Radiation-induced mammary carcinogenesis in virgin, pregnant, lactating, and postlactating rats.

Little attention has been paid to the influence of the repro ductive condition of the rat at the time of irradiation upon Xray-induced mammary carcinogenesis. At 12 weeks of age, 160 virgin female Sprague-Dawley rats were mated, and an other 140 were maintained as virgins. On Days 14 to 16 of pregnancy, 36 mated females and 40 virgins were whole-body irradiated with 200 R of X-rays. On Days 2 to 5 postpartum, 40 additional mated females and 40 additional virgins were irra diated. At this time, all pups were removed from their mothers; 2 weeks later, 40 additional parous females and 40 virgins were irradiated. The remaining animals: parous females; vir gins; and mated, but nonparous females were kept as unirradiated controls. Breast tissue biopsies were taken from 25% of the animals in each irradiated group on the day of irradiation, and the biopsies were examined microscopically for mammary tissue development. The experiment was terminated 10 months after irradiation. Despite marked differences in mammary tissue development among the groups at the time of irradiation, there were no significant differences among the irradiated groups for the incidence of rats with mammary adenocarcinomas or the number of mammary adenocarcinomas per rat. There were no significant differences among the irradiated groups for the incidence of rats with mammary fibroadenomas. There was an increase in the number of fibroadenomas per rat which was associated with increased age at the time of death. In contrast to mammary carcinogenesis produced by polycyclic aromatic hydrocarbons in previous studies, in the current investigation the physiological status of the host at the time of irradiation had little effect on: (a) the final incidence of mammary neopla sia; (b) the number of mammary adenocarcinomas produced; or (c) the type of mammary neoplasms produced.