Stable chromosome aberrations among A-bomb survivors: an update.

Analysis of data on stable chromosome aberrations collected between 1968 and 1985 by the Radiation Effects Research Foundation (RERF) on 1703 individuals exposed to A-bomb radiation in Hiroshima and Nagasaki, Japan, reveals different dose-response relationships in the two cities, as well as significant effects of both time of assay and age at exposure. In Hiroshima, the proportion of cells with aberrations increased by 0.080 per sievert at low doses, assuming a constant neutron radiation RBE of 10 relative to gamma radiation, for assays performed during the latest period (1981-1985). In Nagasaki, the low-dose increase was 0.0126 per sievert. There was evidence that radiation exposure was more effective for producing stable aberrations at some younger ages at exposure, although the interpretation of this interaction is difficult. Modeling neutron and gamma-ray components of dose separately in a way which allows the neutron RBE to vary with dose yielded an estimated low-dose limiting value of RBE of 707 (95% confidence bound 200-infinity), with a low-dose response of approximately 0.008 aberrations per sievert. This RBE is much higher than the published RBEs for induction of aberrations in vitro. The high estimated RBE and the differences in dose response by city both are suggestive of systematic dose estimation errors in which either neutrons were underestimated in Hiroshima or gamma rays were overestimated in Nagasaki.