Mimicry in the Colon: Endometriosis-Associated Carcinoma in a Post-hysterectomy Patient.

Endometriosis is a benign yet chronic gynecologic condition that, in rare cases, may undergo malignant transformation. Although endometriosis-associated malignancies most commonly arise in the ovary, extragonadal sites may also be affected. Endometriosis-associated intestinal tumors (EAITs) are exceptionally rare and often difficult to diagnose. We report the case of a 73-year-old woman with a remote history of total hysterectomy and bilateral salpingo-oophorectomy for endometriosis who presented with rectal bleeding and constipation. Imaging and colonoscopy revealed a nearly obstructing mass in the sigmoid colon, initially suspected to be primary colorectal carcinoma. Biopsy demonstrated a poorly differentiated carcinoma with squamous features. Immunohistochemical analysis supported a Müllerian origin, an unexpected finding given the patient's prior gynecologic surgery. Sigmoidectomy subsequently revealed a high-grade endometrioid carcinoma with extensive squamous differentiation arising from intestinal endometriosis. This rare case highlights that endometrioid carcinoma can develop from intestinal endometriosis decades after definitive gynecologic surgery. Accurate diagnosis requires the integration of clinical history, histopathological findings, and immunohistochemical results to differentiate this entity from primary colorectal carcinoma. Increased awareness of EAITs is essential to ensure accurate diagnosis and appropriate management.