Prevalence and incidence of colorectal adenomas and cancer in asymptomatic persons.

The prevalence of colorectal adenomatous polyps varies widely from country to country and is highly correlated with colorectal cancer incidence rates in each country. The prevalence of adenomas reported in older studies was based on autopsy findings and is higher than that in more recent studies based on endoscopy findings. Among asymptomatic, average-risk patients, adenoma prevalence averages approximately 10% in sigmoidoscopy studies and more than 25% in colonoscopy studies, whereas the prevalence of colorectal cancer among these patients is less than 1%. The cumulative incidence of new adenomas within 3 years after normal endoscopy averages about 7% by flexible sigmoidoscopy and 27% by colonoscopy.