Mammary carcinogenesis in different rat strains after irradiation and hormone administration.

Radiation carcinogenesis of the rat mammary gland was investigated with the objective of investigating the combined effect of oestrogen administration and irradiation. Three rat strains, Sprague-Dawley, Wistar WAG/Rij and Brown Norway, with different susceptibilities to the induction of mammary cancer, have been irradiated with X-rays and mono-energetic neutrons. Increased hormone levels were obtained by subcutaneous implantation of pellets with oestradiol-17 beta (E2). The tumour incidence results were corrected for competing risks and were analysed with a continuous failure time distribution. The latency period for the hormone-treated animals is considerably shorter than for animals with normal endocrinological levels. Administration of the hormone results in an appreciable increase in the proportion of rats with malignant tumours. At the level of hormone administration applied in this study, radiation and hormones appear to produce an additive effect. The effect of hormone administration and irradiation for mammary tumourigenesis is equal for hormone administration one week prior to, or 12 weeks after irradiation. The RBE values for induction of mammary carcinomas after irradiation with 0.5 MeV neutrons have a maximum value of 20 and are not strongly dependent on the hormone levels.