The Effects of Fractionated Gamma Irradiation on Induction of Mammary Carcinoma in Normal and Estrogen-Treated Rats

Abstract Bartstra, R. W., Bentvelzen, P. A. J., Zoetelief, J., Mulder, A. H., Broerse, J. J. and van Bekkum, D. W. The Effects of Fractionated Gamma Irradiation on Induction of Mammary Carcinoma in Normal and Estrogen-Treated Rats. The effects of dose fractionation on induction of mammary carcinoma were studied in normal and estrogen-treated female rats of the inbred WAG/Rij strain. Groups of 40 animals received total-body doses of 1 or 2 Gy of 137Cs γ radiation, administered in fractions of 2.5, 10 or 40 mGy with intervals of 12 h, or in fractions of 10 mGy with intervals of 2, 5 or 24 h. The irradiations were started at the age of 8 weeks. Estrogen treatment was accomplished by implantation of a pellet containing estrogen at the age of 6 weeks. All mammary tumors were resected and classified histologically as carcinoma or fibroadenoma. The age-specific incidence of mammary carcinoma was compared with that in control groups of unirradiated normal or estrogen-treated rats and was expressed as excess norma...