Calculations of Mean Quality Factors and Their Implications for Organ-specific Relative Biological Effectiveness (RBE) in Analysis of Radiation-related Risk in the Atomic Bomb Survivors

Past and current estimates of relative biological effectiveness (RBE) from the cohort analyses of atomic bomb survivors suggested not only that RBE may be much higher than those assessed by the United Nations Scientific Committee on Effects of Atomic Radiation (UNSCEAR) and International Commission on Radiological Protection (ICRP), but also that RBE may differ by organ and organ depth. This is at least partly due to how the ratio of neutron to gamma-ray dose changes with organ depth because of the more rapid attenuation of neutrons in tissue. Additionally, the RBE estimates from Life Span Study (LSS) data depend on the total dose and the neutron/gamma ratio. To further examine this issue, we calculated the mean quality factor based on Linear Energy Transfer (LET) distributions for representative organs and exposure scenarios of A-bomb survivors using Particle and Heavy Ion Transport code System (PHITS) simulation and the radiation quality factor [Q(L) relationship] defined by ICRP, as well as the Quality Factor (QF) function defined by the National Aeronautics and Space Administration (NASA). This is done in the context of the adult male phantom of the J45 series, which was created to precisely reproduce the anatomy of the Japanese population in 1945. We also investigate the depth dependence of the mean quality factors in the International Commission on Radiation Units and Measurements (ICRU) sphere irradiated by mono-energetic neutrons. Both the results from the human phantom, and from the ICRU sphere phantom suggest that the mean quality factors are approximately 15 and independent of the organ type, body depth, city and ground range when the contributions from the secondary γ rays are excluded from the neutron doses. We also discuss reasons that RBE estimates from cohort analyses are generally much larger than those based on the mean quality factors.