Susceptibility of lactating rat mammary glands to gamma-ray-irradiation-induced tumorigenesis.

Lactating rats of the Wistar-MS strain were irradiated with 260 cGy of gamma rays 21 days after parturition (day 21). Diethylstilbestrol (DES) pellets were implanted one month after termination of nursing and were allowed to remain for one year. A significantly higher incidence (96.4%) of mammary tumors was observed in these rats irradiated during late lactation than in virgin irradiated animals (30.4%). A control group of lactating animals irradiated during late lactation but not treated with DES was also observed for one year; the final incidence of mammary tumors in this group was 35.3%. The latency period was shortest in the DES-treated group irradiated during late lactation. Histological examination showed that the mammary glands of lactating rats were highly developed, with alveoli filled with milk. Five days after weaning, there was degeneration of alveolar tissue, concomitant with a marked decrease in the concentration of estrogen and prolactin receptors. A considerable amount of epithelial tissue remained in the mammary glands during the process of atrophy. When the rats were irradiated 5 days after weaning, and then were treated with DES for one year, the incidence of mammary tumors was 73.3%, significantly higher than that in virgin irradiated rats. However, this incidence was not significantly different from that in animals irradiated during late lactation. These results suggested that the induction of mammary tumors by gamma irradiation before or after weaning was more dependent upon the stage of differentiation in mammary glands than upon the proliferative activity of epithelial cells, and that DES is essential as a promoter for radiation-induced mammary tumorigenesis.