Bilateral
oophorectomy is often performed during
hysterectomy for benign conditions and can reduce
breast cancer risk by 20%-50% when performed at younger ages. Accuracy of estimating the decrease in
breast cancer risk associated with bilateral
oophorectomy could be affected by common conditions that lead to surgery, such as
uterine fibroids or
endometriosis. The authors examined the potential for confounding by nonmalignant indications for surgery on
breast cancer risk estimates in a population-based case-control study of invasive
breast cancer newly diagnosed in 1992-1995.
Breast cancer cases (N = 4,935) aged 50-79 years were identified from Wisconsin, Massachusetts, and New Hampshire
tumor registries; similarly aged controls (N = 5,111) were selected from driver's license and Medicare lists. Reproductive and medical history was obtained from structured telephone interviews. Odds ratios and 95% confidence intervals were estimated with multivariate logistic regression. Women who underwent bilateral
oophorectomy with
hysterectomy at age ≤ 40 years had significantly reduced odds of
breast cancer (odds ratio = 0.74, 95% confidence interval: 0.60, 0.90) compared with women with intact ovaries and uterus. Effect estimates were virtually unchanged after adjustment for
uterine fibroids or
endometriosis history. Results indicate that
breast cancer risk reductions conferred by bilateral
oophorectomy are not strongly confounded by failure to account for nonmalignant indications for surgery.