Hyperplastic Lesions of Sprague-Dawley Rat Mammary Glands After X Irradiation 1,2

SUMMARY-This study reports that X irradiation, which produced mammary tumors in Sprague-Dawley rats, also produced hyperplastic lesions of the mammary gland. These lesions were similar morphologically to the preneoplastic lesions of the mouse mammary gland and were more numerous in X-irradiated rats than in animals of comparable age that were not exposed to irradiation. The lesions were more frequent as the time after irradiation increased in both irradiated and non irradiated rats; how-ever, the rate of increase was greater in the irradiated animals. The hyper-plastic lesions induced in the rat mammary glands were not shown to be preneoplastic. However, the greater number of lesions encountered under conditions which increased the incidence of tumors is suggestive infor-mation.-J Nat Cancer Inst 39: 449-458,1967. SPRAGUE-DAWLEY rats develop mammary tumors after a single exposure to either ionizing radiation or polynuclear aromatic hydrocarbons. The chemical carcinogens induce mammary adeno-carcinomas rapidly and efficiently. Huggins et al. (1) have shown that a single feeding of either 3-methylcholanthrene (MeA) or 7,I2-dimethyl-benz[a]anthracene (DMBA) to young, female Sprague-Dawley rats is followed within approxi-mately 100 days by the appearance of at least one mammary adenocarcinoma in each rat. A single exposure of the young, female Sprague-Dawley rat to a sublethal dose of either X rays or gamma rays produced one or more mammary neoplasms within 1 year in 60-80 % of the rat