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Sessile Serrated Polyps - Cory A. Roberts, M.D.

 

SESSILE SERRATED POLYPS by Cory A. Roberts, M.D.

It is difficult to find a more rapidly evolving and confusing topic than that of serrated polyps. This topic has the unenviable position of being in such flux that it is contentious and confounding for both pathologists and gastroenterologists. Not only do we struggle to recognize the morphologic differences and attempt to understand the subsequent clinical significance, we also disagree on the correct nomenclature once we can reliably distinguish the various serrated polyps. Traditionally, we have had a small group from which we could choose a name to apply to any particular polyp. The basic groups have been adenomas, hyperplastic polyps, and the relatively uncommon serrated adenoma (note the absence of the word "sessile"). The adenomatous polyps (AP) of course have low grade dysplasia by definition and are readily identified and agreed upon with little struggle. Hyperplastic polyps (HP) were always considered devoid of dysplasia, lacking the typical adenomatous cytology and featuring the classic serrated glandular architecture with prominent goblet cells. HPs were frequently small, left sided, and generally considered nonneoplastic. We did recognize that, on occasion, polyps with much of the appearance of HP were larger, right sided and sometimes given the term "atypical HP", or "giant HP". Lastly, a polyp termed "serrated adenoma" (SA) was considered to combine the serrated low-power architecture of a HP with the adenomatous cytology of an AP.

We now know that there is a group of polyps with a serrated architecture which have distinct morphologic and molecular differences from traditional HP although they share the general low-power serrated glandular lumina. These polyps are generally termed "sessile serrated adenomas" or "sessile serrated polyp" (SSP).

SSP were first described in 1996 by Torlakovic and Snover (1) although the idea did not immediately take hold. However, in 2003 they published more of their findings and theories (this time in the pathology literature) and the world of polyps has not been the same(2).

The morphologic differences between traditional HP and SSP are many, making them quite recognizable if one is armed with the knowledge of their existence. SSP tend to have dilated basal crypts, unlike HP which tend to have long, narrow crypt bases. They also often show more prominent serrations and serrations extending deeper in the crypts relative to HP. In addition to this, the bases of the crypts in SSA are often lined by goblet cells or gastric mucinous cells. HP, in contrast, lacks both and the basal crypt area often features an undifferentiated lining cell. Perhaps the most recognizable feature of SSP is the flattening of the base of the crypt against the muscularis mucosae, often resembling the look of the letter L (or backwards L) or an upside down T.

The SSP generally lack the typical features of cytologic dysplasia that we are comfortable with in adenomatous polyps, yet they carry a risk of malignancy which may be significant. They do not appear to follow the usual dysplasia to carcinoma sequence via the adenomatous polyposis coli (APC) gene of most colorectal carcinomas. Rather, they appear to follow a microsatellite instability pathway (MSI) that results in the loss of expression of many of the mismatch repair genes, demonstrated by Immunohistochemistry. These are all important points, as these polyps can occur anywhere in the colon and are not restricted to the right colon. Moreover, they are not all large, as many are much less than 10 mm.

While there is no uniformly agreed upon course of action, most agree that these polyps ought to be completely excised as they do carry a risk for malignancy and may even grow and progress more quickly than typical AP. Endoscopic follow-up would also seem reasonable, but again, there is not a widely agreed upon interval. It is difficult to imagine that we will know many of the answers to these questions in a short time, as we need to reproducibly recognize these lesions and have extended clinical follow-up to determine the natural course of these polyps. In the meantime, we need to be clear in our terminology and provide reports with the most up-to-date literature as guiance when offering this diagnosis to avoid any (more) confusion.

References:
1. Torlakovic E, Snover DC. Serrated adenomatous polyposis in humans. Gastroenterology. 1996 Mar; 110(3):748-755.
2. Torlakovic E, Skovland E, Snover DC, et al. Morphologic reappraisal of serrated colorectal polyps. Am J Surg Pathol. 2003 Jan; 27(1):6581.
3. Goldstein N. Small Colonic Microsatellite Unstable Adenocarcinomas and High-Grade Epithelial Dysplasia in Sessile Serrated Adenoma Polypectomy Specimens. Am J Clin Pathol. 2006; 125:132-145.
4. Sheridan TB, Fenton H, et al. Sessile Serrated Adenomas With Low-and High-Grade Dysplasia and Early Carcinomas. Am J Clin Pathol. 2006; 126:564-571.
5. Cunningham KS, Riddell, RH. Serrated mucosal lesions of the colorec tum. Curr Opin Gastroenterol. 2006 Jan; 22(1):48-53.