Rectal mucosal endometriosis primarily misinterpreted as adenocarcinoma: a case report and review of literature.

Endometriosis involving intestinal mucosa is relatively uncommon. It poses a diagnostic challenge for clinicians and pathologists. We herein report a case of colonoscopic specimen revealing rectal mucosal endometriosis. A 39-year-old woman complained of red rectal bleeding and intermittent abdominal pain. Colonoscopic examination showed a rectal mass with ulceration and circum wall involvement. Biopsy was processed in the suspicious of carcinoma. Morphologically, irregular glands replaced residual colorectal ones, displayed mucin depletion, nuclear stratification and subtile subnuclear vacuoles. The stroma was full of spindle cells with abundant pink cytoplasm and unclear boundary. Due to subjectively interpreting as dysplastic glands in desmoplastic setting, primary rectal adenocarcinoma was firstly raised. Immunohistochemically, CK7, ER and CD10 identified the essence of ectopic endometrium. CK20 and CDX2 highlighted residual glands. In case of misdiagnosis, any pathologists should be aware of intestinal endometriosis for each female's colorectal biopsy, especially for that morphology not typical for primary adenocarcinoma or endometriosis. Reading slides carefully combined with a panel of immunomarkers would solve the pitfall.