The Occurrence of Mammary Tumors in the Rat after Sublethal Whole-Body Irradiation

Abstract

A total of 110 female Sprague-Dawley rats, 40 to 41 days old, received whole-body irradiation as a single exposure with and without [beta]-radiation to the skin of the back. The rats were observed for 11 months post-irradiation for the occurrence of mammary tumors. Fourteen nonirradiated rats of the same age were used as controls. Since local [beta]-radiation had no demonstrable effect on the occurrence of mammary tumors, the various experimental groups were combined on the basis of the dose of whole-body irradiation received. Fifty-six percent of the rats exposed to 400 r of gamma-radiation developed one or more tumors, 43% of the rats exposed to 400 r of X-ray developed one or more mammary tumors, and 31% (19 of 48) of the rats exposed to 200 r of gamma-radiation developed one or more mammary tumors. No tumors developed in 48 control rats. All tumors arising in the 11 month period were mammary tumors, except one adamantinoma and one squamous cell carcinoma of the skin of the ear. Seventy-six mammary tumors were observed in 48 whole-body-irradiated rats including 43 adenofibromas, 20 adeno-carcinomas, 5 fibroadenomas, 3 fibrosarcomas, and 5 not classified. An increased incidence of mammary tumors with a short latent period after sublethal whole-body radiation exposure was not previously defined in this species. Although the mechanism of tumor induction is not understood, this observation should add a valuable technique to both radiobiology and oncology.