The Risks for Ovarian, Endometrial, Breast, Colorectal, and Other Cancers in Women With Newly Diagnosed Endometriosis or Adenomyosis: A Population-Based Study.

Objective: Recent studies report a link between endometriosis and ovarian cancer (OC). Using a population-based cohort study to confirm the association between endometriosis and cancer is desirable. We thus examined the magnitude of the risks of OC, endometrial cancer (EC), breast cancer, colorectal cancer (CRC), and other cancers in women with newly diagnosed endometriosis or adenomyosis (internal endometriosis).

Methods/materials: Women older than 20 years with claims data between 2003 and 2005 were identified from the Longitudinal Health Insurance Dataset containing 1 million individuals randomly sampled from the National Health Insurance Research Database. Those with preexisting malignancies, hysterectomy, or oophorectomy were excluded. The endometriosis cohort (n = 2266, including 768 cases of pure adenomyosis) and comparison cohort (n = 9064), formed by 1:4 matching, were followed up until incidence cancer, dropout, or December 31, 2008. Outcome measures included cancer incidence and adjusted hazard ratio by Cox model adjusted for age group, comorbidities, and endometriosis medication use.

Results: With 9842 person-years of follow-up in endometriosis cohort and 36,274 person-years of follow-up in comparison cohort, there were increased risks of all cancers (adjusted hazard ratio, 1.8; 95% confidence interval, 1.4-2.4), OC (4.56, 1.72-12.11), and EC (4.05, 1.20-13.66). The ovarian endometriosis group was associated with increased risk of subsequent OC (4.37, 1.07-17.83). The adenomyosis group was strongly associated with both OC (5.50, 1.95-15.50) and EC (5.13, 1.36-19.40). Increased risk of subsequent CRC was observed in women with adenomyosis with coexistent endometriosis at other sites (13.04, 2.21-77.04). However, no statistically significant increased risk of breast or other cancers was observed.

Conclusions: Having limitations such as lacking of parity information which may affect the magnitude of risk estimates, this study demonstrates that ovarian endometriosis has a 4-fold increased risk of OC. Adenomyosis may associate with a 4- to 5-fold increased risk of OC and EC, and unexpectedly, a 13-fold increased risk of CRC.