Serrated polyps with "intermediate features" of sessile serrated polyp and microvesicular hyperplastic polyp: a practical approach to the classification of nondysplastic serrated polyps.

Sessile serrated polyps (SSPs) have been implicated in the pathogenesis of proximal colonic carcinomas, but they lack well-defined diagnostic criteria and their features overlap considerably with those of microvesicular hyperplastic polyps (MVHPs). We have noted that morphologic features of SSPs are often present in small, distally located MVHPs, suggesting that these polyps represent points on a continuum, rather than distinct entities. We evaluated the molecular features of diminutive (<1 cm) nondysplastic serrated polyps that met at least 4 of the 7 "SSP-like" morphologic criteria, but occurred throughout the colorectum, and compared them with SSPs and MVHPs. Fifty nondysplastic serrated polyps (6 SSPs, 31 study polyps, and 13 MVHPs) were evaluated for Ki-67, O6-methylguanine methyltransferase, MUC2, and MUC5AC expression, and also their BRAF and KRAS mutational status. The study polyps and SSPs were similar; 52% and 50% expressed MUC5AC, and 87% and 100% harbored BRAF mutations, respectively, compared with 15% and 46% of MVHPs (P < or = 0.05, all comparisons). O6-methylguanine methyltransferase expression in the study polyps (29%) was intermediate between that of SSPs (83%, P=0.02) and MVHPs (15%, P=0.04). We conclude that the pathologic and molecular features of diminutive, distally located nondysplastic serrated polyps are often indistinguishable from proximally located SSPs, although convincing evidence linking the former to appreciable colorectal cancer risk is entirely lacking. Thus, we propose that, at present, the term "sessile serrated polyp" be restricted to large (> or = 1 cm), proximally located polyps with a presumed biologic risk, until prospective data regarding the natural history of small, distal lesions are available.