Sasaki Masao S, Endo Satoru, Hoshi Masaharu, Nomura Taisei
Kyoto University and National Institute of Biomedical Innovation, Health and Nutrition, 17-12 Shironosato, Nagaokakyo-shi, Kyoto 617-0835, Japan
Quantum Energy Application, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8527, Japan.
J Radiat Res. 2016 Nov;57(6):583-595. doi: 10.1093/jrr/rrw079. Epub 2016 Sep 10.
The calculated risk of cancer in humans due to radiation exposure is based primarily on long-term follow-up studies, e.g. the life-span study (LSS) on atomic bomb (A-bomb) survivors in Hiroshima and Nagasaki. Since A-bomb radiation consists of a mixture of γ-rays and neutrons, it is essential that the relative biological effectiveness (RBE) of neutrons is adequately evaluated if a study is to serve as a reference for cancer risk. However, the relatively small neutron component hampered the direct estimation of RBE in LSS data. To circumvent this problem, several strategies have been attempted, including dose-independent constant RBE, dose-dependent variable RBE, and dependence on the degrees of dominance of intermingled γ-rays. By surveying the available literature, we tested the chromosomal RBE of neutrons as the biological endpoint for its equivalence to the microdosimetric quantities obtained using a tissue-equivalent proportional counter (TEPC) in various neutron fields. The radiation weighting factor, or quality factor, Q, of neutrons as expressed in terms of the energy dependence of the maximum RBE, RBE, was consistent with that predicted by the TEPC data, indicating that the chromosomally measured RBE was independent of the magnitude of coexisting γ-rays. The obtained neutron RBE, which varied with neutron dose, was confirmed to be the most adequate RBE system in terms of agreement with the cancer incidence in A-bomb survivors, using chromosome aberrations as surrogate markers. With this RBE system, the cancer risk in A-bomb survivors as expressed in unit dose of reference radiation is equally compatible with Hiroshima and Nagasaki cities, and may be potentially applicable in other cases of human radiation exposure.
人类因辐射暴露而患癌症的计算风险主要基于长期随访研究，例如对广岛和长崎原子弹幸存者的寿命研究（LSS）。由于原子弹辐射由γ射线和中子混合而成，如果一项研究要作为癌症风险的参考，那么对中子的相对生物效能（RBE）进行充分评估至关重要。然而，相对较小的中子成分阻碍了对LSS数据中RBE的直接估计。为了规避这个问题，人们尝试了几种策略，包括剂量无关的恒定RBE、剂量依赖的可变RBE以及对混合γ射线主导程度的依赖。通过查阅现有文献，我们测试了中子的染色体RBE作为生物学终点，以确定其在各种中子场中与使用组织等效正比计数器（TEPC）获得的微剂量学量的等效性。以最大RBE（RBE）的能量依赖性表示的中子的辐射权重因子或品质因数Q与TEPC数据预测的一致，这表明通过染色体测量的RBE与共存γ射线的大小无关。利用染色体畸变作为替代标志物，所获得的随中子剂量变化的中子RBE在与原子弹幸存者癌症发病率的一致性方面被确认为最适当的RBE系统。使用这个RBE系统，以参考辐射单位剂量表示的原子弹幸存者的癌症风险在广岛和长崎市同样适用，并且可能潜在地适用于其他人类辐射暴露情况。
