Purpose: Given the relatively high incidence of both endometriosis and breast cancer, investigating the potential connection between these gynecological diseases is of substantial clinical significance. However, there is no clear consensus in the literature on the extent to which the risk of breast cancer is increased in patients with endometriosis. Therefore, we conducted a large-scale observational study investigating the association between endometriosis and breast cancer risk.

Methods: This study included women aged ≥ 18 years with an initial endometriosis diagnosis from one of 315 office-based gynecologists in Germany between January 2005 and December 2021. Non-endometriosis patients were matched 1:1 to patients with endometriosis based on age, index year, average yearly consultation frequency, and predefined co-diagnoses within 12 months before or on the index date, including obesity and benign breast disorders. The association between endometriosis and the 10-year incidence of breast cancer was studied using Kaplan-Meier curves and log-rank tests. Finally, a univariable Cox regression analysis was conducted to assess the association between endometriosis and breast cancer.

Results: Over a follow-up period of up to 10 years, no significant difference was observed between the endometriosis (2.4%) and the matched non-endometriosis group (2.5%) with regard to breast cancer diagnoses. Furthermore, the regression analysis revealed no significant association between endometriosis and subsequent breast cancer.

Conclusion: In summary, our comprehensive 10-year study involving a substantial sample of women indicates that endometriosis is not significantly associated with an increased risk of subsequent breast cancer.