Colon Polyp – A Premalignant Lesion ?
Regardless of the cause, the majority of colon cancer is thought to arise from adenomatous polyps. Throughout the entire population, therefore, the most usual premalignant phenotype of bowel cancer is the polyp. The diagnosis of a polyp, either during screening or as a response to symptoms, should initiate examination of the entire colon mucosa, with subsequent serial follow-up. In addition, patients diagnosed with and treated for sporadic colon or rectal cancer are at a high risk for synchronous and metachronous bowel tumors (predominantly polyps). These patients benefit from routine endoscopic screening to diagnose and preempt the progression of polypoid tumors in what is probably “initiated” but not “promoted” colonic mucosa.The National Polyp Study confirmed that colonoscopic polypectomy in patients with adenomas resulted in a lower probability of the development of colon cancer compared with a reference group of individuals with polyps that had not been removed and a population-based registry (SEER), of whom most did not have polyps.This study effectively validated the colon adenoma-to-adenocarcinoma sequence.
Although questions continue concerning the possible premalignant potential of hyperplastic polyps, in general, hyperplastic mucosal proliferation of the large bowel is not thought to be preneoplastic. Only adenomas are clearly premalignant, and only a minority of adenomas ever develop into cancer.The genetic events involved in the malignant transformation of normal colon mucosa to adenoma to carcinoma have been elucidated ( Fig. 16-3 ) and provide a framework and potential targets for novel diagnostic and therapeutic interventions. Adenomatous polyps are more likely to become malignant if they are sessile rather than pedunculated, villous rather than tubular, and large (>1.5 cm) rather than small. After the detection of an adenomatous polyp, the entire large bowel should be visualized endoscopically, because synchronous lesions are found 35 to 40% of the time. The frequency of subsequent colonoscopic surveillance is hard to pinpoint, because there must be a balance between the cost of the endoscopic procedure and the fact that more than 5 years is probably required for an adenomatous polyp to grow to the size at which neoplastic transformation is likely.It is, however, recommended that two successive colonoscopic examinations be conducted to ensure clearance of the entire bowel mucosa before the endoscopic interval is lengthened in patients who have already been diagnosed as having colon cancer or sporadic polyp formation.
