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

Totol-body exposure of 40-day-old female SpragueDawley rats tc 400 r of 250-Up x rays delivered in a single dose was followed by a 79% tumor incidence in rals with one or more neoplasms of the breast at the end of 10 months postexposure. The 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. These results were interpreted to mean that total-body radiation or exposure of all of the animal except the ovaries ormore » the head induces neoplasia cf the breast. The presence of functioning ovary is required for the maximum neoplastic response of the breast to radiation. Reasons were presented for believing that 400 r of total-body radiation both hastens the onset of neoplasia of the breast and induces a higher incidence of neoplasia cf the breast than would be expected to occur if the nonexposed rats were allowed to live out their life span. The histological types of neoplasms of the breast were, in decreasing order of occurrence, adenofibroma, aderocarcinoma, fibroadenomaa, and fibrosarcoma. When multlple neoplasms were found in the same animal, the neoplasms were usually of the same type. Although the adenecarcinomas were noted to appear somewhat earlier and the adenofibromas somewhat later, no further correlation cf histological type with any radiation, shielding, or surgical treatment was found. The relatively short period cf 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 stadies of neoplasia induction by total-body radiation. (auth)« less