SUSCEPTIBILITY OF FETAL, VIRGIN, PREGNANT AND LACTATING RATS FOR THE INDUCTION OF MAMMARY-TUMORS BY GAMMA-RAYS

Pregnant Wistar-MS rats received a whole-body irradiation of 0-2.6 Gy gamma rays at day 20 of pregnancy. The mother rats were implanted with a diethylstilbestrol (DES) pellet 30 days after weaning, and the fema le pups delivered by the irradiated mother were treated with DES after maturation. Lactating rats were irradiated with gamma rays 21 days af ter parturition and then treated with DES. Virgin rats 70 days of age were also irradiated and then administered DES. The rats which receive d intrauterine irradiation did not develop mammary tumors at doses les s than 2.1 Gy and showed a low incidence of tumors at 2.6 Gy. In virgi n rats, the maximum tumor incidence was obtained with 1 Gy. The incide nce of total mammary tumors in the mother rats and lactating rats incr eased in a dose-dependent manner with increasing doses of gamma rays u p to 2.1 Gy. With 0.1-1 Gy, the incidence of adenocarcinoma in the mot her rats was significantly lower than that observed in the lactating r ats. 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 gamma-ray doses. These findings suggest that the suscep tibility of the mammary glands to radiation depends upon the different iation at the time of exposure. (C) 1996 by Radiation Research Society