Objectives: Opportunistic bilateral oophorectomy (BO) is often considered in patients undergoing benign hysterectomy, but the degree to which BO reduces the incidence of ovarian cancer in this population remains unclear and is important for patient decision-making. Methods: We performed a population-based cohort study of all women (≥20 years) undergoing hysterectomy for benign indications from 1996-2015 in Ontario, Canada, followed to December 2019. Patients with ovarian pathology at hysterectomy, previous breast or gynecologic cancer, or evidence of genetic susceptibility to malignancy were excluded. Ovarian cancers and deaths were ascertained from the Ontario Cancer Registry. Inverse probability of treatment weighted Fine & Gray subdistribution hazard models were used to examine the effect of BO on ovarian cancer incidence and death, while accounting for competing risks and adjusting for demographic factors, gynecologic conditions, and comorbidities. Results: We identified 249,924 patients (BO 61,203 [25%]; ovarian conservation 188,721 [75%]) with median age 45 years (interquartile range [IQR] 40-52). Over median follow-up of 14 years (IQR 9-19), 584 women developed ovarian cancer (0.2%) and 17,052 died (6.8%) from any cause. Baseline characteristics were balanced after IPT weighting (standardized differences <0.1). BO was associated with a decrease in the incidence of ovarian cancer (HR 0.23, 95% CI 0.17-0.32, p<0.001) and ovarian cancer death (HR 0.30, 95% 0.19-0.47, p<0.001). At 20 years follow-up, the weighted cumulative incidence of ovarian cancer was 0.08% and 0.42% in BO and ovarian conservation groups respectively, yielding an absolute risk reduction [ARR] of 0.34% with BO (number needed to treat [NNT] 291). In sensitivity analyses restricted to women ≥50 years at hysterectomy, the ARR was 0.55% (NNT 182). Conclusions: BO results in a small absolute reduction in incident ovarian cancer among women undergoing benign hysterectomy. Population-average risk estimates derived in this study are generalizable, directly address the limitations of previous work, and should be used to counsel patients preoperatively.
