Pregnancy-dependent initiation in tumorigenesis of Wistar rat mammary glands by 60Co-irradiation.

Pregnant Wistar rats received whole body irradiation with 260 cGy gamma-rays at days 7, 14 and 20 of pregnancy and then were treated with diethylstilbestrol (DES) for 1 year. The highest incidence (92.9%) for tumorigenesis of mammary glands was observed in the rats irradiated in late pregnancy. Histological examination showed that tumors were classified as fibroadenoma and adenocarcinoma. To determine the reasons for specific induction of mammary tumors by irradiation in late pregnancy, hormone concentrations in serum and estrogen receptors in mammary glands during pregnancy were measured. Concentrations of estradiol, progesterone, 11-deoxycorticosterone and placental lactogen at day 20 were higher than at days 7 and/or 14, but no difference was observed in the concentrations of prolactin and thyroid-stimulating hormone during pregnancy. The estrogen receptor in mammary glands at day 20 was indicated to have the highest affinity and the highest binding capacity during pregnancy. Normal mammary glands at day 20 were suggested to have more abundant epithelial cells in the mammary lobes than those at days 7 and 14. The data suggest that the critical requirements for the initiation of tumorigenesis by gamma-rays are dependent upon the differentiated state of mammary glands exposed to various hormones, and that the concentration and persistence of the synthetic estrogen (DES) are necessary for the promotion of tumorigenesis of the irradiated mammary glands.