Effects of X-irradiation, ovariohysterectomy and estradiol-17 beta on incidence, benign/malignant ratio and multiplicity of rat mammary neoplasms--a preliminary report.

An overview is given of the effects of X-irradiation, ovariohysterectomy and estradiol-17 beta administration on mammary tumorigenesis in females of 3 rat strains, viz. the WAG/Rij, BN/BiRij and SD. The 3 rat strains differed significantly in their spontaneous mammary tumor incidence. Female SD rats had the highest incidence (47%) and female BN/BiRij rats the lowest (17%). Female WAG/Rij rats had an intermediate incidence of 29%. The benign/malignant ratio in female WAG/Rij, BN/BiRij and SD rats was 1.0, 2.0 and 7.3, respectively. The average number of mammary gland neoplasms per untreated tumor-bearing female was 1.2 in the WAG/Rij, 1.0 in the BN/BiRij and 1.6 in the SD, whereas the respective maximum numbers were 2, 1 and 5. Ovariohysterectomy almost entirely prevented mammary tumor formation in all 3 rat strains, whereas estrogen treatment enhanced it. In addition, estrogen treatment resulted in an increased number of mammary tumors per tumor-bearing female and changed the benign/malignant ratio into the direction of malignant. X-irradiation increased the mammary tumor incidence in all 3 rat strains, especially of the benign tumors. Estrogen potentiated the effect of irradiation. An effect of irradiation on mammary tumorigenesis was not observed in ovariohysterectomized females of all 3 rat strains.