The effect of changes in dosimetry on cancer mortality risk estimates in the atomic bomb survivors.

In the spring of 1986 the Radiation Effects Research Foundation (RERF) received a new atomic bomb dosimetry system. This report presents the comparisons of leukemia and nonleukemia cancer mortality risk estimates under the old and new dosimetries. In terms of total kerma (essentially whole-body gamma plus neutron exposure), risk estimates for both classes of cancer are 75-85% higher with the new dosimetry. This and other summary comparisons allow for possible nonlinearity at high estimated doses. Changes are also considered in relation to organ doses and assumptions about the relative biological effectiveness (RBE) of neutrons. Without regard to RBE, the risk estimates for total organ dose are essentially unchanged by the dosimetry revision. However, with increasing assumed values of RBE, the estimated low-LET risk decreases much less rapidly under the new dosimetry, due to the smaller neutron component. Thus at an assumed constant RBE of 10, for example, the effect of the dosimetry revision is to increase organ dose risk estimates, relative to those based on the old dosimetry, by 30% for nonleukemia and 80% for leukemia. At an RBE of 20 these increases are 72 and 136%, respectively. A number of other issues are discussed. The city difference in dose is no longer statistically significant, even at an RBE of one. Estimation of RBE is even less feasible with new dosimetry. There is substantial question of the linearity in dose response, in the sense of a leveling off at higher doses. Finally, some indication is given of how risks estimated from this dosimetry and the current data may compare to widely used estimates based largely on the RERF data with the previous dosimetry.