeting in Washington, D.C. doi: 10.1118/1.4957421
  15. Deshpande, S., Geurts, M., Hansen, C. R., Metcalfe, P., Vial, P., Holloway, L. (2016), Clinical significance of treatment delivery errors for helical tomotherapy (HT) lung stereotactic ablative radiotherapy (SABR) plans – a dosimetric simulation study. Presented at the 18th International Conference on the use of Computers in Radiation Therapy in London, England.
  16. Rosenberg, S.A., Wojcieszynski, A., Hullett, C., Geurts, M., Lubner, S.J., LoConte, N.K., Deming, D.A., Mulkerin, D.L., Cho, C.S., Weber, S.M., Winslow, E., Bradley, K.A., Bayouth, J., Harari, P.M., Bassetti, M.F. (2016), Real-time MRI-guided radiotherapy for pancreatic cancer. Presented at the 35th ESTRO Annual Meeting in Turin, Italy. doi: 10.1016/S0167-8140(16)31460-8
  17. Geurts, M. (2015), Semi-automated dose tracking and NTCP analysis for abdominal SBRT. Presented at the MIM Software User Group Meeting in San Antonio, TX.
  18. Geurts, M. (2015), Mobius3D – Making the switch to 3D plan QA at the UW Carbone Cancer Center. Presented as the ASTRO Annual Meeting in San Antonio, TX.
  19. Hullett, C.R., Labby, Z.E., Wojcieszynski, A.P., Rosenberg, S.A., Bayliss, R.A., Geurts, M., Brower, J.V., Hill, P.M., Paliwal, B.R., Bayouth, J., Bassetti, M. (2015), Quantitative differences in planned and delivered dose for liver SBRT using MRI guided delivery. Presented at the ASTRO Annual Meeting in San Antonio, TX. doi: 10.1016/j.ijrobp.2015.07.053
  20. Rosenberg, S.A., Labby, Z.E., Wojcieszynski, A.P., Hullett, C.R., Geurts, M., Bayliss, R.A., Hill, P.M., Paliwal, B.R., Bayouth, J., Bassetti, M. (2015), First reported real-time MRI guided liver stereotactic body radiation therapy treatments: experience and clinical implications. Presented at the ASTRO Annual Meeting in San Antonio, TX. doi: 10.1016/j.ijrobp.2015.07.051
  21. Wojcieszynski, A., Rosenberg, S., Hullett, C., Geurts, M., Labby, Z., Hill, P., Bayliss, R. A., Paliwal, B., Bayouth, J., Bassetti, M. (2015), Is it really what’s on the inside that counts? External surrogates for real-time liver tumor motion during radiation therapy. Presented at the ASTRO Annual Meeting in San Antonio, TX. doi: 10.1016/j.ijrobp.2015.07.2057
  22. Wojcieszynski, A., Hill, P., Rosenberg, S., Hullett, C., Labby, Z., Paliwal, B., Geurts M., Bayliss, R. A., Bayouth, J., Bassetti, J., Baschnagel, A. (2015), A dosimetric comparison of MRI guided cobalt-60 to linear accelerator based stereotactic body radiation therapy lung cancer plans. Presented at the ASTRO Annual Meeting in San Antonio, TX. doi: 10.1016/j.ijrobp.2015.07.2049
  23. Hill, P., Labby, Z., Bayliss, R. A., Geurts, M., Bayouth, J. (2015), A plan comparison tool to ensure robustness and deliverability in online-adaptive radiotherapy. Presented at the 57th AAPM Annual Meeting in Anaheim, CA. doi: 10.1118/1.4925202
  24. Szczykutowicz, T., Hermus, J., Geurts, M., Smilowitz, J. (2015), Intensity modulated imaging? clinical workflow for fluence field modulated CT on a TomoTherapy system. Presented at the 57th AAPM Annual Meeting in Anaheim, CA. doi: 10.1118/1.4926305
  25. Geurts, M. (2014), Commissioning and fast treatment planning with TomoHDA. Presented at the 56th AAPM Annual Meeting in Austin, TX.
  26. Geurts, M. (2014), UW Experience Commissioning 2x TomoHDA Systems. Presented at the Frontiers of Image Guidance: Margins, Motion, and Adaptation meeting in Minneapolis, MN.
  27. Geurts, M. (2014), TomoTherapy Treatment Planning with VoLO. Presented at the Frontiers of Image Guidance: Margins, Motion, and Adaptation meeting in Minneapolis, MN.
  28. Geurts, M. (2014), Exit-detector based dose reconstruction as a supplement for TomoTherapy IMRT QA. Presented at the Frontiers of Image Guidance: Margins, Motion, and Adaptation meeting in Minneapolis, MN.
  29. Geurts, M., Nelson, G., Thapa, B., Bayliss, R. A. (2014), Total Skin TomoTherapy for treatment of mycosis fungoides. Presented at the Frontiers of Image Guidance: Margins, Motion, and Adaptation meeting in Minneapolis, MN.
  30. Geurts, M. (2014), TomoHDA: New innovations, improving outcomes. Presented at the 2014 TomoTherapy User Symposium in Toledo, OH, and at the Oncology Outlook 2014 conference in Point Clear, AL.
  31. Geurts, M. and Smilowitz, J. (2013), AAPM MPPG 5: Commissioning and QA of external beam TPS dose calculations. Presented at the Fall NCCAAPM meeting in Madison, WI.
  32. Yang, Y.M., Geurts, M., Smilowitz, J.B., Sterpin, E., Bednarz, B. (2013), Exploiting the rotational symmetry of tomotherapy to reduce dose perturbations from MRI-guided radiotherapy: a Monte Carlo investigation. Presented at the 55th AAPM Annual Meeting in Indianapolis, IN. doi: 10.1118/1.4814932
  33. Geurts, M. (2013), Dose painting using the TomoHDA treatment system. Presented at the TomoTherapy User Symposium in St. Louis, MO.
  34. Deshpande, S., George, A., Xing, A., Holloway, L., Metcalfe, P., Vial, P., Geurts, M. (2013), Sensitivity of three commercial dosimeters to delivery errors for pre-treatment QA in helical tomotherapy. Presented at the 32nd ESTRO Annual Meeting in Amsterdam, The Netherlands. doi: 10.1016/S0167-8140(15)33085-1
  35. Geurts, M. (2012), Modern TomoTherapy treatments with TomoEDGE. Presented at the 2012 Advancing Radiation Oncology Forum in Dallas, TX.
  36. Geurts, M. (2012), Challenges in radiation therapy. Presented at the 2012 Advancing Radiation Oncology Forum in Dallas, TX.
  37. Differding, S., Sterpin, E., Lee, J.A., Geets, X., Geurts, M., Grégoire, V. (2012), Feasibility study on FDG-PET based escalated dose painting in head and neck tumors with helical tomotherapy. Presented at the 31st ESTRO Annual Meeting in Barcelona, Spain. doi: 10.1016/S0167-8140(12)71121-0
  38. Geurts, M. (2011), Advances in IMRT optimization and dose calculation. Presented at the ESTRO IMRT Congress in Reggio Emilia, Italy.
  39. Geurts, M. (2011), Image guided IMRT with TomoTherapy. Presented at the Asociación Latinoamericana de Terapia Radiante Oncológica Annual Meeting in Panama City, Panama.
  40. Geurts, M. (2011), Efficient clinical implementation of adaptive radiotherapy. Presented at the Annual TomoTherapy User Symposium in Charleston, SC.
  41. Geurts, M. (2011), Positioning strategies for helical tomotherapy. Presented at the Annual Patient Positioning Symposium in Fajardo, PR.
  42. Geurts, M. (2011), Safety considerations for helical tomotherapy. Presented at the Annual QA & Dosimetry Symposium in Orlando, FL.
  43. Geurts, M. (2011), Clinical use of 2D/3D arrays for IMRT and arc delivery. Presented at the Annual QA & Dosimetry Symposium in Orlando, FL.
  44. Geurts, M., (2010), Challenges in radiation therapy. Presented at the Caribbean Oncology Outlook in San Juan, PR.
  45. Geurts, M., (2010), Partial and whole breast radiotherapy: addressing clinical needs in the Caribbean. Presented with TomoTherapy Inc. at the 52nd AAPM Annual Meeting in Philadelphia, PA.
  46. Geurts, M., (2010), Accelerated partial breast irradiation with tomotherapy. Presented at the Annual TomoTherapy User Symposium in San Diego, CA.
  47. Geurts, M., (2010), Longitudinal study using a diode phantom for helical tomotherapy IMRT QA. Presented at the Annual TomoTherapy Inc. User Symposium in San Diego, CA.
  48. Geurts, M. (2007), A simple high performance MCNP5 based PET simulation application for the analysis of Y-90 quantification. Presented at the 2007 University of Wisconsin Monte Carlo Practicum in Madison, WI.
  49. Geurts, M., Thomadsen, B., and Selwyn, R. (2007), PET quantification of non-pure positron emitting radioisotopes: Y-90 case study. Presented at the 28th Annual American Brachytherapy Society meeting in Chicago, IL. doi: 10.1016/j.brachy.2007.02.131

Radiation Therapy Technologist Education

In addition to my clinical and research focus, I enjoy teaching the future of our field. Since 2015, I have taught the physics introduction course Radiation Therapy Physics to University of Wisconsin–La Crosse radiation therapy technologist students completing their internships at the University of Wisconsin Hospital. This course focuses on preparing students for their ASRT certification exam, including the following core topics:


  • Radiation Physics and Biology
    • Sources of Radiation
    • Basic Properties of Radiation
    • Interactions with Matter
    • Biological Effects of Radiation
    • Measurement of Radiation
  • Principles, Radiation Monitoring, and Environmental Protection
  • Equipment Use and Quality Assurance
    • CT Simulators
    • Linear Accelerators
    • Quality Assurance
    • Brachytherapy
  • Monitor Unit Calculations


In addition to the above didactic content, students are encouraged to apply their knowledge in a series of labs. Students gain experience using basic physics measurement equipment, including ion chambers, electrometers, water tanks, image quality phantoms and diode array phantoms. The following lab exercises are scheduled throughout the semester:

  • Half value layer measurements
  • Linear accelerator output calibration (AAPM TG-51)
  • Radiotherapy CT simulator quality assurance
  • Patient specific IMRT QA measurement
  • Linear accelerator mechanical quality assurance

Medical Physics Residency Education

In addition to technologist education, I participate in the medical physics residency program as a mentor for radiation shielding and TomoTherapy. I strive to achieve a healthy balance between didactic education and hands-on experience with the goal of preparing physicists for their board certification and subsequent clinical practice.


The radiation shielding rotation explores the latest methods for performing shielding calculations and places an emphasis on practicing these methods through a series of common radiation therapy shielding scenarios. Other facility design goals, including room layout, construction cost management, electrical/HVAC considerations and radiation surveys are also be discussed.


Having more than 10 years of experience working with TomoTherapy systems, I teach an in-depth understanding of TomoTherapy system operation starting with an exploration of the many unique technologies that comprise this system, then a review of potential mechanical and user failure modes and their effects and finally how to build a comprehensive quality management program. Treatment planning is also a key focus of this rotation with residents honing their skills through plan challenges and clinical cases.

  • A New Era of Image Guidance with Magnetic Resonance-guided Radiation Therapy for Abdominal and Thoracic Malignancies. Cureus
    Mittauer K, Paliwal B, Hill P, Bayouth JE, Geurts MW, Baschnagel AM, Bradley KA, Harari PM, Rosenberg S, Brower JV, Wojcieszynski AP, Hullett C, Bayliss RA, Labby ZE, Bassetti MF
    2018 Apr 04; 10 (4): e2422
    • More

      Magnetic resonance-guided radiation therapy (MRgRT) offers advantages for image guidance for radiotherapy treatments as compared to conventional computed tomography (CT)-based modalities. The superior soft tissue contrast of magnetic resonance (MR) enables an improved visualization of the gross tumor and adjacent normal tissues in the treatment of abdominal and thoracic malignancies. Online adaptive capabilities, coupled with advanced motion management of real-time tracking of the tumor, directly allow for high-precision inter-/intrafraction localization. The primary aim of this case series is to describe MR-based interventions for localizing targets not well-visualized with conventional image-guided technologies. The abdominal and thoracic sites of the lung, kidney, liver, and gastric targets are described to illustrate the technological advancement of MR-guidance in radiotherapy.

      View details for PubMedID 29872602
  • Clinical implementation of an exit detector-based dose reconstruction tool for helical tomotherapy delivery quality assurance. Med Phys
    Deshpande S, Xing A, Metcalfe P, Holloway L, Vial P, Geurts M
    2017 Oct; 44 (10): 5457-5466
    • More

      PURPOSE: The aim of this study was to validate the accuracy of an exit detector-based dose reconstruction tool for helical tomotherapy (HT) delivery quality assurance (DQA).

      METHODS AND MATERIAL: Exit detector-based DQA tool was developed for patient-specific HT treatment verification. The tool performs a dose reconstruction on the planning image using the sinogram measured by the HT exit detector with no objects in the beam (i.e., static couch), and compares the reconstructed dose to the planned dose. Vendor supplied (three "TomoPhant") plans with a cylindrical solid water ("cheese") phantom were used for validation. Each "TomoPhant" plan was modified with intentional multileaf collimator leaf open time (MLC LOT) errors to assess the sensitivity and robustness of this tool. Four scenarios were tested; leaf 32 was "stuck open," leaf 42 was "stuck open," random leaf LOT was closed first by mean values of 2% and then 4%. A static couch DQA procedure was then run five times (once with the unmodified sinogram and four times with modified sinograms) for each of the three "TomoPhant" treatment plans. First, the original optimized delivery plan was compared with the original machine agnostic delivery plan, then the original optimized plans with a known modification applied (intentional MLC LOT error) were compared to the corresponding error plan exit detector measurements. An absolute dose comparison between calculated and ion chamber (A1SL, Standard Imaging, Inc., WI, USA) measured dose was performed for the unmodified "TomoPhant" plans. A 3D gamma evaluation (2%/2 mm global) was performed by comparing the planned dose ("original planned dose" for unmodified plans and "adjusted planned dose" for each intentional error) to exit detector-reconstructed dose for all three "Tomophant" plans. Finally, DQA for 119 clinical (treatment length <25 cm) and three cranio-spinal irradiation (CSI) plans were measured with both the ArcCHECK phantom (Sun Nuclear Corp., Melbourne, FL, USA) and the exit detector DQA tool to assess the time required for DQA and similarity between two methods.

      RESULTS: The measured ion chamber dose agreed to within 1.5% of the reconstructed dose computed by the exit detector DQA tool on a cheese phantom for all unmodified "Tomophant" plans. Excellent agreement in gamma pass rate (>95%) was observed between the planned and reconstructed dose for all "Tomophant" plans considered using the tool. The gamma pass rate from 119 clinical plan DQA measurements was 94.9% ± 1.5% and 91.9% ± 4.37% for the exit detector DQA tool and ArcCHECK phantom measurements (P = 0.81), respectively. For the clinical plans (treatment length <25 cm), the average time required to perform DQA was 24.7 ± 3.5 and 39.5 ± 4.5 min using the exit detector QA tool and ArcCHECK phantom, respectively, whereas the average time required for the 3 CSI treatments was 35 ± 3.5 and 90 ± 5.2 min, respectively.

      CONCLUSION: The exit detector tool has been demonstrated to be faster for performing the DQA with equivalent sensitivity for detecting MLC LOT errors relative to a conventional phantom-based QA method. In addition, comprehensive MLC performance evaluation and features of reconstructed dose provide additional insight into understanding DQA failures and the clinical relevance of DQA results.

      View details for PubMedID 28737014
  • Sensitivity evaluation of two commercial dosimeters in detecting Helical TomoTherapy treatment delivery errors. Phys Med
    Deshpande S, Geurts M, Vial P, Metcalfe P, Holloway L
    2017 May; 37: 68-74
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      PURPOSE: To assess the sensitivity of two commercial dosimetry systems in detecting Helical TomoTherapy (HT) delivery errors.

      METHOD: Two commercial dosimeters i) MatriXXEvolution and ii) ArcCHECK® were considered. Ten retrospective nasopharynx HT patients were analysed. For each patient, error plans were created by independently introducing systematic offsets in: a) Jaw width error ±1, ±1.5 and ±2mm, b) Couch speed error ±2%, ±2.5, ±3% and ±4%, and c) MLC Leaf Open Time (MLCLOT) errors (3 separate MLC errors: either leaf 32 open or leaf 42 remains open during delivery, and 4% random reductions in MLCLOT). All error plans, along with the no error plan for each patient, were measured using both dosimeters in the same session. Gamma evaluation (3%/3mm) was applied to quantitatively compare the measured dose from each dosimeter to the treatment planning system. The error sensitivity was quantified as the rate of decrease in gamma pass rate.

      RESULTS: The gamma pass rate decreases with increase in error magnitude for both dosimeters. ArcCHECK was insensitive for couch speed error up to 2.5% and jaw width error up to -1.5mm while MatriXXEvolution was found to be insensitive to couch speed error up to 2% and couch speed up to -1mm. Both of the detectors show similar sensitivity to all the MLCLOT errors that were clinically relevant.

      CONCLUSION: No statistically significant (p>0.05) differences exist in detecting the simulated delivery errors between MatriXXEvolution and ArcCHECK dosimeter systems for HT plans. Both dosimeters were able to pick up clinically relevant delivery errors.

      View details for PubMedID 28535917
  • Long-term dosimetric stability of multiple TomoTherapy delivery systems. J Appl Clin Med Phys
    Smilowitz JB, Dunkerley D, Hill PM, Yadav P, Geurts MW
    2017 May; 18 (3): 137-143
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      The dosimetric stability of six TomoTherapy units was analyzed to investigate changes in performance over time and with system upgrades. Energy and output were tracked using monitor chamber signal, onboard megavoltage computed tomography (MVCT) detector profile, and external ion chamber measurements. The systems (and monitoring periods) include three Hi-Art (67, 61, and 65 mos.), two TomoHDA (31 and 26 mos.), and one Radixact unit (11 mos.), representing approximately 10 years of clinical use. The four newest systems use the Dose Control Stability (DCS) system and Fixed Target Linear Accelerator (linac) (FTL). The output stability is reported as deviation from reference monitor chamber signal for all systems and/or from an external chamber signal. The energy stability was monitored using relative (center versus off-axis) MVCT detector signal (beam profile) and/or the ratio of chamber measurements at 2 depths. The clinical TomoHDA data were used to benchmark the Radixact stability, which has the same FTL but runs at a higher dose rate. The output based on monitor chamber data of all systems is very stable. The standard deviation of daily output on the non-DCS systems was 0.94-1.52%. As expected, the DCS systems had improved standard deviation: 0.004-0.06%. The beam energy was also very stable for all units. The standard deviation in profile flatness was 0.23-0.62% for rotating target systems and 0.04-0.09% for FTL. Ion chamber output and PDD ratios supported these results. The output stability on the Radixact system during extended treatment delivery (20, 30, and 40 min) was comparable to a clinical TomoHDA system. For each system, results are consistent between different measurement tools and techniques, proving not only the dosimetric stability, but also these quality parameters can be confirmed with various metrics. The replacement history over extended time periods of the major dosimetric components of the different delivery systems (target, linac, and magnetron) is also reported.

      View details for PubMedID 28464517
  • Dosimetric Comparison of Real-Time MRI-Guided Tri-Cobalt-60 Versus Linear Accelerator-Based Stereotactic Body Radiation Therapy Lung Cancer Plans. Technol Cancer Res Treat
    Wojcieszynski AP, Hill PM, Rosenberg SA, Hullett CR, Labby ZE, Paliwal B, Geurts MW, Bayliss RA, Bayouth JE, Harari PM, Bassetti MF, Baschnagel AM
    2017 Jun; 16 (3): 366-372
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      PURPOSE: Magnetic resonance imaging-guided radiation therapy has entered clinical practice at several major treatment centers. Treatment of early-stage non-small cell lung cancer with stereotactic body radiation therapy is one potential application of this modality, as some form of respiratory motion management is important to address. We hypothesize that magnetic resonance imaging-guided tri-cobalt-60 radiation therapy can be used to generate clinically acceptable stereotactic body radiation therapy treatment plans. Here, we report on a dosimetric comparison between magnetic resonance imaging-guided radiation therapy plans and internal target volume-based plans utilizing volumetric-modulated arc therapy.

      MATERIALS AND METHODS: Ten patients with early-stage non-small cell lung cancer who underwent radiation therapy planning and treatment were studied. Following 4-dimensional computed tomography, patient images were used to generate clinically deliverable plans. For volumetric-modulated arc therapy plans, the planning tumor volume was defined as an internal target volume + 0.5 cm. For magnetic resonance imaging-guided plans, a single mid-inspiratory cycle was used to define a gross tumor volume, then expanded 0.3 cm to the planning tumor volume. Treatment plan parameters were compared.

      RESULTS: Planning tumor volumes trended larger for volumetric-modulated arc therapy-based plans, with a mean planning tumor volume of 47.4 mL versus 24.8 mL for magnetic resonance imaging-guided plans ( P = .08). Clinically acceptable plans were achievable via both methods, with bilateral lung V20, 3.9% versus 4.8% ( P = .62). The volume of chest wall receiving greater than 30 Gy was also similar, 22.1 versus 19.8 mL ( P = .78), as were all other parameters commonly used for lung stereotactic body radiation therapy. The ratio of the 50% isodose volume to planning tumor volume was lower in volumetric-modulated arc therapy plans, 4.19 versus 10.0 ( P < .001). Heterogeneity index was comparable between plans, 1.25 versus 1.25 ( P = .98).

      CONCLUSION: Magnetic resonance imaging-guided tri-cobalt-60 radiation therapy is capable of delivering lung high-quality stereotactic body radiation therapy plans that are clinically acceptable as compared to volumetric-modulated arc therapy-based plans. Real-time magnetic resonance imaging provides the unique capacity to directly observe tumor motion during treatment for purposes of motion management.

      View details for PubMedID 28168936
  • Clinical significance of treatment delivery errors for helical TomoTherapy nasopharyngeal plans - A dosimetric simulation study. Phys Med
    Deshpande S, Geurts M, Vial P, Metcalfe P, Lee M, Holloway L
    2017 Jan; 33: 159-169
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      PURPOSE: Develop a framework to characterize helical TomoTherapy (HT) machine delivery errors and their clinical significance.

      METHOD AND MATERIALS: Ten nasopharynx HT plans were edited to introduce errors in Jaw width (JW), couch speed (CS), gantry period (GP), gantry start position (GSP), multi leaf collimator leaf open times (MLC LOT). In case of MLC LOT only, both systematic and random delivery errors were investigated. Each error type was simulated independently for a range of magnitudes. Dose distributions for the clinical reference plans and the error simulated plans were compared to establish the magnitude for each error type which resulted in a change in clinical tolerance, defined as 5% variation in D95 of PTV70, D0.1cc of spinal cord, D0.1cc of brainstem and the smallest value of either a 10% or 3.6Gy dose variation in mean parotid dose.

      RESULTS: Dose variation from systematic delivery errors in JW ±0.5mm, CS ranges between -1% to 1.5%, GP ±1s, GSP ranges between -2(0) to 2.5(0) and MLC LOT random error up to 2% from the planned value relative to the clinical reference plan was within the set tolerance values for all the patient cohorts. GSP errors and the random MLC LOT errors with up to 10% standard deviation were found to be relatively insensitive compared to other delivery errors.

      CONCLUSION: This work has established a framework to characterize HT machine delivery errors. This framework could be applied to any patient dataset to determine clinically relevant HT QA tolerances.

      View details for PubMedID 28110824
  • AAPM Medical Physics Practice Guideline 5.a.: Commissioning and QA of Treatment Planning Dose Calculations - Megavoltage Photon and Electron Beams. J Appl Clin Med Phys
    Smilowitz JB, Das IJ, Feygelman V, Fraass BA, Kry SF, Marshall IR, Mihailidis DN, Ouhib Z, Ritter T, Snyder MG, Fairobent L, AAPM Medical Physics Practice Guideline Task Group
    2015 09 08; 16 (5): 14–34
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      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 8,000 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. The following terms are used in the AAPM practice guidelines:• Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline.• Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances.

      View details for PubMedID 26699330
  • Gadoxetate for direct tumor therapy and tracking with real-time MRI-guided stereotactic body radiation therapy of the liver. Radiother Oncol
    Wojcieszynski AP, Rosenberg SA, Brower JV, Hullett CR, Geurts MW, Labby ZE, Hill PM, Bayliss RA, Paliwal B, Bayouth JE, Harari PM, Bassetti MF
    2016 Feb; 118 (2): 416-8
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      SBRT is increasingly utilized in liver tumor treatment. MRI-guided RT allows for real-time MRI tracking during therapy. Liver tumors are often poorly visualized and most contrast agents are transient. Gadoxetate may allow for sustained tumor visualization. Here, we report on the first use of gadoxetate during real-time MRI-guided SBRT.

      View details for PubMedID 26627702
  • Simulation study of high-dose-rate brachytherapy for early glottic cancer. Brachytherapy
    Hoffman MR, McCulloch TM, Mohindra P, Das R, Geurts M, Harari PM
    2016 Jan-Feb; 15 (1): 94-101
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      PURPOSE: External beam radiation therapy (EBRT) is effective for early glottic cancers, with cure rates of ∼90% for T1 tumors. EBRT has strengths but also disadvantages including radiation to healthy tissues and duration of 5-7 weeks. With advances in laryngeal framework surgery, new devices can provide reliable, minimally invasive access to the larynx. Such devices could be modified to insert brachytherapy catheters. Brachytherapy could provide focused radiation while limiting dose to normal structures in the larynx and neck. As a preliminary step, we performed simulations comparing EBRT to high-dose-rate brachytherapy to assess if this approach could provide dosimetric advantage.

      METHODS AND MATERIALS: One- and 2-catheter brachytherapy simulations were performed for 3 patients with T1 glottic carcinoma. Percentage of dose delivered to the target and adjacent structures was compared with conventional EBRT using 3D and intensity-modulated radiation therapy approaches.

      RESULTS: Percentage of structures exposed to 50% of the dose was lower for brachytherapy compared with 3D EBRT and intensity-modulated radiation therapy, particularly for the cricoid and contralateral arytenoid. Dose was also lower for the carotid-internal jugular vein complexes compared with 3D EBRT. Dose profiles did not differ significantly between 1- and 2-catheter simulations.

      CONCLUSION: Brachytherapy can decrease radiation to normal tissues including laryngeal cartilages and carotid-internal jugular vein complexes. Recent advancements allowing catheter placement may afford the potential to decrease radiation to healthy tissues with decreased treatment time. However, careful, stepwise evaluation of feasibility and outcomes in model systems is required before recommending this approach for such high cure rate cancers in humans.

      View details for PubMedID 26614234
  • Realization of fluence field modulated CT on a clinical TomoTherapy megavoltage CT system. Phys Med Biol
    Szczykutowicz TP, Hermus J, Geurts M, Smilowitz J
    2015 Sep 21; 60 (18): 7245-57
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      The multi-leaf collimator (MLC) assembly present on TomoTherapy (Accuray, Madison WI) radiation therapy (RT) and mega voltage CT machines is well suited to perform fluence field modulated CT (FFMCT). In addition, there is a demand in the RT environment for FFMCT imaging techniques, specifically volume of interest (VOI) imaging. A clinical TomoTherapy machine was programmed to perform VOI. Four different size ROIs were placed at varying distances from isocenter. Projections intersecting the VOI received 'full dose' while those not intersecting the VOI received 30% of the dose (i.e. the incident fluence for non VOI projections was 30% of the incident fluence for projections intersecting the VOI). Additional scans without fluence field modulation were acquired at 'full' and 30% dose. The noise (pixel standard deviation) and mean CT number were measured inside the VOI region and compared between the three scans. Dose maps were generated using a dedicated TomoTherapy treatment planning dose calculator. The VOI-FFMCT technique produced an image noise 1.05, 1.00, 1.03, and 1.05 times higher than the 'full dose' scan for ROI sizes of 10 cm, 13 cm, 10 cm, and 6 cm respectively within the VOI region. The VOI-FFMCT technique required a total imaging dose equal to 0.61, 0.69, 0.60, and 0.50 times the 'full dose' acquisition dose for ROI sizes of 10 cm, 13 cm, 10 cm, and 6 cm respectively within the VOI region. Noise levels can be almost unchanged within clinically relevant VOIs sizes for RT applications while the integral imaging dose to the patient can be decreased, and/or the image quality in RT can be dramatically increased with no change in dose relative to non-FFMCT RT imaging. The ability to shift dose away from regions unimportant for clinical evaluation in order to improve image quality or reduce imaging dose has been demonstrated. This paper demonstrates that FFMCT can be performed using the MLC on a clinical TomoTherapy machine for the first time.

      View details for PubMedID 26348406
  • Measurement-guided volumetric dose reconstruction for helical tomotherapy. J Appl Clin Med Phys
    Stambaugh C, Nelms B, Wolf T, Mueller R, Geurts M, Opp D, Moros E, Zhang G, Feygelman V
    2015 Mar 08; 16 (2): 5298
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      It was previously demonstrated that dose delivered by a conventional linear accelerator using IMRT or VMAT can be reconstructed - on patient or phantom datasets - using helical diode array measurements and a technique called planned dose perturbation (PDP). This allows meaningful and intuitive analysis of the agreement between the planned and delivered dose, including direct comparison of the dose-volume histograms. While conceptually similar to modulated arc techniques, helical tomotherapy introduces significant challenges to the PDP formalism, arising primarily from TomoTherapy delivery dynamics. The temporal characteristics of the delivery are of the same order or shorter than the dosimeter's update interval (50 ms). Additionally, the prevalence of often small and complex segments, particularly with the 1 cm Y jaw setting, lead to challenges related to detector spacing. Here, we present and test a novel method of tomotherapy-PDP (TPDP) designed to meet these challenges. One of the novel techniques introduced for TPDP is organization of the subbeams into larger subunits called sectors, which assures more robust synchronization of the measurement and delivery dynamics. Another important change is the optional application of a correction based on ion chamber (IC) measurements in the phantom. The TPDP method was validated by direct comparisons to the IC and an independent, biplanar diode array dosimeter previously evaluated for tomotherapy delivery quality assurance. Nineteen plans with varying complexity were analyzed for the 2.5 cm tomotherapy jaw setting and 18 for the 1 cm opening. The dose differences between the TPDP and IC were 1.0% ± 1.1% and 1.1% ± 1.1%, for 2.5 and 1.0 cm jaw plans, respectively. Gamma analysis agreement rates between TPDP and the independent array were: 99.1%± 1.8% (using 3% global normalization/3 mm criteria) and 93.4% ± 7.1% (using 2% global/2 mm) for the 2.5 cm jaw plans; for 1 cm plans, they were 95.2% ± 6.7% (3% G/3) and 83.8% ± 12% (2% G/2). We conclude that TPDP is capable of volumetric dose reconstruction with acceptable accuracy. However, the challenges of fast tomotherapy delivery dynamics make TPDP less precise than the IMRT/VMAT PDP version, particularly for the 1 cm jaw setting.

      View details for PubMedID 26103199
  • Monte Carlo simulations of patient dose perturbations in rotational-type radiotherapy due to a transverse magnetic field: a tomotherapy investigation. Med Phys
    Yang YM, Geurts M, Smilowitz JB, Sterpin E, Bednarz BP
    2015 Feb; 42 (2): 715-25
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      PURPOSE: Several groups are exploring the integration of magnetic resonance (MR) image guidance with radiotherapy to reduce tumor position uncertainty during photon radiotherapy. The therapeutic gain from reducing tumor position uncertainty using intrafraction MR imaging during radiotherapy could be partially offset if the negative effects of magnetic field-induced dose perturbations are not appreciated or accounted for. The authors hypothesize that a more rotationally symmetric modality such as helical tomotherapy will permit a systematic mediation of these dose perturbations. This investigation offers a unique look at the dose perturbations due to homogeneous transverse magnetic field during the delivery of Tomotherapy(®) Treatment System plans under varying degrees of rotational beamlet symmetry.

      METHODS: The authors accurately reproduced treatment plan beamlet and patient configurations using the Monte Carlo code geant4. This code has a thoroughly benchmarked electromagnetic particle transport physics package well-suited for the radiotherapy energy regime. The three approved clinical treatment plans for this study were for a prostate, head and neck, and lung treatment. The dose heterogeneity index metric was used to quantify the effect of the dose perturbations to the target volumes.

      RESULTS: The authors demonstrate the ability to reproduce the clinical dose-volume histograms (DVH) to within 4% dose agreement at each DVH point for the target volumes and most planning structures, and therefore, are able to confidently examine the effects of transverse magnetic fields on the plans. The authors investigated field strengths of 0.35, 0.7, 1, 1.5, and 3 T. Changes to the dose heterogeneity index of 0.1% were seen in the prostate and head and neck case, reflecting negligible dose perturbations to the target volumes, a change from 5.5% to 20.1% was observed with the lung case.

      CONCLUSIONS: This study demonstrated that the effect of external magnetic fields can be mitigated by exploiting a more rotationally symmetric treatment modality.

      View details for PubMedID 25652485
  • Longitudinal study using a diode phantom for helical tomotherapy IMRT QA. Med Phys
    Geurts M, Gonzalez J, Serrano-Ojeda P
    2009 Nov; 36 (11): 4977-83
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      PURPOSE: The Caribbean Radiation Oncology Center acquired a DELTA4 diode phantom for helical tomotherapy IMRT QA and presents the results of their first 264 clinical cases.

      METHODS: The validation consisted of several case studies comparing existing ionization chamber and Gafchromic film IMRT QA results to diode phantom results, along with a longitudinal study analyzing the IMRT QA results against other machine QA procedures for a complete sample of IMRT patients.

      RESULTS: The case studies resulted in a maximum observed difference of 0.7% between the diode phantom and the ionization chamber measurements in low dose-gradient regions. Over the longitudinal study, every IMRT QA plan passed a gamma specification of a 3%/3 mm and 98% of the diodes yielded a value of less than 1. In addition, the mean 90% isodose absolute difference for all plans was 0.05% with a (lsigma) standard deviation of 1.19%.

      CONCLUSIONS: The phantom measurements closely match the planned dose distributions in high and low dose-gradient regions. In addition, a significant positive statistical correlation was determined between the IMRT QA, daily QA, and rotational variation output measurements. Together, these results signify high degree of accuracy of both the DELTA4 phantom as well as the TomoTherapy Hi-Art system.

      View details for PubMedID 19994506
  • Portal imaging practice patterns of children's oncology group institutions: Dosimetric assessment and recommendations for minimizing unnecessary exposure. Int J Radiat Oncol Biol Phys
    Olch AJ, Geurts M, Thomadsen B, Famiglietti R, Chang EL
    2007 Feb 01; 67 (2): 594-600
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      PURPOSE: To determine and analyze the dosimetric consequences of current portal imaging practices for pediatric patients, and make specific recommendations for reducing exposure from portal imaging procedures.

      METHODS AND MATERIALS: A survey was sent to approximately 250 Children's Oncology Group (COG) member institutions asking a series of questions about their portal imaging practices. Three case studies are presented with dosimetric analysis to illustrate the magnitude of unintended dose received by nontarget tissues using the most common techniques from the survey.

      RESULTS: The vast majority of centers use double-exposure portal image techniques with a variety of open field margins. Only 17% of portal images were obtained during treatment, and for other imaging methods, few centers subtract monitor units from the treatment delivery. The number of monitor units used was nearly the same regardless of imager type, including electronic portal imaging devices. Eighty-six percent imaged all fields the first week and 17% imaged all fields every week. An additional 1,112 cm3 of nontarget tissue received 1 Gy in one of the example cases. Eight new recommendations are made, which will lower nontarget radiation doses with minimal impact on treatment verification accuracy.

      CONCLUSION: Based on the survey, changes can be made in portal imaging practices that will lower nontarget doses. It is anticipated that treatment verification accuracy will be minimally affected. Specific recommendations made to decrease the imaging dose and help lower the rate of radiation-induced secondary cancers in children are proposed for inclusion in future COG protocols using radiation therapy.

      View details for PubMedID 17236976

Contact Information

Mark Geurts, MS

600 Highland Avenue, K4/B100,
Madison, WI 53792