Colonization of Intestinal Endometriosis by Benign Colonic Mucosa: A Pattern Potentially Misdiagnosed as Invasive Mucinous Carcinoma

Endometriosis is well known for creating diagnostic pitfalls for pathologists. It may produce masses mimicking neoplasms or cause diagnostic quandaries, particularly when the patient age, location, and/or epithelial appearance are atypical. This study reports a patient with endometriosis causing rectal bleeding and involving the cecum. It produced a mass clinically considered appendiceal. The endometriosis was focally lined by intestinal epithelium including Paneth cells. In the deep endometriotic glands embedded within intestinal wall, direct fusion of the intestinal and the endometrial epithelium—the benign intestinal epithelium apparently colonizing the endometriotic foci—was found. The mass effect, plus deep-seated intestinal epithelium, closely mimicked invasive well-differentiated mucinous carcinoma. This is yet another peculiar presentation of endometriosis with potential for misinterpretation as a more serious condition, specifically well-differentiated mucinous carcinoma of the cecum or appendix.