St Aubin J, Keyvanloo A, Fallone B G
Department of Medical Physics, Cross Cancer Institute, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada and Department of Oncology, Medical Physics Division, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada.
Department of Medical Physics, Cross Cancer Institute, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada.
Med Phys. 2016 Jan;43(1):195. doi: 10.1118/1.4937933.
The advent of magnetic resonance imaging (MRI) guided radiotherapy systems demands the incorporation of the magnetic field into dose calculation algorithms of treatment planning systems. This is due to the fact that the Lorentz force of the magnetic field perturbs the path of the relativistic electrons, hence altering the dose deposited by them. Building on the previous work, the authors have developed a discontinuous finite element space-angle treatment of the linear Boltzmann transport equation to accurately account for the effects of magnetic fields on radiotherapy doses.
The authors present a detailed description of their new formalism and compare its accuracy to geant4 Monte Carlo calculations for magnetic fields parallel and perpendicular to the radiation beam at field strengths of 0.5 and 3 T for an inhomogeneous 3D slab geometry phantom comprising water, bone, and air or lung. The accuracy of the authors' new formalism was determined using a gamma analysis with a 2%/2 mm criterion.
Greater than 98.9% of all points analyzed passed the 2%/2 mm gamma criterion for the field strengths and orientations tested. The authors have benchmarked their new formalism against Monte Carlo in a challenging radiation transport problem with a high density material (bone) directly adjacent to a very low density material (dry air at STP) where the effects of the magnetic field dominate collisions.
A discontinuous finite element space-angle approach has been proven to be an accurate method for solving the linear Boltzmann transport equation with magnetic fields for cases relevant to MRI guided radiotherapy. The authors have validated the accuracy of this novel technique against geant4, even in cases of strong magnetic field strengths and low density air.
磁共振成像（MRI）引导放疗系统的出现要求将磁场纳入治疗计划系统的剂量计算算法中。这是因为磁场的洛伦兹力会干扰相对论电子的路径，从而改变它们沉积的剂量。基于之前的工作，作者开发了一种线性玻尔兹曼输运方程的间断有限元空间角处理方法，以准确考虑磁场对放疗剂量的影响。
作者详细描述了他们的新形式体系，并将其准确性与geant4蒙特卡罗计算结果进行比较，该计算针对的是场强为0.5和3 T、磁场平行和垂直于辐射束的情况，使用的是包含水、骨和空气或肺的非均匀三维平板几何体模。作者新形式体系的准确性通过采用2%/2 mm标准的伽马分析来确定。
对于所测试的场强和方向，超过98.9%的分析点通过了2%/2 mm伽马标准。作者在一个具有挑战性的辐射输运问题中，将他们的新形式体系与蒙特卡罗方法进行了基准测试，该问题中高密度材料（骨）紧邻极低密度材料（标准温度和压力下的干燥空气），磁场效应主导碰撞。
对于与MRI引导放疗相关的情况，间断有限元空间角方法已被证明是求解含磁场的线性玻尔兹曼输运方程的一种准确方法。作者已针对geant4验证了这种新技术的准确性，即使在强磁场强度和低密度空气的情况下也是如此。
