Accurate Monte Carlo simulation of megavoltage beams coupled to external magnetic fields

一种将磁共振成像与巨量放射治疗机相结合的新技术在图像引导方面具有独特的优势，与基于X射线的技术相比，具有高对比度的靶区清晰度，并且对患者没有成像辐射剂量。随着这些技术的出现，传统放射治疗的临床常规必须完全适应，以确保最佳治疗和患者安全。需要解决的一个重要问题是辐射剂量测定。在磁共振成像中使用的强度下，磁场可以对能量沉积产生重大影响，这是由于电子束运动的带电粒子所受的力所致。模拟这些效应以获得不同几何形状的剂量分布需要使用蒙特卡罗方法，因为其他商业上可用的技术在这些条件下是过时的。磁场对能量沉积的一个主要影响是剂量分布的空间失真，除其他因素外，它随几何形状和束流相对于磁场的取向而变化。人们认识到，蒙特卡罗方法有可能在磁场存在的情况下重现兆伏光子束的能量沉积，达到比临床要求更好的精度水平。该研究计划的主要目的是改进和验证蒙特卡罗算法，以模拟在外磁场存在的情况下，兆伏辐射在非均匀几何结构中的输运和能量沉积。在接下来的十年里，MRI综合放射治疗有望在临床上得到广泛应用。因此，这项研究计划预计将对放射治疗产生重大兴趣和影响，实现对耦合到磁场的兆伏束流进行准确的辐射剂量建模。该计划的结果将有助于达到射束校准、射束建模和患者剂量计算方面的相同标准，就像目前在没有磁场的情况下使用的技术和方法所实现的那样，例如传统的放射治疗直线加速器。

A novel technology integrating magnetic resonance imaging to megavoltage radiotherapy machines promises unique advantages for image guidance over x-ray based techniques, with high-contrast for target definition and no imaging radiation dose to the patient. With the advent of such technologies, the clinical routine of conventional radiotherapy must be completely adapted to assure optimal treatment and patient safety. One important topic that needs to be addressed is radiation dosimetry. At strengths used in magnetic resonance imaging, magnetic fields can have a significant impact on the energy deposition due to the force acting charged particles set in motion by the beam. Modeling these effects to obtain dose distributions in heterogeneous geometries requires using the Monte Carlo method, since other techniques commercially available are obsolete in these conditions. One main effect of magnetic fields on energy deposition is a spatial distortion of the dose distribution, which among other factors varies with the geometry and the orientation of the beam with respect to the field. It is recognized that the Monte Carlo method has the potential to reproduce the energy deposition of megavoltage photon beams in the presence of magnetic fields to an accuracy level better than clinical requirements. The main purpose of the research program is to improve and validate Monte Carlo algorithms for simulating the transport and energy deposition of megavoltage radiation in heterogeneous geometries in the presence of external magnetic fields. Over the next decade, MRI-integrated radiotherapy is expected to become widely used in clinics. Therefore this research program anticipates significant interest and impact in radiotherapy, achieving accurate radiation dosimetry modeling of megavoltage beams coupled to magnetic fields. The outcomes of the program will contribute to meeting the same standards in beam calibration, beam modeling and patient dose calculation as currently achieved with the technology and methods currently available for modalities used in the absence of magnetic fields, such as conventional radiotherapy linear accelerators.