Institute of Medical Physics and Radiation Protection, University of Applied Sciences Giessen (THM), Giessen, Germany; Department of Radiotherapy and Radiation Oncology, University Medical Center Giessen and Marburg, Marburg, Germany.
Institute of Medical Physics and Radiation Protection, University of Applied Sciences Giessen (THM), Giessen, Germany; Department of Radiotherapy and Radiation Oncology, University Medical Center Giessen and Marburg, Marburg, Germany; Marburg Ionbeam Therapycenter (MIT) Marburg, Germany.
Z Med Phys. 2023 Nov;33(4):499-510. doi: 10.1016/j.zemedi.2022.07.002. Epub 2022 Aug 25.
Monte Carlo simulations are crucial for calculating magnetic field correction factors k for the dosimetry in external magnetic fields. As in Monte Carlo codes the charged particle transport is performed in straight condensed history (CH) steps, the curved trajectories of these particles in the presence of external magnetic fields can only be approximated. In this study, the charged particle transport in presence of a strong magnetic field B→ was investigated using the Fano cavity test. The test was performed in an ionization chamber and a diode detector, showing how the step size restrictions must be adjusted to perform a consistent charged particle transport within all geometrical regions.
Monte Carlo simulations of the charged particle transport in a magnetic field of 1.5 T were performed using the EGSnrc code system including an additional EMF-macro for the transport of charged particle in electro-magnetic fields. Detailed models of an ionization chamber and a diode detector were placed in a water phantom and irradiated with a so called Fano source, which is a monoenergetic, isotropic electron source, where the number of emitted particles is proportional to the local density.
The results of the Fano cavity test strongly depend on the energy of charged particles and the density within the given geometry. By adjusting the maximal length of the charged particle steps, it was possible to calculate the deposited dose in the investigated regions with high accuracy (<0.1%). The Fano cavity test was performed in all regions of the detailed detector models. Using the default value for the step size in the external magnetic field, the maximal deviation between Monte Carlo based and analytical dose value in the sensitive volume of the ion chamber and diode detector was 8% and 0.1%, respectively.
The Fano cavity test is a crucial validation method for the modeled detectors and the transport algorithms when performing Monte Carlo simulations in a strong external magnetic field. Special care should be given, when calculating dose in volumes of low density. This study has shown that the Fano cavity test is a useful method to adapt particle transport parameters for a given simulation geometry.
蒙特卡罗模拟对于计算外磁场中剂量学的磁场校正因子 k 至关重要。由于在蒙特卡罗代码中，带电粒子的输运是在直凝聚历史（CH）步骤中进行的，因此只能近似外磁场中这些粒子的弯曲轨迹。在这项研究中，使用 Fano 腔测试研究了强磁场 B→下的带电粒子输运。该测试在电离室和二极管探测器中进行，展示了如何调整步长限制以在所有几何区域内进行一致的带电粒子输运。
使用 EGSnrc 代码系统进行了磁场为 1.5 T 的带电粒子输运的蒙特卡罗模拟，该代码系统包括一个用于在电磁场中传输带电粒子的附加 EMF-宏。将电离室和二极管探测器的详细模型放置在水模体中，并使用所谓的 Fano 源照射，该源是一种单能、各向同性的电子源，其中发射的粒子数与局部密度成正比。
Fano 腔测试的结果强烈依赖于带电粒子的能量和给定几何形状内的密度。通过调整带电粒子步长的最大长度，可以以高精度（<0.1%）计算所研究区域内的沉积剂量。在详细探测器模型的所有区域都进行了 Fano 腔测试。在外磁场中使用步长的默认值，在离子室和二极管探测器的灵敏体积中，蒙特卡罗计算和分析剂量值之间的最大偏差分别为 8%和 0.1%。
Fano 腔测试是在强外磁场中进行蒙特卡罗模拟时对建模探测器和传输算法进行关键验证的方法。在计算低密度体积中的剂量时应特别注意。本研究表明，Fano 腔测试是一种适应给定模拟几何形状的粒子输运参数的有用方法。
