由于辐射暴露而计算出的人类癌症风险主要基于长期随访研究,例如,广岛和长崎的原子弹(A-bomb)幸存者的寿命研究(LSS)。由于A炸弹辐射由γ射线和中子的混合物组成,如果一项研究要作为癌症风险的参考,那么必须充分评估中子的相对生物学有效性(RBE).然而,相对较小的中子分量阻碍了LSS数据中RBE的直接估计。为了避免这个问题,已经尝试了几种策略,包括剂量独立常数RBE,剂量依赖性变量RBE,以及对混合γ射线的优势程度的依赖性。通过调查现有文献,我们测试了中子的染色体RBE作为生物学终点,其与使用组织等效比例计数器(TEPC)在各种中子场中获得的微剂量测定量的等效性。辐射加权因子,或质量因素,Qn,以最大RBE的能量依赖性表示的中子,RBEm,与TEPC数据预测的一致,表明染色体测量的RBE与共存γ射线的大小无关。得到的中子RBE,随着中子剂量的变化,被证实是最适当的RBE系统,与A炸弹幸存者的癌症发病率一致,使用染色体畸变作为替代标记。有了这个RBE系统,以参考辐射的单位剂量表示的A炸弹幸存者的癌症风险与广岛和长崎城市同等兼容,并且可能适用于其他人类辐射暴露的情况。
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, Qn, of neutrons as expressed in terms of the energy dependence of the maximum RBE, RBEm, 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.