nine months post exposure. The results suggest that the severity of opacification is dependent upon dose. At nine months, the animals with the most severe opacification are those that had received the highest doses of irradiation.Al so, the results suggest that the latency is dependent upon the dose. At nine months post irradiation only one animal in the 10 rad group had begun to develop lens opacification. We therefore evaluated the animals again after a period of 18 months (Table l). At that time, all mice given 10 rad showed lensepacifications. The average opacification score was similar (2.6-2.9) at that time, in mice that received 10, 25, or 50 rad, and we assume that the lack of dose-dependence is due to a plateauing of the response by this time. We will continue to examine these animals to determine if the cataractogenic process proceeds further in these mice. Table I Average Cataract Scores of the Posterior Lens in LAF^ Female Mice at 18 Months After Exposure to 40Ar ion (4 cm SOBP) Experiment IV compares the effects of plateau irradiation of animals irradiated with argon, neon, carbon and also x-ray. At the onset of the experiment it was postulated that HZE particles might have an RBE of 10 in comparison with x-radiation. Six months after the animals had been irradiated, only minimal lens changes were observed. No dose-response relationship was apparent, and no significant differences emerged among the lenses irradiated with the different particles. This situation changed at 9 months after irradiation. As is apparent from Figure 1, the degree of opacification is strongly dependent upon the ion used. The dose scale for x-irradiated mice must be multiplied by 10. At the highest dose (90 rad), all types of particles produced unmistakable opacification, the extent of which is correlated with the expected LET dependence of the response. That is, the degree of opacification increased progressively with increasing estimated values of LET; namely argon, neon and then carbon. From these data, one cannot accurately determine the RBE for HZE particles in relation to x-radiation. There is no question that the RBE is less than 10. Whereas,the x-ray animals given 900 rad had developed total lens opacification at 9 months, the 90 rad argon-induced cataracts were approximately grade 3, the neon about grade 2 and carbon grade 1. If one considers the data in Figure 1, one observes that 60 rad of argon radiation induces about as much opacification as 300 rads of x-rays; thus, the RBE may be of the order 5. The RBE for carbon would then be about 159/90 or less than 2.0 at 9 months. If average cataract scores plateau with time, estimates of RBE will be time dependent. Preliminary results will be presented concerning radiation-induced life span shortening after exposure to 400 MeV spread peak 12C or G0Co gamma radiation. At one year after a single dose of 240 rad, the cumulative mortality is similar to that produced in the same mouse strain given the same dose of fission
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