Matsumoto Shinnosuke, Koba Yusuke, Kohno Ryosuke, Lee Choonsik, Bolch Wesley E, Kai Michiaki
*Graduate school, Oita University of Nursing and Health Sciences. Oita city, Oita 870-1201, Japan; †Medical Exposure Research Project, National Institute of Radiological Sciences. Chiba city, Chiba 263-8555, Japan; ‡Division of Particle Therapy, National Cancer Center Hospital East. Kashiwa city, Chiba 277-8577, Japan; §Division of Cancer Epidemiology and Genetics, National Cancer Institute, National Institute of Health, Rockville, MD 20850, USA; **Department of Radiology, University of Florida, Gainesville, FL 32611, USA.
Health Phys. 2016 Apr;110(4):380-6. doi: 10.1097/HP.0000000000000461.
Proton therapy has the physical advantage of a Bragg peak that can provide a better dose distribution than conventional x-ray therapy. However, radiation exposure of normal tissues cannot be ignored because it is likely to increase the risk of secondary cancer. Evaluating secondary neutrons generated by the interaction of the proton beam with the treatment beam-line structure is necessary; thus, performing the optimization of radiation protection in proton therapy is required. In this research, the organ dose and energy spectrum were calculated from secondary neutrons using Monte Carlo simulations. The Monte Carlo code known as the Particle and Heavy Ion Transport code System (PHITS) was used to simulate the transport proton and its interaction with the treatment beam-line structure that modeled the double scattering body of the treatment nozzle at the National Cancer Center Hospital East. The doses of the organs in a hybrid computational phantom simulating a 5-y-old boy were calculated. In general, secondary neutron doses were found to decrease with increasing distance to the treatment field. Secondary neutron energy spectra were characterized by incident neutrons with three energy peaks: 1×10, 1, and 100 MeV. A block collimator and a patient collimator contributed significantly to organ doses. In particular, the secondary neutrons from the patient collimator were 30 times higher than those from the first scatter. These results suggested that proactive protection will be required in the design of the treatment beam-line structures and that organ doses from secondary neutrons may be able to be reduced.
质子治疗具有布拉格峰这一物理优势，与传统的X射线治疗相比，它能提供更好的剂量分布。然而，正常组织的辐射暴露不容忽视，因为这可能会增加患继发性癌症的风险。评估质子束与治疗束流线结构相互作用产生的次级中子是必要的；因此，需要在质子治疗中进行辐射防护优化。在本研究中，使用蒙特卡罗模拟从次级中子计算器官剂量和能谱。名为粒子与重离子传输代码系统（PHITS）的蒙特卡罗代码被用于模拟质子传输及其与治疗束流线结构的相互作用，该结构模拟了国立癌症中心东医院治疗喷嘴的双散射体。计算了模拟一名5岁男孩的混合计算体模中各器官的剂量。一般来说，发现次级中子剂量会随着与治疗野距离的增加而降低。次级中子能谱以具有三个能量峰（1×10、1和100兆电子伏）的入射中子为特征。准直器和患者准直器对器官剂量有显著贡献。特别是，来自患者准直器的次级中子比来自第一次散射的次级中子高30倍。这些结果表明，在治疗束流线结构的设计中需要积极的防护措施，并且次级中子产生的器官剂量或许能够降低。
