Deep infiltrating endometriosis with mucinous metaplasia of mullerian origin

Background: Endometriosis is a common gynecological condition that often presents with pelvic pain and infertility, but in rare cases may masquerade as a gastrointestinal or gynecologic malignancy. Case: A 49-year-old G0 presented with abdominal pain, fevers, elevated tumor markers, and a large adnexal mass concerning for malignancy. Intra-operatively, mucinous lesions diffusely involved abdominopelvic structures. Intra-operative frozen section (IOFS) revealed invasive cancer and full cytoreductive surgery was completed. Final pathology was downgraded to atypical cystic endometriosis with mucinous metaplasia. Conclusion: Endometriosis, a nonmalignant condition, can present as pelvic masses associated with elevated tumor markers. The case presented here depicts a confounding preoperative and intraoperative picture where endometriosis was falsely identified as malignancy. Endometriosis should always remain a part of the differential diagnosis in a premenopausal patient with presumed gastrointestinal or gynecologic malignancies.