Intestinal metaplasia of appendiceal endometriosis is not uncommon and may mimic appendiceal mucinous neoplasm.

Endometriosis of the appendix can be an incidental finding or a cause of appendicitis, intussusception, perforation or retention mucocele. Intestinal metaplasia of appendiceal endometriosis may occur, which can lead to a misdiagnosis of low-grade appendiceal mucinous neoplasm. On a retrospective search of the pathology database from 2001 to 2015, we identified 78 appendiceal endometriosis cases and intestinal metaplasia was present in 10/78 (13%) cases. In most of the cases (90%), the foci of intestinal metaplasia were mainly localized close to the mucosa. Intestinal and endometrial hybrid glands were present in 9/10 (90%) cases. These cases were often associated with marked appendiceal distortion, luminal obliteration and mass formation, causing concern for a mucinous neoplasm clinically and pathologically. Our findings indicate that intestinal metaplasia in appendiceal endometriosis is not an uncommon phenomenon, which can be mistaken for a mucinous neoplasm. Endometriosis should be kept in mind when a diagnosis of appendiceal mucinous neoplasm is made, especially in a young woman with a clinical history of endometriosis.