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Data from the National Polyp Study often are used to support the contention that colonoscopy and polypectomy prevent as many as 90% of incident colorectal cancers. However, data from case-control studies and from U.S. dietary intervention trials suggest that these procedures actually provide a much lower level of protection. To determine risk for colorectal cancer during colonoscopic surveillance, researchers evaluated data from three U.S. randomized chemoprevention trials involving 2915 patients who had had at least one adenoma removed before study entry.
During a mean of 3.7 years of follow-up, 19 patients developed colorectal cancer, and 7 additional patients developed adenomas with high-grade dysplasia; 16 cancers were stage I or II. Two…