COMMON, BUT PREVALENCE IS NOT CLEARLY ESTABLISHED
The histologic criteria for sessile serrated polyps and traditional serrated adenomas have been elucidated,4–7 but the epidemiology of these serrated polyps is not clear. Small studies have shown that sessile serrated polyps account for 2% to 9% of all polyps removed at colonoscopy8–10; however, larger studies are needed to determine the prevalence because detection by an endoscopist and pathologic diagnosis of these polyps are both operator-dependent.
Traditional serrated adenomas are the least common type of serrated polyp, with a reported prevalence of 0.3%.7 Hyperplastic polyps are by far the most common, accounting for 20% to 30% of all polyps removed at colonoscopy.9,11 Sessile serrated polyps have a predilection for the proximal colon and are associated with female sex and with smoking, 12,13 but no consistent effect of other factors on their formation has been reported. In contrast, Wallace et al13 found that obesity, cigarette smoking, dietary fat intake, total caloric intake, and the consumption of red meat were associated with an increased risk of distal (but not proximal) serrated polyps, including hyperplastic polyps, sessile serrated polyps, and traditional serrated adenomas.
Hyperplastic polyps usually occur in the rectosigmoid colon. They appear as slightly elevated, whitish lesions with a diameter less than 5 mm (Figure 2). Microscopically, the serrated architecture is present in the upper half of their crypts (Figure 3). The proliferative zone is more or less normally located in the basal half of the crypt (the nonserrated portion), with nuclei that are small, uniform, and basally located.14 The bases of the crypts have a rounded contour and do not grow laterally along the muscularis mucosae.
SESSILE SERRATED POLYPS
Endoscopically, sessile serrated polyps are often subtle, appear flat or slightly elevated, and can be covered by yellow mucus (Figure 4). They are typically found in the proximal colon and are usually larger than typical adenomas, with 50% being larger than 10 mm.10
Histologically, the serrations are more prominent than those of hyperplastic polyps and involve the entire length of the crypt (Figure 5). The crypt bases are often dilated and display lateral growth along the lamina muscularis mucosae, resembling a letter t or l. The lamina muscularis mucosae is often thinner than normal. Crypts from sessile serrated polyps are occasionally found beneath the muscularis mucosae, a condition called pseudoinvasion.7
TRADITIONAL SERRATED ADENOMAS
Traditional serrated adenomas are usually left-sided. In contrast to the other types of serrated polyps, they are histologically often villiform and are lined by cells with elongated nuclei and abundant eosinophilic cytoplasm (Figure 6). Unlike those in sessile serrated polyps, the crypt bases do not display an abnormal architecture; rather, traditional serrated adenomas have abundant ectopic crypts (“budding crypts”) in the long, slender villi.7
Traditional serrated adenomas also appear to be genetically distinct from sessile serrated polyps. They are most often characterized by a KRAS (or less commonly, BRAF) mutation and commonly have methylation of the DNA repair gene MGMT (O-6-methylguanine-DNA methyltransferase) rather than hMLH1.