Beneficial effects of subacute and chronic experiments of low dose irradiation in mice

This study examined the beneficial effect of radiation, focusing on survival time and liver tumorigenesis in mice treated with low dose irradiation. In Experiment 1, four groups of B6C3F1 mice were given total doses of 0, 5, 10, and 20 cGy, respectively, at a low-dose rate of 0.1-0.5 cGy/min, and 2 weeks later, each group was exposed to total irradiation of 800, 1000, or 1200 cGy at a high-dose rate of 40-50 cGy/min. Twenty days after high-dose rate irradiation, the animals were observed for survival. All groups previously treated with low-dose irradiation but not treated with the subsequent high-dose irradiation showed a survival rate of 100%. There was no beneficial effect of the subsequent high-dose irradiation with all three doses. In Experiment 2, five species of male and female mice (C3H, C57BL, BCF1, CBF1, and Csb) were treated with total low-dose Cf-252 irradiation (200 or less cGy). Thirteen months after irradiation, the animals were sacrificed and observed for liver tumorigenesis. The incidence of liver tumor was higher with increasing radiation doses in the groups of C3H, BCF1, and CBF1, irrespective of sex. In the group of C57BL, male rats treated with 50 cGy had a V-shaped pattern in the development of liver tumor. The group of C3H treated with 12.5 cGy showed an inverse V-shaped pattern in the effect of low-dose irradiation. (N.K.)