Answer: Endometioid carcinoma arising in endometriosis

Histology: On the serosal surface of the bowel there is a portion of cystic tumor, which is lined by confluent well-formed glands composed of cells with enlarged nuclei and prominent nucleoli. Surrounding the tumor are multiple foci of endometrial type glands and stroma within the muscularis propria (the intestinal mucosa was histologically unremarkable).

Discussion: Intestinal involvement by endometriosis, although not common, is well documented. In most cases the endometriosis is confined to the serosa or subserosa. Although endometriosis is usually identified in the reproductive age group, it has been reported that 7% of patients with symptomatic intestinal endometriosis were postmenopausal. Endometrioid carcinoma is the most common tumor arising within endometriosis. Mesothelial hyperplasia is often seen in cases of endometriosis and can be misinterpreted as invasive tumor. In florid examples, solid, tubular, and papillary patterns may be seen. The mesothelial cells are often eosinophilic with cytoplasmic vacuoles, mild to moderate nuclear pleomorphism, but do not show the stratified columnar cells and confluent glandular pattern of an endometrioid tumor. Gastrointestinal carcinomas are also included in the differential diagnosis of adenocarcinomas with an endometrioid pattern. Fortunately, immunoperoxidase stains for cytokeratins 7 and 20 can be extremely helpful in differentiating colonic adenocarcinomas from endometrial and ovarian carcinomas. Gynecologic carcinomas will typically show strong positive staining with CK 7 and negative staining with CK 20, while adenocarcinomas of the colon will show the reverse staining pattern. Endometrioid carcinomas can also arise in the ovary or in an ovarian remnant, which can be seen in patients with a history of oophorectomy complicated by extensive periovarian adhesions (often a result of endometriosis). In this case, however, there was no evidence of ovarian stroma in the extensively sampled specimen, making the diagnosis of carcinoma of the ovary less likely.