Study of Calculation Method for 3D Dose Distribution and Range Distribution in a Patient Body in Proton Therapy

使用真实的质子治疗获得的CT数据进行了质子照射到患者体内的蒙特卡罗模拟。我们研究的实际能力，这种模拟方法在治疗计划系统中的质子治疗在一个点的计算时间，在质子照射的二次粒子效应的模型，并在使用的剂量分布函数与蒙特卡罗模拟的可能性。将GEANT应用于质子治疗的仿真研究。这是一个基于粒子的完整的模拟代码，包括核相互作用。用体素构造患者结构，体素的属性由CT数据确定。在质子治疗中，模拟需要每个体素中材料的原子序数Z和原子质量A。因此，在质子模拟中，CT数据中的电子密度应该被转换成Z和A。有效原子序数Z_<eff>i和原子质量A_<eff>i定义为：Z_i <eff>=&lt;$ω_iZ_i，A_i <eff>=&lt;$ω_iA_i，ω_i=P_iA_i/&lt;$P_iA_i，其中 ...更多信息 e P_i是材料中第i个组分的数重。它们表示为CT数据的函数，使用从体模数据确定的校准曲线。每层5 mm厚的CT数据具有320×320像素，尺寸为1 mm × 1 mm。患者结构被构造为一组体素，如在治疗计划系统中计划的模拟代码中的设置。我们在Monte Carlo模拟中开发了以下三点的模型：（1）对通过每个体素的质子计算一次能量存款和运动学参数值，（2）制作每个脊过滤器仅质子的空间和运动学分布表以及补偿器前面产生的每个质子能量（团注滤波器），以及（3）在估计穿过体素的初级和次级质子的能量沉积中使用扩展深度剂量分布。将该模型引入到GEANT程序的蒙特卡罗模拟中，在一台PC工作站（Linux OS，Pentium 4）上，在垂直于250 MeV质子束的半径为7 cm的圆内，对1个切片（5 mm × 2 mm）的每个体素横截面的1%精度进行计算，计算时间为1-2小时。蒙特卡罗模拟中各事件计算的独立性使得使用n台并行PC工作站可以减少1/n的计算时间。这些PC工作站的配置使统计在总结事件时变得容易n倍。我们认为，这种方法是有能力的治疗计划系统的质子治疗，与多台并行的PC工作站。少

Monte Carlo simulation of proton irradiation into a patient has been performed using CT data obtained for real proton treatment. We examined practical capability of this simulation method in treatment planning system in proton therapy in a point of calculation time, a model of secondary particle effects in proton irradiation, and possibility in use of dose distribution function produced with the Monte Carlo simulation. The GEANT is applied to the simulation in the proton therapy. It is a particle-based full simulation code including nuclear interaction. A patient structure is constructed with voxels, whose properties are determined by CT data. In the proton therapy, the simulation requires an atomic number Z and an atomic mass A of the material in each voxel. Thus an electron density in the CT data should be converted into Z and A in the proton simulation. Effective atomic number Z_<eff> and atomic mass A_<eff> are defined as : Z_<eff>=Σω_iZ_i, A_<eff>=Σω_iA_i, ω_i=P_iA_i/ΣP_iA_i, wher … More e P_i is a number weight of the i'th component in a material. They are expressed as a function of the CT data using calibration curves determined from phantom data. Each 5mm-thick slice of the CT data has 320×320 pixels in 1mm×1mm size. The patient structure is constructed as a group of voxels, as setup in the simulation code as planned in the treatment planning system. We developed the model with the following three points in the Monte Carlo simulation : (1) calculating energy deposit and kinematical parameter values once for a proton passing through each voxel, (2) making tables of spatial and kinematical distribution of only protons for each ridge filter and each proton energy in production at the front of compensator (bolus filter), and (3) using extended-depth-dose distribution in estimating energy deposits of primary and secondary protons passing through voxels. In introducing this model to the Monte Carlo simulation with the GEANT code, calculation time is 1-2 hours in producing dose distribution in a patient body under the following calculation condition : one PC workstation (Linux OS, Pentium4), 1% accuracy for each voxel cross-section of 1 slice (5mm×2mm), in a circle of 7cm in radius perpendicular to the proton beam at 250MeV. Independence of each event calculation in the Monte Carlo simulation makes it possible to reduce computing time 1/n with using n parallel sets of PC workstation. Clusterization of these PC workstation sets makes statistics n times easily in summing up events. We consider that this method is capable as treatment planning system in proton therapy, with several parallel sets of PC workstation. Less