PURPOSE: The importance of benign ovarian tumors as precursors or risk markers for ovarian cancer is not fully understood. Studies on the association between benign ovarian tumors and ovarian cancer have provided inconclusive results. We examined the overall and histological type-specific risk of ovarian cancer among 158,221 Danish women diagnosed with a benign ovarian tumor during 1978-2016.

METHODS: The study cohort was linked to the Danish Cancer Register to identify all cases of epithelial ovarian cancer, and standardized incidence ratios (SIR) with 95% confidence intervals (CI) were calculated.

RESULTS: After excluding the first year of follow-up, women with benign ovarian tumors did not have an increased risk for overall epithelial ovarian cancer (SIR 1.02; 95% CI 0.93-1.11), as compared with women in the general population. However, we found an increased risk for mucinous ovarian cancer (SIR 2.06; 95% CI 1.67-2.52); both solid and cystic benign ovarian tumors were associated with an increased risk. The risk for mucinous ovarian cancer was increased irrespective of the age at benign ovarian tumors diagnosis and persisted for up to 20 years after the benign ovarian tumor diagnosis. No clear associations for other histological types of ovarian cancer were observed, except for an increased risk for serous ovarian cancer among women diagnosed with benign ovarian tumors at an young age.

CONCLUSIONS: Benign ovarian tumors may be associated with long-term increased risk for mucinous ovarian cancer.