Studies on Radiation-Induced Mammary Gland Neoplasia in the Rat: V. Induction by Localized Irradiation

Abstract

These experiments were designed to determine if the breast tissue of Sprague-Dawley rats must be directly irradiated for maximal neoplasia induction. Groups of 30 females were irradiated on the 55th day of age as follows: 400 r total-body irradiation; 400 r entire left half of body; 400 r to chest; 400 r to lower half of body; 400 r with one hind limb shielded; and 400 r through a grid. With half-body exposure and with exposure through the grid, approximately one-half of the total number of neoplasms seen after total-body irradiation was observed, and more than 90% of all breast neoplasms occurred in tissue exposed directly to the beam. The results indicate that direct radiation injury to the breast is necessary for an increased incidence of radiation-induced neoplasia. Radiation-induced "primary" damage in the target organ is necessary for an increased neoplastic incidence, but the primary damage may lie dormant unless an additional secondary mechanism (presumably cyclic ovarian activity) is operative. The implications of the results with regard to a possible somatic mutation mechanism of radiation neoplasia induction are discussed.