The nature of the RBE-LET relationship for radiation-induced effects in vivo is not well known in the high-LET range above about 100 keV/μm. Here, we compare the cataractogenic effects of acute doses of 190 keV/μm iron ions on the eyes of rats with those of 88 keV/μm argon ions. The RBEs of the two radiations cannot be distinguished statistically, both being between 50 and 200 at our lowest dose of 0.01 Gy and decreasing to between 2 and 14 at our highest dose of 0.5 Gy; these values are consistent with results obtained in vivo, both for cataractogenesis and for oncogenesis. For this end point, therefore, the RBE-LET relationship probably varies very slowly between 88 and 190 keV/μm. On the basis of these studies with acute doses of 88 and 190 keV/μm particles, the detailed distribution in LET of the very high-LET galactic cosmic-ray dose to which astronauts in deep space are exposed may not be critical for the prediction of biological hazard. Such a conclusion might simplify the task of high-LET radiation risk estimation in space.

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