Mixed hyperplastic adenomatous polyps--an underdiagnosed entity. Report of a case of adenocarcinoma arising within a mixed hyperplastic adenomatous polyp.

We report a case of colonic adenocarcinoma arising within a polyp with mixed morphology of a hyperplastic polyp and tubular adenoma. Despite the relatively small size of the polyp, two isolated foci of adenocarcinoma in situ were present and tumor islands invaded the submucosa. Isolated areas, morphologically resembling hyperplastic glands, and varying degrees of atypia. Though rare, some hyperplastic polyps may be precursors of adenomas.