Localized lymphoid hyperplasia of the rectum resembling polypoid mucosa-associated lymphoid tissue lymphoma: a report of three cases.

Histologically, benign lymphoid hyperplasia of the rectum is usually characterized by large lymphoid follicles with active germinal centers and by a narrow surrounding mantle zone and marginal zone (MZ). We report here three cases of benign lymphoid hyperplasia of the rectum associated with prominent marginal zone hyperplasia, which caused serious difficulty in the differential diagnosis from the polypoid type of mucosa-associated lymphoid tissue (MALT) lymphoma. Colonoscopy demonstrated small sessile polyps in all three cases. Histologically, the lesions were characterized by a hyperplastic germinal center and expanded MZs. The expanded MZs contained numerous monocytoid B-cells (MBC) and scattered large transformed B-cells. Initially, combined colonoscopic and histological findings strongly supported a diagnosis of polypoid MALT-type lymphoma of the rectum. However, there were neither colonized lymphoid follicles nor lymphoepithelial lesions in any of the three lesions. MBCs and large transformed B-lymphocytes were CD43- and bcl-2-. Moreover, immunohistochemical and genotypic studies proved the polytypic nature of the B-lymphocytes in all three lesions. The present cases indicated that benign lymphoid hyperplasia of the rectum should be included in the differential diagnosis for polypoid MALT-type lymphoma of the rectum.