Endometriosis is associated with an increased risk of type I ovarian cancer, yet the prognostic relevance of concurrent endometriosis at diagnosis remains unclear. This cohort study evaluated whether endometriosis independently influences survival outcomes in patients with ovarian cancer and investigated the clinicopathologic characteristics of endometriosis-associated ovarian tumors.

This retrospective cohort study included patients treated for primary malignant or borderline ovarian tumors between January 2014 and July 2025 at a tertiary ESGO-accredited German academic center. Concurrent endometriosis was defined by histologic confirmation at primary surgery and classified as endometriosis-correlated ovarian tumors when histopathologic a transitional lesion from endometriosis to tumor was identified, or as endometriosis-incidental ovarian tumors when endometriosis was identified without transitional lesions. An age-matched reference cohort without endometriosis (1:5) was used for comparison. Survival analyses were restricted to patients with epithelial ovarian cancer and performed using Kaplan–Meier estimates and multivariable Cox regression, adjusting for age, histologic subtype, FIGO stage, endometriosis status, and surgical outcomes.

Among 2,164 eligible patients, 176 (8.1%) had histologically confirmed endometriosis, including 22 (1.0%) endometriosis-correlated ovarian tumors and 154 (7.1%) endometriosis-incidental ovarian tumors. Patients with endometriosis were significantly younger and more frequently diagnosed with early-stage disease and endometrioid or clear cell histology compared with patients without endometriosis. Endometriosis-correlated ovarian tumors showed a high prevalence of ovarian endometriosis, the absence of p53 aberrations, and no BRCA1/2 mutations. Median overall survival was not reached for patients with endometriosis-correlated ovarian tumors or endometriosis-incidental ovarian tumors. After multivariable adjustment, neither endometriosis-correlated ovarian tumors nor endometriosis-incidental ovarian tumors was independently associated with overall or disease-free survival. Advanced FIGO stage, older age, and incomplete cytoreduction were independently associated with worse survival.

Endometriosis-associated ovarian tumors show favorable clinicopathologic features but concurrent endometriosis is not an independent prognostic factor for survival in patients with ovarian cancer. Prognosis appears to be driven primarily by tumor biology, stage, and surgical outcome rather than the presence of endometriosis itself.