252Cf relative biological effectiveness and inheritable effect of fission neutrons in mouse liver tumorigenesis

To determine the relative biological effectiveness of 252Cf fission neutrons versus 60Co gamma-rays for inducing liver tumorigenesis, C57BL/6NCrj x C3H/NCrj F1, hereafter called B6C3F1, mice were irradiated once either with 252Cf or 60Co and the tumorigenicity of the two types of radiation was studied. Individual groups of B6C3F1 mice (about 30 mice/group) were irradiated once with 252Cf at doses of 0, 3, 6, 12.5, 50, and 200 cGy for males and 0, 12.5, 50, and 200 cGy for females or with 60Co at doses of 0, 12.5, 50, and 200 cGy for both sexes. The groups were observed for 13 months after irradiation. The incidence of hepatic tumors in nonirradiated controls was 3.8% in males and 3.2% in females. 252Cf irradiation increased the incidence dose dependently in males and females, but less efficiently in females. The mean number and size of hepatic tumors were well correlated with tumor incidence. In contrast, 60Co was less efficient at inducing tumors. The relative biological effectiveness of 252Cf compared to 60Co was quite high in the low-dose range. Overall, the average relative biological effectiveness of 252Cf versus 60Co was 15.2 in males and 2.5 in females. In another experiment, 7-week-old male C3H mice, which are known to be carriers of hepatic tumorinogens, and that were the fathers of the B6C3F1 mice, were irradiated with 0, 50, and 200 cGy of 252Cf, and 2 weeks later they were mated with nonirradiated female C57BL mice. The control mice had nonirradiated fathers. The incidence of hepatic tumors in the offspring of irradiated fathers was 43% in males and 1.7% in females, whereas in the offspring of control mice with nonirradiated fathers the incidence was 3.2% in males and 3.3% in females. The multiplicity of the hepatic tumors was similar to the tumor incidence. Thus our results indicated a strong hepatocarcinogenic potential for 252Cf neutrons compared to 60Co gamma-rays, and that the cancer-prone genetic trait activated by 252Cf irradiation was inherited by first-generation offspring, especially in males.