Effects of radiation type and dose and the role of glucocorticoids, gonadectomy, and thyroidectomy in mammary tumor induction in mammotropin-secreting pituitary tumor-grafted rats.

Three experiments on the induction of mammary neoplasms by total-body 11- to 100-rad neutron or 50- to 500-rad gamma-radiation of female Fischer or W/Fu rats are reported. Grafts of mammotropin-secreting pituitary tumor were used to elevate mammotropic hormone levels. The results (a) confirm and extend previous reports that neutrons are more efficient in carcinoma induction than are gamma-rays (the neutron relative biological effectiveness for first carcinomas was 3.68) and (b) demonstrate that the potentiation of carcinoma induction by adrenalectomy is reversed by glucocorticoid replacement. Statistical analysis of the data by procedures that take into account time at risk as well as tumor frequency indicates that multiple mammary tumors do not occur independently (i.e., a first mammary neoplasm significantly increases the probability of development of another neoplasm). The statistical procedure used in this analysis is presented.