Comparative stochastic effects from low level exposure of mice to tritiated water

C57B1 mice were exposed to single or fractionated d(50)-Be neutron or /sup 137/Cs gamma ray exposure at 12 weeks of age and were followed for life shortening and disease incidence. The data were analyzed namely by the Kaplan-Meier procedure using as criteria cause of death and possible causes of death. Individual groups were compared by a modified Wilcoxon test according to Hoel and Walburg. No significant difference was found in survival time of C57B1 mice between a single gamma and neutron exposure. Gamma fractionation was clearly less effective in reducing survival than single exposure. On the contrary, fractionation of neutrons was slightly, although not significantly more effective in reducing lifespan than a single exposure. The data on causes of death show that malignant tumors, particulary leukemias and non-stochastic lung diseases were the principal causes of life shortening after a high single gamma exposure. Exposure delivered in 8 fractions 3 hours apart was more effective in causing leukemias and all carcinomas and sarcomas than one delivered in a single session. Following a single neutron exposure, leukemias and all carcinomas and sarcomas appeared to increase somewhat more rapidly with dose than after gamma irradiation. No significant difference in the incidence of leukemias and all carcinomas and sarcomas was noted between a single and a fractionated neutron exposure