252CfRelative Biological Effectiveness and Inheritable Effect of Fission Neutrons in Mouse Liver Tumorigenesis1

To determine the relative biological effectiveness of 252Cffission neu trons versus *°Co 7-rays for inducing liver tumorigenesis, C57BL/6NCrj x C3H/NOJ F,, hereafter called B6C3F,, mice were irradiated once either with 252Cfor "Co and the tumorigenicity of the two types of radiation was studied. Individual groups of B6C3F] mice (about 30 mice/group) were irradi ated once with 252Cfat 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 "Co 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. 252Cfirradiation increased the incidence dose dependent!) 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, ''"Co was less efficient at inducing tumors. The relative biological effectiveness of 2S2Cfcompared to MCo was quite high in the low-dose range. Overall, the average relative biological effective ness of 252Cfversus MCo 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 B6C3F, 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 252Cfneutrons compared to *°Co 7-rays, and that the cancer-prone genetic trait activated by 252Cfirradiation was inherited by first-generation off spring, especially in males.