目的:使用蒙特卡洛方法模拟在碳离子放疗过程中产生的次级中子对人体主要器官的吸收剂量和当量剂量.方法:基于中国科学院近代物理研究所(IMP)的重离子深层肿瘤治疗的束流配送系统治疗头,使用MCNPX对该系统的初级准直器、脊型过滤器、射程移位器以及多叶准直器进行建模,模拟计算400 MeV/u碳离子均匀照射野经过IMP被动式束流配送系统,入射到RPI-Adult男性体模后,统计全身主要器官内的次级中子能谱和吸收剂量与当量剂量.结果:当脑垂体瘤接受治疗并给予50 Gy处方剂量时,不同器官内的中子能谱显示所产生的次级中子能量范围比较大,最大能量高这几百MeV;而且脑部部分器官的中子当量剂量相对比较高,大脑、头盖骨和眼晶体当量剂量为53.18、32.43、33.20 mSv;离脑部较远的器官,如胸部、肺以及前列腺剂量很低,都小于0.4 mSv.结论:利用蒙特卡洛方法和计算机仿真人体模型模拟了碳离子放疗过程,并统计了全身大部分器官内次级中子能谱和受到的剂量.本研究计算的结果和结论,再结合相关资料,可以为临床上研究碳离子放疗的远期效应提供参考.%Objective To calculate the absorbed secondary neutron dose and the equivalent dose in the major organs during carbon ion radiotherapy using the Monte Carlo method.Methods Based on the beam delivery system for heavy ion deep tumor treatment developed by the Institute of Modem Physics (IMP),Chinese Academy of Sciences,we constructed the models of the primary collimator,ridge filter,range shifter,and multi-leaf collimator using MCNPX code.We recorded the absorbed secondary neutron dose,equivalent dose and energy spectrum in the major organs when the RPI-Adult male phantom was exposed to a beam of 12C ions with an energy level of 400 MeV/u delivered via the IMP passive beam delivery system.Results For treatment of pituitary tumors,a prescribed dose at 50 Gy resulted in a wide energy range in the organs,as shown by the neutron energy spectrum,and the maximum energy could reach even several hundreds of MeV.The secondary neutron equivalent doses were also high in the organs close to the target including the brain,cranium and eye lens (53.18,32.43 and 33.20 mSv,respectively),but in the more distal organs (such as the breasts,lungs and prostate),the equivalent doses were rather low (all below 4 mSv).Conclusion We successfully simulated the process of carbon ion radiotherapy using the Monte Carlo method and computational voxelized human phantom.The secondary neutron energy spectrum and dose distribution in the major organs in this simulation can provide a reference for clinical evaluation of the long-term effects of the carbon ion radiotherapy.
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