Development of Monte Carlo techniques for radiation transport simulations and application to radiotherapy physics and dosimetry

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The 2011 AAPM summer school for medical physicists was on the subject of Uncertainties in External Beam Radiation Therapy. The opening plenary speaker argued that the issues that were most likely to impact patient outcomes were dose calculation accuracy and calibration accuracy because these issues affect every one of a patient's many treatments. The goal of this research program is to improve the dosimetry of radiotherapy delivery by providing accurate research tools for calculating dose distributions from traditional and emerging technologies and by providing new information needed to accurately use different types of radiation dosimeters.The program consists of four broadly linked areas of research.For over 25 years the PI has been deeply involved with the development of the EGS system which is used to simulate the transport of radiation through matter. EGSnrc is considered the gold standard for such software. However, with the emergence of combined MRI-radiotherapy systems, it is essential to extend the EGSnrc system to work in the presence of magnetic fields which can have a strong influence both on the dose delivered and on some of the instruments used to measure it. Other improvements to be made will maintain the code's accuracy and significantly increase its efficiency. Measurements will be made in collaboration with the standards lab at NRC to verify the accuracy of the calculations. These improvements will be made available to researchers around the world so they may independently verify the new MRI-therapy systems and more efficiently study many other problems.The second research area is the calculation of correction factors for ion chambers which are used to measure the dose in radiation therapy treatments. Values recently calculated by the PI's group for use in linac photon beams have been recommended for clinical use throughout North America. In this program, calculations and measurements will be done for the more complex case of electron beams and corrections for low-energy x-ray beams will be calculated.The third research area is to extend our recent work on TLD dosimeters to a wide range of clinical detectors. With the software now available, we are able to much more accurately model detector response and investigate the effects of realistic models of the detectors. Within the conceptual framework popularized by the 2009 AAPM summer school, it is now possible to more clearly characterize detector's response. Our research on TLD detectors showed that there are many subtleties which most clinical physicists were unaware of. The goal is to use Monte Carlo simulations to clarify these types of issues for a wide variety of detectors (e.g., diodes, OSL detectors, MOSFETS, radiochromic film) so they may be used with confidence to ensure accurate radiotherapy.The final component is the extraction of photon spectra from measured depth-dose curves. Previous approaches suffered from a variety of problems but, by introducing an accurate correction for electron contamination, using dual detectors with different properties to measure the depth-dose curves, using our recently developed functional form to represent the bremsstrahlung spectra and explicitly accounting for the detector's response with depth in the phantom, this research program will overcome the previous problems and provide a robust way to accurately determine clinical accelerator spectra which are needed for the calculation of dose distributions in the patient.While these four components of the research program will not provide any dramatic improvements in cancer treatment, collectively they will have a significant impact on how well radiation therapy is delivered and hence improve patient care by improving our physics understanding of dosimetry.