重离子加速器示范装置被动式束流配送系统次级中子特征的蒙特卡罗研究

在碳离子放射治疗中，碳离子束在剂量配送过程中会与束流输运线相互作用，形成以中子辐射为主的外辐射场。由于中子是高LET射线，具有较高的相对生物学效应，减少碳离子放疗中产生的次级中子有助于降低放疗后正常组织并发症几率及二次肿瘤风险。利用蒙特卡罗方法对保守情况(能量为400 MeV/u，多叶光栅完全闭合)下碳离子治疗被动式束流配送系统引发的外源性中子在等中心点周围不同位置的注量谱及剂量当量谱进行模拟计算，评估了次级中子辐射的剂量当量贡献；同时探究了多元限束装置即多叶光栅不同开口大小对次级中子剂量当量的影响。模拟结果显示:高能中子(≥15 MeV)对所有模拟位置的中子剂量当量占比最高可达26%。实际治疗条件下，当多叶光栅开口由闭合状态扩大至15 cm×15 cm，等中心点和横向远端位置的次级中子剂量当量强度下降约85%，而束流方向远端位置下降约70%。这些结果可用于指导不同治疗条件下次级中子的研究，为治疗条件下评估患者所受非治疗辐射提供依据。

Monte Carlo Study on the Characteristics of Secondary Neutrons in Passive Beam Delivery System of Heavy Ion Medical Machine

In carbon-ion radiotherapy, a variety of secondary particles are produced when carbon-ion beams pass through beam-line components during the dose delivery process and form an external radiation field. The predominant part of external radiation is from neutrons. As high LET radiation, low-dose neutron exposures have high relative biological effectiveness. Reducing the secondary neutrons can help to curb the risks of normal tissue complications and secondary cancers. The fluence and dose equivalent spectrums of exogenous neutrons induced by the passive beam delivery system of carbon ion therapy at different locations around the isocenter were simulated using the Monte Carlo method to evaluate the dose equivalent contribution of neutron radiation; The influence of the different collimating aperture sizes on the secondary neutron dose equivalent was also investigated. The simulation results show that high-energy neutrons had a certain contribution to the total neutron dose equivalent of the simulated locations, which account for up to 26%. Under real treatment conditions, as the collimating aperture size increased from closed to 15 cm×15 cm, the neutron dose equivalent decreased by approximately 85% at the isocenter and laterally distal locations, and about 70% at the distal locations in the beam direction. These results can be used to guide the study of secondary neutrons under different treatment conditions, and provide a basis for assessing non-therapeutic radiation received by patients under treatment conditions.