Breast tumor incidence in rats after partial mammary resection

The use of subcutaneous mastectomy in women at high risk for breast cancer is based on the assumption that surgical reduction of the tissue at risk results in a corresponding reduction in risk. To assess the validity of this assumption in an animal model, we subjected 120 female Sprague-Dawley rats to the mammary carcinogen 7,12-dimethylbenz(alpha)anthracene and a 20% fat diet. Two days later, one mammary ridge was excised in half of the animals (mastectomy group), and a midline incision was made without excision of tissue in the other half (control group). Seventy-nine rats survived dosing and operation and were observed for 77 weeks. After 15 weeks, 17 of 39 controls and seven of 40 animals with mastectomies had histologically confirmed neoplastic tumors of the breast. However, after 77 weeks, 32 of 39 controls and 35 of 40 animals in the mastectomy group had neoplastic tumors. The incidence of carcinogen-induced mammary neoplasms in rats progressed with time; whereas surgical reduction of breast tissue was followed by early reduction in neoplasias, the difference was not maintained.