质子治疗机房中次级中子反射分布的模拟研究

Simulation study of secondary neutron reflection distribution in proton therapy room

100~250 MeV的质子治疗机房的复杂次级辐射中,中子是屏蔽设计的重点。目前多采用混凝土内嵌入铁板以降低中子透射,但这样会造成显著的中子反射(即朝向患者方向)。使用蒙特卡罗方法探究245 MeV质子治疗机房中铁板的不同嵌入深度对次级中子反射分布的影响,并使用ICRP(International Commission on Radiological Protection)116号报告给出的中子外照射剂量转换系数评估患者受到的有效剂量。结果显示:主屏蔽墙对机房内次级中子的反射贡献大于95%,铁板嵌入深度为0(裸露在机房内)时机房外辐射剂量水平最低,但患者在AP(Antero-posterior)、PA(Posterior-anterior)、LAT(Lateral)三种照射方式下由反射中子造成的有效剂量率最高,分别为1.99μSv‧s^(−1)、1.37μSv‧s^(−1)和1.00μSv‧s^(−1);随着铁板嵌入深度增加,反射中子造成的有效剂量率逐渐降低,但趋势越来越缓慢。在机房屏蔽设计时需要考虑这一特征。

[Background]The protection of secondary neutrons in the complex secondary radiation of the 100~250 MeV proton therapy room has always been the focus of shielding design.Currently,iron plates embedded in concrete are often used to reduce neutron transmission,but this will cause significant neutron reflection(toward the patient).And neutrons have a high radiation weighting factor which may cause harmful effects on patients.[Purpose]The study aims to investigate the reflection distribution of secondary neutrons in the proton therapy room and its impact on patients.[Methods]First of all,a simple model of the proton therapy room was established,and Monte Carlo program FLUKA was used to calculate the effects of the secondary neutron reflection from different directions and at different embedding positions of the iron shield in the 245 MeV proton therapy room.Then,under the three irradiation modes of AP(Antero-posterior),PA(Posterior-anterior)and LAT(Lateral),the effective dose of the patient was calculated according to the energy of the reflected neutrons reaching the position of the patient whilst the neutron external radiation dose conversion coefficient given in report of ICPR(International Commission on Radiological Protection)Publication 116 was used.[Results]The simulation results show that the main shielding wall contributes more than 95%to the reflection distribution of the secondary neutrons in the treatment room.The radiation dose level outside the treatment room is the lowest when the embedded depth of the iron plate in the wall is 0(the iron plate is exposed in the room),but highest effective dose rates caused by reflected neutrons are 1.99μSv‧s^(-1),1.37μSv‧s^(-1 ) and 1.00μSv‧s^(-1),respectively,under the three irradiation modes of AP,PA and LAT.With the increase of embedding depth of iron plate,the effective dose rate of reflected neutron decreased gradually,but the decreasing trend becomes slower and slower.[Conclusions]The structure of the main shielding wall of the treatment room and the embedded position of the iron plate have a significant impact on the reflected secondary neutron energy spectrum and the patient's radiation health.This feature needs to be considered in the shielding design of the proton therapy room.