体元大小对基于MonteCarlo的剂量计算的影响

目的：在放射治疗计划系统中，剂量计算之前需要对人体密度数据体元化。对于蒙特卡罗方法的模拟过程，当一个自由程跨过体元界面时，会应用自由程近似。选取的体元越小，将导致越多的自由程近似。本文采用蒙特卡罗方法模拟一个虚拟射线源入射到水箱中的反应，计算水箱中的剂量分布，通过比较水箱分层和不分层两种情况下中心轴百分深度剂量分布和离轴比分布，来探讨选用不同大小的体元对剂量分布的影响。方法：本文以6MeV的方形电子射线源为外照射源、以三维水箱为介质模型。使用PENELOPE程序包模拟电子束垂直入射到水箱中引起的电子与物质的相互作用。比较水箱在分层和不分层情况下中心轴百分深度剂量和离轴比分布。结果：通过比较水箱在分层和不分层情况下中心轴百分深度剂量和离轴比分布，发现差异很小。结论：选用不同大小的体元，蒙特卡罗近似处理自由程对剂量计算精度的影响很小。研究结果对蒙特卡罗方法在放射治疗中的临床应用具有指导意义。

Effects Induced by Choosing Different Voxels on Dose Calculation Based on Monte Carlo Method

Objective:In treatment planning system,human density data should be voxelized before dose calculation.When using Monte Carlo method to simulate the procedure,if the free path steps over the interface of the voxel,the free path will be processed approximately.The smaller the voxel is,the more approximate processions there will be.The article is to simulate the reactions in the water phantom that caused by a virtual radiation source based on Monte Carlo method,and calculate the dose distribution in it.By comparing the distribution of the PDD and ORA under two cases,that is,when the water phantom is stratified,we deal with the free path of the particles approximately when it crossing the interfaces and when the water phantom isn’t stratified.The impact of the distribution by choosing different sizes of voxels wll be discussed.Methods: Set 6 MeV electron square fields as external beam source,3-dimensional water phantom as media and simulate the reactions between electron and material in the water phantom when electron beam enter into it vertically by using Penelope program.We stratify the water phantom and compare the central axis Percent Depth Dose(PDD) and Off-Axis Ratio(OAR).Results: Comparing the central axis Percent Depth Dose(PDD) and Off-Axis Ratio(OAR) in these two situations,the influence is little.Conclusions: Choosing different voxels has little influence on the dose accuracy when using Monte Carlo method to process the free path approximately.The results of the research have an instructive effect on the clinical radiotherapy based on Monte Carlo simulation.