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

2011年AAPM医学物理学家暑期班的主题是外照射治疗中的不确定因素。开幕式全体演讲者认为，最有可能影响患者结果的问题是剂量计算准确性和校准准确性，因为这些问题影响到患者许多治疗中的每一个。这项研究计划的目标是通过提供精确的研究工具来计算传统和新兴技术中的剂量分布，并通过提供准确使用不同类型的辐射剂量计所需的新信息来改进放射治疗的剂量学。该计划由四个广泛联系的研究领域组成。25年来，PI一直深度参与EGS系统的开发，该系统用于模拟辐射通过物质的传输。EGSnrc被认为是此类软件的黄金标准。然而，随着MRI-放射治疗联合系统的出现，有必要将EGSnrc系统扩展到在磁场存在的情况下工作，磁场可以对所提供的剂量和用于测量剂量的一些仪器产生强烈的影响。将进行的其他改进将保持代码的准确性，并显著提高其效率。测量将与NRC的标准实验室合作进行，以验证计算的准确性。这些改进将提供给世界各地的研究人员，以便他们可以独立验证新的核磁共振治疗系统，并更有效地研究许多其他问题。第二个研究领域是用于测量放射治疗中剂量的电离室的修正系数的计算。PI小组最近计算的用于直线加速器光子束的数值已被推荐用于整个北美的临床使用。在这个项目中，将对更复杂的电子束情况进行计算和测量，并将计算低能X射线束的修正。第三个研究领域是将我们最近在热释光剂量计方面的工作扩展到更广泛的临床探测器。有了现在可用的软件，我们能够更准确地模拟探测器的响应，并调查探测器的真实模型的影响。在2009年AAPM暑期班推广的概念框架内，现在有可能更清楚地描述探测器的反应。我们对热释光探测器的研究表明，存在许多大多数临床物理学家没有意识到的微妙之处。我们的目标是使用蒙特卡罗模拟来澄清各种探测器(例如二极管、OSL探测器、MOSFET、辐射变色薄膜)的这些类型的问题，以便它们可以放心地使用，以确保精确的放射治疗。最后一个组成部分是从测量的深度-剂量曲线中提取光子光谱。以前的方法存在各种问题，但是，通过引入对电子污染的精确校正、使用具有不同性质的双探测器来测量深度-剂量曲线、使用我们最近开发的函数形式来表示韧致辐射光谱并明确考虑探测器对模体中深度的响应，该研究计划将克服先前的问题，并提供一种可靠的方法来准确确定计算患者剂量分布所需的临床加速器光谱。虽然研究计划的这四个组成部分不会在癌症治疗中提供任何显著的改进，总的来说，它们将对放射治疗的效果产生重大影响，从而通过提高我们对剂量学的物理理解来改善患者护理。

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.