Bilateral oophorectomy and rate of colorectal cancer: A prospective cohort study.

Worldwide, colorectal cancer is the second most common cancer and third cause of cancer death in women. Estrogen exposure has been inversely associated with colorectal cancer. Oophorectomy reduces circulating estrogen, but the effect on colorectal cancer remains uncertain. The aim of this study was to examine the association between unilateral and bilateral oophorectomy and subsequent risk of colorectal cancer, and whether this association varied by menopausal status at time of oophorectomy, use of hormone replacement therapy (HRT) at baseline, hysterectomy and baseline body mass index (BMI). The study included 25 698 female nurses (aged ≥45 years) participating in the Danish Nurse Cohort. Nurses were followed from baseline until date of colorectal cancer, death, emigration or end of follow-up at December 31, 2018, whichever came first. We examined the association between oophorectomy and colorectal cancer (all ages and stratified by menopausal status). The potential modifying effects of hysterectomy, HRT use at baseline and BMI were investigated. During 542 140 person-years of follow-up, 863 (3.4%) nurses were diagnosed with colorectal cancer. Bilateral oophorectomy was associated with a 79% increased colorectal cancer rate, adjusted rate ratio (aRR) (95% confidence interval [CI]): 1.79 (1.33-2.42). Effect estimates following unilateral oophorectomy also showed higher rate of colorectal cancer, although less pronounced and nonstatistically significant (aRR) (95% CI): 1.25 (0.86-1.82). Similar results were seen when stratifying by menopausal status. The association was not modified by baseline HRT use, hysterectomy or BMI. Oophorectomy was associated with increased rate of colorectal cancer, with highest rates among women with bilateral oophorectomy.