SURVIVAL AND DISEASES IN C57BL MICE EXPOSED TO X-RAYS OR 3.1 MEV NEUTRONS AT AN AGE OF 7 OR 21 DAYS

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 to tal of 1287 animals were used in the experiments. Survival of irradiat ed animals was reduced significantly only in the mice receiving the hi ghest doses (1 Gy neutrons, 3 Gy X rays). Mice exposed to the lowest d oses (0.125 Gy neutrons, 0.5 Gy X rays) lived significantly longer tha n controls, apparently reflecting a reduction in non-neoplastic lung a nd liver diseases. All malignant tumors increased significantly from ( and including) doses of 0.5 Gy neutrons and 1 Gy X rays. Hepatocellula r carcinoma was the principal contributor to the increase in tumor inc idence, at least after exposure to neutrons. No significant increase i n hepatocellular carcinoma was seen in 21-day-old mice exposed to X ra ys. An increase, especially after 3 Gy X rays, was also observed for a ll 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 fr om cancer much more than exposure of adults, although such exposure, e specially to neutrons, causes more hepatocellular carcinoma. Due to th e nonlinearity of the dose-effect relationships, it is difficult to ca lculate 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. (C) 1996 by Radiation Research Society