Rat differences in mammary tumor induction with estrogen and neutron radiation

Young adult female rats of either the Sprague-Dawley stock or the ACI strain were each given an implant of a compressed pellet of 5 mg diethylstilbestrol (DES) and 15 mg cholestrol 2 days before irradiation with 0.4, 1.3, or 4.0 rads of 0.43-MeV neutrons. These rats were studied, along with appropriate irradiated and nonirradiated controls, until death or for a maximum of 48 weeks. Response differences between the strain and stock included the following: DES produced both pituitary tumors and mammary adenocarcinomas (MAC) in ACI rats only. Neutron radiation increased mammary fibroadenoma (MFA) formation in Srague-Dawley rats only. No interactions between DES and radiation on MAC formation in Sprague-Dawley rats or MFA formation in ACI rats were demonstrated. However, when DES and neutron radiation were combined, DES appeared to inhibit the MFA response to radiation in Sprague-Dawley rats. In contrast, DES appeared to act synergistically with neutron radiation on MAC formation in ACI rats. These results clearly demonstrate rat differences in mammary gland carcinogenesis in response to estrogen, to radiation, or to a combination of both agents.