Family adenomatous polyposis come across dome type adenocarcinoma: a case report and literature review.

Dome-type carcinoma (DC), also referred as Gut-associated lymphoid tissue (GALT) carcinoma, is a rare variant of colorectal adenocarcinoma which has been seldomly reported up to now. We report a case of a DC lesion developed in a 33-year-old male diagnosed with family adenomatous polyposis (FAP). A 1.5 × 1.5 cm well-demarcated lesion exhibited a 0-Is + IIc figure was detected near the anastomotic stoma during regular colonoscopic polypectomy. Surgical specimen showed well-differentiated adenocarcinoma consisted of dilated cystic glands and the lymphoid stroma with reactive germinal centers exhibited a destructive manner of infiltration into SM2 level. The immunohistochemical findings revealed MUC1 positive but MUC2 negative of the carcinomas epithelial which retained all the 4 mismatch repair proteins (MMRs) (MLH1, PMS2, MSH2, and MSH6) and was negative for EBV-encoded small RNA-1 (EBER). Considered a rare category of colorectal adenocarcinoma, more cases will help uncover the nature of GALT/dome-type carcinoma. Clinicians and pathologists should be aware of recognizing this special type of carcinoma and making necessary differential diagnostics.