Adenomas and serrated polyps may be non-polypoid (flat, slightly elevated, or depressed) (eFigure 15–84), sessile, or pedunculated (containing a stalk). They are present in 30% of adults over 50 years of age. Their significance is that over 95% of cases of adenocarcinoma of the colon are believed to arise from these lesions. It is proposed that there is a polyp → carcinoma sequence whereby nonfamilial colorectal cancer develops through a continuous process from normal mucosa to adenomatous or serrated polyp to carcinoma. The majority of cancers arise in adenomas after inactivation of the APC gene leads to chromosomal instability and inactivation or loss of other tumor suppressor genes. By contrast, cancers arising in the serrated pathway appear to have either Kras (traditional serrated adenomas) mutations or BRAF oncogene activation (sessile serrated adenomas) with methylation of CpG-rich promoter regions that leads to inactivation of tumor suppressor genes or mismatch repair genes (MLH1) with microsatellite instability.
Large flat (sessile) polyp. This 15-mm flat (sessile) adenomatous polyp was identified in a middle-aged woman during screening colonoscopy. To remove this polyp, the endoscopist first used a submucosal saline injection to lift the lesion away from the muscularis, followed by snare cautery resection. (Used, with permission, from Michelle Nazareth, MD.)
Most adenomas are smaller than 1 cm and have a low risk of becoming malignant; less than 5% of these enlarge with time. Adenomas and serrated polyps are classified as “advanced” if they are 1 cm or larger or contain villous features or high-grade dysplasia. Advanced lesions are believed to have a higher risk of harboring or progressing to malignancy. It has been estimated from longitudinal studies that it takes an average of 5 years for a medium-sized polyp to develop from normal-appearing mucosa and 10 years for a gross cancer to arise. The prevalence of advanced adenomas is 6% and colorectal cancer 0.3%. The role of aspirin and NSAIDs for the chemoprevention of adenomatous polyps is discussed in Chapter 39-14, in the section on Colorectal Cancer.
Most sessile serrated polyps and traditional serrated adenomas are believed to arise from hyperplastic polyps. It is believed that sessile serrated polyps and traditional serrated adenomas harbor an increased risk of colorectal cancer similar or greater to that of adenomas. Many pathologists cannot reliably distinguish between hyperplastic polyps and sessile serrated polyps. Hyperplastic polyps smaller than 5 mm located in the rectosigmoid region are of no consequence, except that they cannot reliably be distinguished from adenomatous lesions other than by biopsy. Hyperplastic polyps located in the proximal colon (ie, proximal to the splenic flexure) are associated with an increased risk of neoplasia, particularly those larger than 1 cm.
Most patients with adenomatous and serrated polyps are completely asymptomatic. ...