Inverted hyperplastic polyposis of the colon.

AIMS To describe and evaluate two apparently unique cases of inverted hyperplastic (metaplastic) polyposis of the colon. METHODS The cases were analysed by standard histopathological, histochemical, and immunohistochemical techniques and the findings compared with those of regular hyperplastic polyps of the colorectum. RESULTS Both patients were middle-aged men with concurrent adenocarcinoma of the proximal large intestine. The inverted polyps numbered 18 and 12, measured between 0.4 and 2.5 cm in diameter, and all were present in the proximal ascending colon. The polyps had characteristic macroscopic features: they were positioned on the apex of mucosal folds and demonstrated surface pitting and mucus hypersecretion. Histologically, inversion and misplacement of hyperplastic epithelium was related to lymphoglandular complexes. The polyps showed all the histochemical and immunohistochemical features of regular hyperplastic polyps. CONCLUSIONS Inverted hyperplastic polyps are an unusual but distinctive polyp of the proximal colon, may be multiple, and share the phenotypic changes of regular hyperplastic polyps. The pathogenesis of epithelial inversion probably relates to misplacement of epithelium through anatomical defects in the muscularis mucosae due to mechanical forces. The polyps may mimic both adenomas and carcinomas. The neoplastic potential of inverted hyperplastic polyposis is likely to be very low: one polyp only showed adenomatous change.