RBE FOR CARCINOGENESIS FOLLOWING EXPOSURE TO HIGH-LET RADIATION

Stochastic radiation effects following exposure to heavy ions and othe r high linear energy transfer (LET) radiation in space are a matter of concern when the long-term consequences of space flights are consider ed. This paper is an overview of the relevant literature, emphasizing uncertainties entailed from estimates of relative biological effective ness (RBE) for different experiment endpoints, making the choice of a single weighting factor for the pre-diction of cancer risk in man extr emely difficult. Life-span-shortening studies in mice exposed to heavy ions and ongoing large-scale experiments in monkeys exposed to proton s suggest that RBEs for all cancers are lower than 5. This does not ex clude a much higher RBE for rare tumors such as brain tumors in monkey s or promoted Harderian gland tumours in mice at LET >80 keV/mu m. Ski n cancer studies in rats exposed to neon or argon resulted in similar RBE. Exposure to fission neutrons led to high RBE in all species, not excluding values much higher than 20 for specific cancers such as lung tumors in mice and all cancers in rats. The estimate of maximal RBE i s, however, extremely dependent on the hypothesis made on the shape of the dose-response curves in the lower range of doses. These results s uggest that neutrons may be the most hazardous component of high-LET r adiation. There is only limited evidence from cancer experiments that LET >150 keV/mu m results in highly decreased efficiency, but this has been found for bone cancer induction following exposure to fission fr agments.