1985 Volume 76 Issue 6 Pages 449-456
The present study was undertaken to assess the mammary carcinogenic effect of low doses of fission and thermal neutrons in female W/Fu rats. Only 2 of 62 (3.2%) rats exposed to various doses of fission radiation alone developed mammary tumors (MT) in a 12-month observation period, whereas 21 of 63 (33.3%) similarly irradiated rats developed MT if further treated with prolactin; the lowest effective dose was 4.1 rad, which contained only 1.7 rad of fission spectrum neutrons with an average energy of 2.0MeV. The long survival of radiation-initiated potentially malignant cells was suggested by the observation that excess numbers of MT developed in irradiated rats in whom prolactin treatment was started as late as 12 months after irradiation. The negligible contribution of gamma-rays, one component of the reactor radiations, to the rat mammary carcinogenesis was proven by a simulation experiment with 50Co gamma-rays. Thermal neutrons with an average energy of 0.025eV were less effective than fission neutrons. The rat mammary carcinogenic effects of 180 kVp X-rays, 14.1MeV fast neutrons, 0.025eV thermal neutrons and 2.0MeV fission neutrons were estimated in such a way as to compare the dose of each radiation that gave an MT incidence of 40% of that in irradiated, prolactin-treated rats. The efficiency of fission spectrum neutrons is much higher than those of other radiations; the relative biological effectiveness (RBE) of fission spectrum neutrons was 17.8 against X-rays. Based on these findings, the relevance of animal data to tumor induction in atomic bomb survivors is discussed.
