Survival and causes of mortality were studied in 7- or 21-day-old male C57BL/Cnb mice exposed to 0.5, 1 or 3 Gy of 250 kVp X rays or 0.125, 0.25, 0.5 or 1 Gy of accelerator neutrons (modal energy 3.1 MeV). A total of 1287 animals were used in the experiments. Survival of irradiated animals was reduced significantly only in the mice receiving the highest doses (1 Gy neutrons, 3 Gy X rays). Mice exposed to the lowest doses (0.125 Gy neutrons, 0.5 Gy X rays) lived significantly longer than controls, apparently reflecting a reduction in non-neoplastic lung and liver diseases. All malignant tumors increased significantly from (and including) doses of 0.5 Gy neutrons and 1 Gy X rays. Hepatocellular carcinoma was the principal contributor to the increase in tumor incidence, at least after exposure to neutrons. No significant increase in hepatocellular carcinoma was seen in 21-day-old mice exposed to X rays. An increase, especially after 3 Gy X rays, was also observed for all leukemias. Controls in the present study lived significantly longer than those in our earlier studies of irradiated adult mice, making a direct comparison of the radiation-induced effects in adult and infant mice difficult. Based on percentage life shortening, it appears that exposure during infancy does not shorten total survival or survival from cancer much more than exposure of adults, although such exposure, especially to neutrons, causes more hepatocellular carcinoma. Due to the nonlinearity of the dose-effect relationships, it is difficult to calculate the RBE of neutrons. For survival time at higher doses an RBE of about 3 is obtained. When the incidence of all malignant tumors and of hepatocellular cancer is fitted to a linear or a linear-quadratic function, an RBE from 5 to 8 is obtained. No RBE can be estimated for hepatocellular carcinoma in mice of an age of 21 days because exposure to X rays does not seem to cause this tumor at that age.

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