多层平板电离室优化设计

Optimization design of a multilayer ionization chamber

多层平板电离室(Multilayer Ionization Chamber,MLIC)能够降低质子束流剂量深度分布的测量时间,对提高质子治疗日常剂量验证的效率具有实际意义。影响MLIC测量精度的因素主要有组成材料、敏感面积等。基于这些影响因素,用蒙特卡罗方法计算束流在MLIC的剂量深度分布,与标准剂量分布对比评估其测量精度。优化后的MLIC包含180层电离室,组成材料为FR-4,敏感面积为12cm×12cm,极板间隙为1mm,采用自由空气作为敏感探测器气体,其剂量深度测量精度在±5%以内,射程测量精度在±1mm以内。

[Background] Multilayer ionization chamber (MLIC) can significantly reduce the measurement time of proton pencil beam dose depth distribution, which is of practical significance for improving the efficiency of daily quality assurance of proton therapy.[Purpose] This study aims to optimize the design of a MLIC to realize fast and accurate dose verification.[Methods] Based on Monte-Carlo method, simulation of the MLIC was performed by using Geant4 codes. To evaluate the measurement uncertainties of the MLIC, materials of FR-4, polymethylmethacrylate (PMMA), Al were applied and the depth dose distribution in each material were compared with water phantom to evaluate the uncertainties caused by different material. Then the depth dose distribution was corrected by the collection efficiency of each layer of the MLIC in different sensitive area, and the deviation compared with water phantom after correction was calculated.[Results] The final measurement accuracy of the optimized MLIC in the depth dose distribution is within ±5%, and range accuracy is within ±1 mm.[Conclusion] The optimized MLIC could replace the water tank to achieve fast daily quality assurance in clinical use.