Studies on Radiation-Induced Mammary Gland Neoplasia in the Rat: I. The Role of the Ovary in the Neoplastic Response of the Breast Tissue to Total- or Partial-Body X-Irradiation

Abstract

Total-body exposure of 40-day-old female Sprague-Dawley rats to 400 r of 250-kvp X-rays delivered in a single dose was followed by a 79% tumor incidence with one or more neoplasms of the breast at 10 months postexposure; incidence of breast neoplasms in nonexposed rats of the same age was 2%. The over-all incidence of nonbreast tissue neoplasms was 2% and distributed in all groups with and without radiation exposure. Exteriorizing and shielding the ovaries, or transplanting nonexposed ovaries into ovariectomized irradiated rats, or shielding the head did not reduce the incidence of breast neoplasms below 69%. Shielding the ovarian area reduced the incidence slightly to 40%. Removal of the ovaries before or after total-body radiation reduced, but did not eliminate, neoplasia of the breast in 19 to 23%. A further reduction to 25% occurred when the day of exposure was delayed to 120 days of age, although this group was studied for only 6 months postexposure. Exposure of the ovaries alone or ovariectomy or sham ovariectomy did not induce neoplasia of the breast. The histo-logical types of neoplasms of the breast were, in decreasing order of occurrence, adenofibroma, adenocarcinoma, fibroadenoma, and fibro-sarcoma. When multiple neoplasms were found in the same animal, the neoplasms were usually of the same type. Adenocarcinomas were noted to appear somewhat earlier and the adenofibromas somewhat later. The relatively short period of time between exposure and the development of a high incidence of breast neoplasms suggests that the young female Sprague-Dawley rat offers many advantages in studies of neoplasia induction by total-body radiation.