吸收体材料水等效厚度比的能量依赖对MLIC测量结果影响的模拟研究

Simulation study of the influence of water equivalent ratio energy dependence of absorber materials on measurement of multilayer ionization chamber

多层平板电离室(Multilayer Ionization Chamber,MLIC)能够对质子深度剂量分布进行快速测量。研究吸收体水等效厚度比(Water Equivalent Ratio,WER)的能量依赖,对提高MLIC的测量精度有重要意义。采用上海先进质子治疗装置(Shanghai Advanced Proton Facility,SAPT)固定束治疗室的束流源参数,使用蒙特卡罗方法,通过与实测积分深度剂量曲线进行对比,验证模拟环境的准确性,并计算铝、PMMA和FR-4三种吸收体材料在不同能量和不同厚度下的WER。利用不同能量质子束入射采用不同吸收体材料的MLIC,计算相应的积分深度剂量分布及特征参数。模拟计算结果表明:在临床质子放疗能量范围内,WER的能量依赖会影响MLIC获得的深度剂量分布射程参数的测量,其影响超过60%,而对布拉格峰宽度和远端剂量跌落区的影响较小。采用合适的WER值可减小不同吸收体MLIC与参考MLIC在深度剂量分布特征参数上的差异,其中聚甲基丙烯酸甲酯(Polymethylmethacrylate,PMMA)的射程差异可减小至0.220 mm。研究结果对测量质子治疗深度剂量分布的MLIC相关设备的开发设计和实验测试具有重要的指导价值。

[Background]The Multilayer Ionization Chamber(MLIC)is an instrument in rapidly measuring the proton depth dose distribution,which is crucial for enhancing the efficiency of beam commissioning and daily quality assurance in treatment rooms.[Purpose]This study aims to investigate the impact of the Water Equivalent Ratio(WER)energy dependence of various absorber materials on MLIC measurements,thereby improving the accuracy of depth dose distribution measurements.[Methods]Based on the fixed beam source parameters in the beam therapy room of Shanghai Advanced Proton Facility(SAPT),a physical model of MLIC was constructed using Monte-Carlo method.The simulation environment was validated by comparison of the measured and simulated integrated depth dose curve.The WER for three absorber materials i.e.,Aluminum,PMMA,and FR-4,was calculated by simulation across different energies and thicknesses.Then,proton pencil beams of varying energies were simulated incident on MLIC,and the depth dose distribution of MLIC made from these materials was analyzed whilst the MLIC composed of water absorber was served as a reference.[Results]Simulation results show that the energy dependence of WER significantly influences the range parameters of the depth dose distribution,which was measured by MLIC within the clinical proton radiotherapy energy spectrum,with an impact exceeding 60%,and has a lower effect on the width and the distal dose falling region length of the Bragg peak.By adopting the appropriate WER values,the disparities in depth dose distribution parameters between MLIC made from different absorber materials and that composed of water absorber can be greatly reduced.Notably,for PMMA(Polymethylmethacrylate),the range discrepancy is minimized to 0.220 mm.[Conclusions]The depth dose distribution measured by MLIC is notably affected by the energy dependence of WER,underscoring the importance of considering WER's energy dependence in clinical proton therapy.The study is valuable for guiding experiment tests and optimized design of MLIC.