Endometriosis and risks for ovarian, endometrial and breast cancers: A nationwide cohort study

A growing body of evidence suggests that endometriosis increases the risk for ovarian cancer, but it is less well studied whether the excess risk is confined to certain histotypes. Furthermore, it is not fully resolved if endometriosis is associated with endometrial- and breast cancer. The aim was to study overall- and histotype-specific risks for these hormone-dependent cancers in women with endometriosis.

In the Danish National Patient Register, we identified 45,790 women with a clinical diagnosis of endometriosis during 1977-2012. We linked the cohort to the Danish Cancer Register and calculated standardized incidence ratios (SIRs) with corresponding 95% confidence intervals (CIs).

Endometriosis was associated with increased risks for ovarian cancer (SIR 1.34; 95% CI: 1.16-1.55), due primarily to endometrioid (SIR 1.64; 95% CI: 1.09-2.37) and clear-cell types (SIR 3.64; 95% CI: 2.36-5.38). An excess risk was also observed for endometrial cancer (SIR 1.43; 95% CI: 1.13-1.79), primarily of type 1 (SIR 1.54; 95% CI: 1.20-1.96); and the risk for breast cancer was increased among women aged ≥50years at first diagnosis of endometriosis (SIR 1.27; 95% CI: 1.12-1.42).

The results corroborate previous findings of increased risks for endometrioid and clear-cell ovarian cancer in women with endometriosis. As the first cohort study to date, we observed a significantly increased risk for endometrial cancer in women with a diagnosis of endometriosis. The increased breast cancer risk among women with endometriosis diagnosed at ≥50years of age should be studied further.