In the past few years much physical evidence has accumulated that the A-bomb survivors at Hiroshima were exposed to significant doses of neutrons, in contrast to the predictions of the current DS86 dosimetry. We discuss some biological measurements of exchange-type chromosomal aberrations in survivors at Hiroshima, which also strongly imply that the survivors received a significant neutron dose. Specifically, the ratio of translocations (an interchromosomal aberration) to pericentric inversions (intrachromosomal interarm aberration), the F value, was significantly smaller than would be expected from a γ-ray exposure, and was consistent with the majority of the effective dose coming from neutrons. If this biological evidence and the previous physical evidence are correct, the effective neutron dose at relevant locations at Hiroshima dominated the total effective dose, from which it may be concluded that (1) the risk coefficient for γ rays may have been considerably overestimated, and (2) there is a possibility of deriving from the A-bomb data, with reasonable confidence limits, the relative biological effectiveness (RBE) for carcinogenesis by neutrons.

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