基于TOPAS计算的磁场下质子辐射剂量分析

摘要

Objective To study the impact of magnetic field on the dose distribution of proton beam in phantom,providing a data reference for magnetic resonance imaging (MRI)-guided proton therapy.Methods Monte Carlo code TOPAS were implemented to calculate the dose distribution in a water phantom with the impact of transverse magnetic field on proton beam.In addition,a water phantom with an air-gap (water-air-water phantom) was also used to study the impact of the electron return effect (ERE) under the magnetic field on the proton dose at the interface of water and air.Results When the intensity of the magnetic field was less than 0.5 T,and the proton energy was less than 150 MeV,the shift of the field in depth direction was 1 mm at the Bragg peak,but the lateral shift of high dose area of XZ plane which was parallel to the incidence direction of proton beams was 4 mm in homogeneous water phantom.For the combination of proton of 150 MeV and magnetic field of 1.5 T,though the shift of the field was 1 mm at the Bragg peak,the lateral shift was larger than 10 mm.At the interface of water and air,no visible effect of the magnetic field on the proton dose distribution was observed.Conclusion The proton radiation dose under the magnetic field can be accurately analyzed by using Monte Carlo method.Transverse magnetic field has little effect on the dose distribution of proton in depth direction,but has obvious impact on the lateral shift.And the lateral shift was proportional to the proton energy and magnetic field strength,but the impacts of ERE on the proton dose distribution at the interface of water and air were nearly negligible.%目的:分析磁场作用下质子束在模体中的剂量分布,为核磁共振引导的质子放疗提供数据参考.方法:采用蒙特卡罗软件TOPAS计算治疗用质子束在核磁共振横向磁场影响下水体模中的剂量分布.同时采用水-空气-水模型研究磁场下的电子回转效应对不同介质交界处质子剂量分布的影响.结果:在均匀水体模中,当磁场强度在0.5T以内,质子能量在150 MeV以下时,质子布拉格峰位置沿深度方向偏移在1mm以内,但与束流入射方向平行的XZ面的高剂量区横向侧移在4mm左右;当磁场强度达到1.5T时,150 MeV的质子布拉格峰位置偏移在1mm以内,但横向侧移达10mm以上.研究结果还发现在磁场作用下,质子在水与空气交界处的剂量无明显变化.结论:利用蒙特卡罗方法可以准确分析磁场下的质子辐射剂量.横向磁场的存在对质子在深度方向的剂量影响较小,但对横向剂量侧移影响较大,且与能量、磁场强度成正比,而电子回转效应对质子在水与空气交界处的剂量影响近似可以忽略.