Pregnant Wistar-MS rats received a whole-body irradiation of 0-2.6 Gy γ rays at day 20 of pregnancy. The mother rats were implanted with a diethylstilbestrol (DES) pellet 30 days after weaning, and the female pups delivered by the irradiated mother were treated with DES after maturation. Lactating rats were irradiated with γ rays 21 days after parturition and then treated with DES. Virgin rats 70 days of age were also irradiated and then administered DES. The rats which received intrauterine irradiation did not develop mammary tumors at doses less than 2.1 Gy and showed a low incidence of tumors at 2.6 Gy. In virgin rats, the maximum tumor incidence was obtained with 1 Gy. The incidence of total mammary tumors in the mother rats and lactating rats increased in a dose-dependent manner with increasing doses of γ rays up to 2.1 Gy. With 0.1-1 Gy, the incidence of adenocarcinoma in the mother rats was significantly lower than that observed in the lactating rats. However, the incidence in the mother rats irradiated with 1.0-1.5 Gy was significantly higher than that of virgin rats treated with the corresponding γ-ray doses. These findings suggest that the susceptibility of the mammary glands to radiation depends upon the differentiation at the time of exposure.

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