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Case Diagnosis and Discussion


Mucosal endometriosis


The histologic and IHC features of this polyp are consistent with endometriosis involving the mucosal surface of the sigmoid colon. No malignancy was present.

Endometriosis is defined as the presence of endometrial tissue outside of the uterus. Up to 10 percent of women are affected by endometriosis, particularly women of childbearing age. The clinical presentation is most commonly abdominal pain.  It is treated with hormones or in severe cases with surgical resection.

The classic histologic features are endometrial glands (typically inactive or proliferative type), endometrial stroma surrounding the glands, and hemosiderin deposition or hemosiderin-laden macrophages.  Typically at least two of these features would need to be present to make the diagnosis of endometriosis.

The glands of endometriosis, like normal endometrial glands, are usually positive for CK7, PAX8, and ER/PR. They are negative for CK20 and CDX2. The stroma is usually CD10 and ER positive, while negative for other spindle cell markers such as S100 and CD117.

Endometriosis most commonly involves the ovaries, uterine ligaments, and other structures within the pelvis, including the rectovaginal septum and pelvic peritoneum. The GI tract is involved in approximately 3-37 percent of women that have endometriosis. The most common sites are the sigmoid and rectum, followed by the ileum, appendix, and cecum. Typically endometriotic lesions are limited to the serosa of the bowel. Mucosal involvement is estimated to occur in up to 30 percent of patients that have GI tract involvement. Clinical symptoms include pain and mild bleeding, however there have been reports of more severe (rare) complications such as severe GI bleeding, bowel obstruction, or perforation.

Mucosal involvement can cause a mass lesion to form, and thus on imaging or colonoscopy could be mistaken for colitis, IBD, or even malignancy.

The histologic appearance of endometriosis involving the GI mucosa is similar to endometriosis elsewhere.  As in normal endometrium, the glands can undergo metaplasia (tubal/ciliated metaplasia being the most common), and in very rare cases even malignant transformation.

The surrounding colonic mucosa can also exhibit changes secondary to the adjacent endometriosis, including surface erosion or ulceration, crypt architecture distortion or abscess formation, and prolapse-like fibromuscular hyperplasia. Indeed, the appearance could mimic that of IBD in some cases.


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Jiang, W, et al.  Endometriosis involving the mucosa of the intestinal tract: a clinicopathologic study of 15 cases.  Modern Pathology. (2013) 26; 1270-1278.

Tipps AM and Weidner N. Colonization of intestinal endometriosis by benign colonic mucosa: a pattern potentially misdiagnosed as invasive mucinous carcinoma. International Journal of Surgical Pathology.  (2011) 2; 259-62.

Yantiss RK, et al. Neoplastic and preneoplastic changes in gastrointestinal endometriosis: a study of 17 cases. American Journal of Surgical Pathology. 2000; 24: 513-524.

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