Negative colon cancer test makes 5-year risk low

NEW YORK (Reuters Health) - If the results of an initial colonoscopy indicates there are no polyps or cells that appear irregular, which may eventually become cancerous, the odds that a malignancy will develop within the next 5 years is "extremely low," according to a report in The New England Journal of Medicine.

In fact, no patients in the new study who had an initial negative result had cancer at follow-up. The results also indicate that it is unlikely that advanced adenoma - the type of polyp most often associated with cancer, will appear after a negative screening result, although men are at greater risk than are women.

"Our findings support a rescreening interval of 5 years or longer after a normal colonoscopic examination," Dr. Thomas F. Imperiale and colleagues conclude. However, the optimal interval in between colonoscopies is unclear, according to the report.

Colonoscopy is a screening test for colorectal cancer in which a thin, flexible tube called a colonoscope is used to examine the colon. The colonoscope has a small video camera attached so the whole colon and the lower part of the small colon can be seen. Tissue samples may be collected for biopsy and abnormal growths can be removed during the procedure.

The US Multisociety Task Force on Colorectal Cancer and the American Cancer Society recommend that it be performed every 10 years, whereas the US Preventive Services Task Force only recommends that it be done, not specifying the interval.

With this uncertainty in mind, Imperiale, from Indiana University Medical Cancer in Indianapolis, and colleagues evaluated the occurrence of adenomas and cancer in 1,256 adults who were reevaluated with colonoscopy 5 years after they received negative exam results.

The average patient's age was 57 years old, and 57 percent were men. The average time between initial colonoscopy and follow-up colonoscopy was 5.34 years.

None of the patients had colorectal cancer on the follow-up colonoscopy, the report indicates. Although 201 patients had at least one adenoma, just 16 (1.3 percent) had an advanced adenoma.

Polyps with irregular tissue, or "hyperplastic polyps," detected on the first colonoscopy did not significantly affect the risk of having advanced adenoma on follow-up colonoscopy.

Men were 1.88-times more likely than women to have any adenoma on follow-up, and 3.31-times more likely to have advanced adenoma.

The findings suggest that after an initial negative screening result, colonoscopy does not need to be repeated sooner than 5 years. "However," they add, "the follow-up interval for this study was only 5 years; we did not assess the appropriateness of the recommended 10-year rescreening interval for colonoscopy."

SOURCE: The New England Journal of Medicine, September 18, 2008.