Risk of breast cancer among women with benign ovarian tumors: a Danish nationwide cohort study.

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To assess the association between benign ovarian tumors and subsequent risk of breast cancer, and to examine this association according to type of benign ovarian tumors. This nationwide cohort study comprised all Danish women diagnosed with a benign ovarian tumor during 1978-2016 (n = 158,221) identified through the Danish National Patient Register. The cohort was linked to the Danish Cancer Registry to identify all cases of breast cancer, and standardized incidence ratios (SIR) and 95% confidence intervals (CI) were calculated. Overall, women with a benign ovarian tumor were at significantly increased risk of breast cancer. The risk was confined to women with a solid ovarian tumor (SIR 1.09; 95% CI 1.05-1.13), particularly in women ≥ 50years at benign tumor diagnosis (SIR 1.19; 95% CI 1.12-1.26). The risk remained increased up to 20years or more after the diagnosis of a solid ovarian tumor (SIR 1.11; 95% CI 1.04-1.18), and women with a solid tumor were at increased risk of ductal, lobular, and other types of breast cancer, although most consistent for the lobular subtype. For cystic tumors, this association was confined to ductal breast cancer in women with the tumor diagnosed at age ≥ 50years. Women with a benign ovarian tumor were at increased risk of breast cancer. This association was largely confined to women with a solid ovarian tumor, and the excess risk was present 20years or more after the ovarian tumor diagnosis. The underlying mechanism is unknown and should be investigated further.