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Guidelines for colorectal cancer screening generally recommend flexible sigmoidoscopy, followed by full colonoscopy only when a distal polyp is found. Because this strategy will obviously miss some proximal lesions, some authorities advocate colonoscopy as the initial screening procedure, even for average-risk people. Two studies examined the yield of screening colonoscopy in asymptomatic people.
The first included 3121 people (mostly men, age 50 to 75) at U.S. veterans hospitals. The prevalence of any neoplasm was 37.5 percent, and 10.5 percent had "advanced disease" (tubular adenoma at least 10 mm, villous adenoma, adenoma with high-grade dysplasia, or invasive carcinoma). The prevalence of advanced disease proximal to the descending colon…