基于蒙特卡罗算法研究补偿膜下空腔的影响

Evaluation of the effects of cavity under bolus using the Monte Carlo algorithm

基于Geant4蒙特卡罗软件包对比不同空腔模型下水模体中剂量分布与无空腔时的差异,探讨空腔对补偿膜下剂量的影响。以Geant4构建加速器机头模型并获取机头下相空间文件,构建上表面位于源轴距、中心在射野中心并与射野中心轴垂直的30 cm×30 cm×30 cm水模体,在近机头一侧分别构建不含或含有不同厚度空腔的0.3 cm、0.5 cm和1.0 cm补偿膜,以相空间文件为粒子源计算不同补偿膜模型下水模体中心轴和浅层离轴剂量,以不含空腔结果为基准,对比不同厚度补偿膜下空腔厚度对剂量沉积的影响。结果表明:空腔厚度越大,对中心轴和浅层离轴剂量影响越大;对于0.3 cm、0.5 cm和1.0 cm的补偿膜,当空腔厚度分别为0.2 cm、0.3 cm和0.5 cm时,对浅表剂量影响均约3%,尚能满足临床需求,之后影响将随空腔厚度增加而迅速增大;空腔厚度相同时,中心轴剂量受影响由小到大依次为0.3 cm、1.0 cm和0.5 cm补偿膜,0.1 cm深度离轴剂量影响由小到大依次为1.0 cm、0.5 cm和0.3 cm补偿膜,受影响深度最小为0.3 cm补偿膜,另两者相似。制定浅表肿瘤放疗计划时,应根据瘤区位置选择合适厚度补偿膜,技术员在放置补偿膜时应努力将空腔厚度控制在补偿膜厚度1/2以内,越小越好。

To investigate the effects of“cavity under bolus”on dose distribution,we compared and analyzed the difference in dose distribution in the water phantom in the presence and absence of cavities using the Geant 4 Monte Carlo package.The model of the linear accelerator head was built using Geant 4,and the phase space file of the radiation particles under the model was obtained by simulating the process of electron hitting the target.A 30 cm×30 cm×30 cm water phantom with a center coinciding with the field center was built;its upper surface was located at the source-axis level and was vertical to the incoming ray.A 0.3-cm,0.5-cm,or 1.0-cm bolus was built on the upper surface of the phantom.For each bolus,there were a control model and several models with cavities.The bolus was close to the phantom in the control model,and there were different cavity thicknesses between the bolus and the phantom in the models with cavities.The central-axis and shallow off-axis doses of the phantom in different models were calculated based on the phase space file;the doses for the models with cavities were compared based on the results of the control model.The results showed that thicker cavities were associated with greater effects on the central-axis and shallow off-axis doses.If the cavity thicknesses were 0.2 cm,0.3 cm,and 0.5 cm for the 0.3-cm,0.5-cm,and 1.0-cm boluses,the largest effect of the lower dose were 2.97%,2.86%,and 3.45%,respectively.The cavity thickness increased with the rapid increase in the magnitudes of the effects.If the thicknesses of the cavities were the same,the effects of the central-axis doses were the greatest on the 0.5-cm bolus,followed by the 1.0-cm and 0.3-cm boluses,and those of the shallow off-axis doses were the greatest on the 0.3-cm bolus,followed by the 0.5-cm and 1.0-cm boluses.When planning,a suitable thickness of the bolus should be selected based on the tumor location.Technicians should be extremely careful when placing the bolus to ensure the cavity thickness is less than half of the bolus thickness.