Discussion of uncertainties and the impact of different neutron RBEs on all solid cancer radiation incidence risks obtained from the Japanese A-bomb survivor data.

The most recent publicly available data on all solid cancer incidence from the Life Span Study (LSS) of Japanese A-bomb survivors provides colon dose contributions weighted with a relative biological effectiveness (RBE) of 10 for neutrons, relative to gammas. However, there is evidence from several investigations that the neutron RBE for A-bomb survivors may be higher than 10. The change in the shape of the corresponding dose-response curves was evaluated by Hafner and co-workers in a previous study by applying sex-specific linear-quadratic dose models to previous LSS data for all solid cancer incidence that include separate neutron and gamma absorbed doses for several organs, in contrast to the most recent data. The resulting curvature change became significantly negative for males at an RBE of 140 for colon, 100 for liver, and 80 for organ averaged dose. For females, the corresponding RBE values were 110, 80, and 60 for colon, liver, and organ averaged doses. The present study compares three different methods to calculate the 95% confidence intervals in an analysis of the curvature with increasing RBE. Further, the impact of a higher neutron RBE on the work of the International Commission on Radiological Protection, and the importance of including uncertainties and performing sensitivity analysis of different parameters in radiation risk assessment are discussed.