Flat, hyperplastic, and sessile serrated polyps

Increasing evidence indicates that colonoscopy offers less reliable protection against proximal versus distal colorectal cancer. Two key factors may explain the occurrence of postcolonoscopy (ie, interval) cancers in the proximal colon, namely endoscopist-dependent factors and biological characteristics of precursor lesions resulting in a more rapid progression. There is increasing evidence that nonpolypoid lesions, of adenomatous or serrated type, are major contributors to interval cancers through endoscopist-dependent factors, as these lesions are preferentially located in the proximal colon and more likely to be overlooked and/or incompletely resected, in particular when predisposing factors (ie, suboptimal bowel preparation or insufficient training) are involved. However, emerging data now indicate that a subset of nonpolypoid adenomas might also display distinct molecular features that may impact on growth as compared with their polypoid counterparts. In this review, we summarize the current literature on classification and biological significance of nonpolypoid colorectal lesions, with special attention to their endoscopic appearance and potential implications for training.