It is generally agreed that SSPs have extension of serrations to lower levels of the crypt and extension of the proliferating cells to higher levels of the crypt rather than being limited to the upper crypt as seen in HPs. The recognition of SSPs as an important cancer precursor is now well established and there is a growing acceptance of the WHO classification of serrated polyps. This review will focus on the detection, eradication and prevention of SSPs. Although TSAs are also thought to have malignant potential they are much less common than SSPs. It is believed that there is a “serrated” pathway to CRC that has a characteristic histologic and molecular progression from a subset of HPs to SSPs to SSPs with cytologic dysplasia (SSP-CD) to serrated CRCs ( Figure 1) and that this pathway is more common in the proximal than distal colon. The WHO suggests the terms SSP and sessile serrated adenoma may be used interchangeably we will use SSP in this review. In an attempt to standardize the terminology, the World Health Organization (WHO) updated their classification of serrated polyps in 2010 into 3 categories: 1) hyperplastic polyps 2) sessile serrated polyps (SSP) with or without cytologic dysplasia (CD) and 3) traditional serrated adenomas. Variability in both terminology and definitions has hindered our understanding of this field. Suffice it to say that the pathologic definitions and terminology used to describe what we now term SSPs has evolved substantially over the last 20 years a sampling of terms that have been used include giant or large hyperplastic polyp, epithelial polyp, mixed hyperplastic/adenomatous polyp, serrated lesion, serrated polyp and serrated adenoma. It is now believed that specific subsets of serrated polyps account for over 30% of colorectal cancers (CRCs). The identification of this lesion, now called a traditional serrated adenoma (TSA), helped lead to a careful re-evaluation of the entire class of serrated colonic polyps by Torlakovic et al. In the 1990s, Longacre and Fenoglo-Preiser et al coined the term “serrated adenoma” to describe a class of polyps that “exhibited the architectural but not the cytologic features of a hyperplastic polyp”. Prior to the 1990s, essentially all serrated polyps were called hyperplastic polyps (HP) and it was thought these lesions had no potential for malignancy. Within the last 2 decades, our knowledge of colorectal SSPs has transformed dramatically and it has led to marked changes in the terminology used to describe the spectrum of serrated polyps. These lesions should be on the endoscopists’ most wanted list. There is no wonder that there is a need to understand these lesions well, learn how best to prevent the colonic mucosa from going down this errant path or, if that fails, to detect these deviants and eradicate them from colonic society. As will be described, these lesions have multiple aliases (serrated adenoma, serrated polyp or serrated lesion among others), they hang out in a bad neighborhood (the poorly prepped right colon), they hide behind a mask of mucus, they are difficult for witnesses (pathologists) to identify, they are difficult for police (endoscopists) to find, they are difficult to permanently remove from society (high incomplete resection rate), they can be impulsive (progress rapidly to CRC) and enforcers (gastroenterologists) don’t know how best to control them (uncertain surveillance recommendations). The sessile serrated polyp (SSP), also known as sessile serrated adenoma, is the evil twin among the colorectal cancer precursors.
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