A Case of Endometrioid Adenocarcinoma Arising from Rectal Endometriosis

We report a case of endometrioid adenocarcinoma arising from rectal endometriosis. A 70-year-old woman with constipation and lower abdominal pain was diagnosed with a submucosal rectal tumor based on colonoscopy, pelvic computed tomography and magnetic resonance imaging. Following low anterior resection, the tumor measured 5.3×3.4×3.5cm. Sectioning showed brown fluid and a white, partially solid mass within the tumor. Well-differentiated adenocarcinoma contiguous with endometriosis extended from the adventitia into the rectal submucosa, CD10 and cytokeratin (CK) immunostaining was done. Spindle cells surrounding the endometrial glands showed reactivity for CD10, which was the marker for endometrial stromal cells. Tumor cells were positive for CK7 but negative for CK20, making the definitive diagnosis endometrioid adenocarcinoma arising from rectal endometriosis. Malignant metastasis from endometoriosis is extremely rare, and immunohistochemical staining in addition to usual histopathology is critical in confirming the diagnosis of this endometoriosis-associated intestinal tumor.