Polycystic ovary syndrome and risk of breast cancer in premenopausal and postmenopausal women: a nationwide population-based cohort study.

Purpose: Although some reproductive and metabolic characteristics of polycystic ovary syndrome (PCOS) are known risk factors for breast cancer, the evidence regarding a potential association between PCOS and breast cancer is scarce. In this population-based cohort study including all 1,719,452 women born in Denmark between 1940 and 1993, we investigated the association between PCOS and breast cancer.

Methods: PCOS diagnoses, cancer diagnoses, covariates, migrations, and vital status were all obtained from national population and health registers. Hazard ratios (HR) and 95% confidence intervals (CI) for breast cancer overall and for histological subtypes separately were calculated based on adjusted cox proportional hazards models.

Results: During a median follow-up of 26 years, 63,078 women were diagnosed with breast cancer. We found an increased risk of breast cancer overall among women with PCOS compared with women without PCOS (HR: 1.21, 95% CI 1.02-1.44). In analyses stratified for menopausal status, the increased risk was restricted to postmenopausal women (HR: 1.63, 95% CI 1.23-2.15). The results for ductal and lobular histological subtypes analyses separately resembled those observed for breast cancer overall.

Conclusion: This is the first study to report an increased risk of breast cancer among women with a history of PCOS. The increased risk was seemingly confined to postmenopausal women. Our results therefore contribute to an increased knowledge of the etiology of breast cancer, but our findings should be further confirmed in other large cohort studies with an appropriately long follow-up period.