Endometriosis has been described in numerous extra-pelvic sites including the abdominal wall, pleura, pericardium, muscle, nerve  and bronchus . Confusing clinical presentations may result. As one example, within the gynecologic tract co-existence of an ovarian leiomyoma with an endometriotic cyst resulted in appendicitis-like symptoms and urgent laparotomy .
Endometriosis may involve the gastrointestinal tract from the small intestine to the rectosigmoid colon. The latter is the most common site, followed, in order, by the proximal colon, small intestine, the appendix and cecum. Ileocolic intussusception due to cecal endometriosis has been documented .
Microscopic foci of endometriosis are encountered in appendices removed for appendicitis or those included in colectomy specimens, and are clinically silent. Conversely, endometriosis may present as acute or chronic appendicitis, with the latter occasionally causing a mucocele. Other presentations of appendicial endometriosis include a mass lesion, perforation during pregnancy (most commonly in the first two trimesters and when the endometriosis is transmural), and intussusception of the appendix .
Endometriosis can display metaplastic changes. In the ovary 12-68% of endometriosis lesions show metaplasia in the form of ciliated, eosinophilic, hobnail, squamous or mucinous change, with endocervical-type metaplasia encountered more commonly than intestinal-type . Although in a review of 44 cases of endometriosis involving the intestinal tract no examples of intestinal-type metaplasia were found , a recent report documents intestinal epithelium “colonizing” endometriosis of the cecum of a 55 year-old woman who had previously undergone appendectomy .
One other case of dysplastic intestinal-type metaplasia involving appendicial endometriosis has been reported . A 39 year-old woman with severe endometriosis underwent uterine myomectomy as well as appendectomy for an incidental 1.6 cm nodule of the distal appendix. Microscopic examination showed endometriosis extending from the serosa to the mucosa of the appendix. Foci of intestinal-type metaplasia, including Paneth cells, were found within the endometriosis with one area showing cytologic atypia consistant with dysplasia. Regarding histogenesis, the authors favored metaplasia of the endometriosis, as opposed to “colonization” by overlying luminal epithelium, citing the presence of ER-positive mucinous cells, the presence of ciliated mucinous cells (not seen in normal intestinal epithelium), absence of direct communication between endometriotic and native appendiceal glands, and the presence of “transitional epithelium”.
Similarly, we believe our case represents intestinal metaplasia rather than colonization, as: 1) no connection between the overlying native epithelium and the foci of endometriosis was found, 2) multiple foci of direct transition between endometriotic epithelium and intestinal-type epithelium were present, 3) ER-positive mucinous cells at sites of transition from endometriotic to mucinous epithelium were identified, and 4) the known potential of endometrium to undergo intestinal metaplasia [10, 11].
The differential diagnosis in the present case is with low-grade appendiceal mucinous neoplasm (LAMN). LAMN is characterized by invasive intestinal-type epithelium with abundant mucin production, minimal architectural complexity and low-grade cytologic atypia . Two features of the present case rule out LAMN: absence of invasive intestinal epithelium and absence of a primary lesion (adenoma or carcinoma in situ) of the appendicial epithelium. Furthermore, there is clinical support against LAMN as at second-look surgery one year following resection of the perforated appendix there was absence of both recurrent neoplasia and diffuse peritoneal mucin.
The decision to perform peritonectomy and HIPEC was based on an initial pathological diagnosis of invasive cancer. Given the uniqueness of our case it is impossible to know what further treatment, if any, would have been warranted had the correct diagnosis initially been made. We therefore suggest that in any appendectomy specimen with endometriosis and suspicion of neoplasia that the possibility of intestinal metaplasia be considered in the differential diagnosis.