Prevention of age-related spontaneous mammary tumors in outbred rats by late ovariectomy

Background: Breast cancer prevention trials have shown that the antiestrogen tamoxifen inhibits development of estrogen receptor (ER)-positive tumors. In Sprague–Dawley rats, removal of ovarian function in young animals can reduce the incidence of spontaneous age-dependent mammary tumors. However, it is not known whether removal of ovaries late in life, before middle age onset, can still prevent mammary tumor development. Methods: In this study we used Hsd:Sprague–Dawley® SD® (Hsd) rats to determine the effect of late ovariectomy on mammary tumor development. Intact, sham-ovariectomized and ovariectomized rats were followed until 110 weeks of age, or over their life span. In some experiments, palpable tumors were surgically removed upon presentation. Results: Removal of ovaries before middle age onset (∼5–7 months) inhibited development of spontaneous mammary tumors by 95%. Only one mammary tumor was observed in 19 late ovariectomized animals while 47 total tumors developed in 42 non-ovariectomized animals. Tumor incidence was reduced from 73.8 to 5.3% (relative risk = 0.05, 95% CI = 0.0072–0.354). The frequency of mammary carcinomas in non-ovariectomized virgin female rats was one in eight rats. Spontaneous rat carcinomas expressed ER and other biomarkers, such as cyclin D1. When palpable tumors were removed by surgical excision, tumor multiplicity increased from 0.76 to 1.61 tumors per rat. Surprisingly, ovariectomy increased the 110-week survival rate and maximum life span of Hsd rats. Conclusion: Late ovariectomy prevents spontaneous mammary tumor development in Hsd rats. This animal model may be useful for evaluating novel interventions in breast cancer prevention.