Answer: Endometriosis

Histology: Bowel mucosa is ulcerated, but an adenomatous component is not seen. Instead, within the musculature of the colon, there are clusters of glands with fairly uniform nuclei set in a background of a spindly stroma. The glands focally have cilia, indicating tubal metaplasia. The bland stroma encircles thin-walled capillaries, characteristic of endometrial stroma. The bowel wall musculature associated with these glands is markedly thickened, and this along with the ulceration simulated a primary colonic adenocarcinoma grossly and clinically.

Discussion: The absence of anaplastic nuclear features, as well as the absence of an adenomatous component, excluded adenocarcinoma. Pancreatic Heterotopias are also characterized by clusters of bland glands within intestinal walls. However, these typically affect the duodenum, and may have collections of acinar or endocrine cells. These would lack the characteristic endometrial stroma and ciliated epithelium seen in the current case.

Endometriosis is one of the great mimickers of metastatic adenocarcinoma, particularly on frozen section. Other mimickers include mesothelial proliferations (adenomatoid tumor and tubulopapillary mesothelial hyperplasia), and herniated, misplaced benign glandular epithelium (cystitis glandularis, adenomyosis of the gallbladder, etc.).