Screening for colorectal cancer with fecal occult blood testing and lower endoscopy with removal of polyps reduce the mortality rate from colorectal cancer. Screening with flexible sigmoidoscopy is becoming standard for asymptomatic persons older than 50 years. Because adenomatous polyps found in the distal colon have been associated with adenomatous polyps in the proximal colon, full colonoscopy is generally recommended for patients with distal adenomas. Small polyps (less than 1 cm) seem to have a lower risk of malignant transformation than do larger polyps and are less likely to be malignant or to have high-grade histologic features. With increasing data, it appears that there is a low prevalence of histologically advanced polyps in the proximal colon among patients with small distal tubular adenomas. Wallace and associates conducted a study to determine the prevalence of advanced adenomatous polyps in the proximal colon among patients with small tubular adenomas found on flexible sigmoidoscopy.

Data were prospectively collected from flexible sigmoidoscopy examinations performed on a cohort of 4,490 average-risk patients. All polyps smaller than 10 mm identified by flexible sigmoidoscopy were biopsied and examined histologically. Polyps were characterized as nonneoplastic or neoplastic by a gastrointestinal pathologist. Patients with single small adenomas (1 to 5 mm) were informed of the implications of a single small distal adenoma found on flexible sigmoidoscopy, and colonoscopy was performed within six months.

Among the 90 patients with a single tubular adenoma 1 to 5 mm in diameter noted on flexible sigmoidoscopy, none was found to have advanced proximal polyps on colonoscopy. Among the 148 patients with multiple tubular adenomas 1 to 5 mm in diameter or any adenoma 6 to 10 mm in diameter, eight patients (5.4 percent) were found to have advanced proximal polyps on colonoscopy. Villous adenomas of any size found on flexible sigmoidoscopy were considered advanced and, therefore, were not grouped with tubular adenomas.

The authors conclude that the prevalence of tubular adenomas larger than 10 mm, villous adenomas and cancer in the proximal colon is very low in asymptomatic patients with a single small adenoma (1 to 5 mm) and who have no other risk factors for colon cancer. These patients may not require follow-up colonoscopy. Patients with multiple adenomatous polyps of any size, tubular adenomas larger than 5 mm or advanced polyps in the distal colon should undergo a colonoscopy.