QUANTITATIVE ASSESSMENT OF THE CATARACTOGENIC POTENTIAL OF VERY-LOW DOSES OF NEUTRONS

We report on the prevalence and relative biological effectiveness (RBE ) for various stages of lens opacification in rats induced by very low doses (2 to 250 mGy) of medium-energy (440 keV) neutrons, compared to those for X rays. Neutron doses were delivered either in a single fra ction or in four separate fractions and the irradiated animals were fo llowed for over 100 weeks. At the highest observed dose (250 mGy) and at early observation times, there was evidence of an inverse dose-rate effect; i.e., a fractionated exposure was more potent than a single e xposure. Neutron RBEs relative to X rays were estimated using a non-pa rametric technique. The results were only weakly dependent on time pos tirradiation. At 30 weeks, for example, 80% confidence intervals for t he RBE of acutely delivered neutrons relative to X rays were 8-16 at 2 50 mGy, 10-20 at 50 mGy, 50-100 at 10 mGy and 250-500 at 2 mCy. The re sults are consistent with the estimated neutron RBEs in Japanese A-bom b survivors, though broad confidence bounds are present in the Japanes e results. Our findings are also consistent with data reported earlier for cataractogenesis induced by heavy ions in rats, mice and rabbits. We conclude from these results that, at very low doses (<10 mGy), the RBE for neutron-induced cataractogenesis is considerably larger than the RBE of 20 commonly used, and use of a significantly larger value f or calculating equivalent dose would be prudent. (C) 1996 by Radiation Research Society