One type of screening for colorectal cancer - sigmoidoscopy - need not be repeated more often than every five years, as is currently recommended. According to a new study, the likelihood of colorectal cancer occurring remains low for at least five years after a negative sigmoidoscopy result.

Guidelines urge screening for colorectal cancer for everyone after age 50, and at a younger age for those with risk factors for the disease such as a family history.

Colonoscopy, in which the entire colon is inspected visually with a long optical tube, is the most definitive screening exam. Sigmoidoscopy, which looks only at the closest segment of the colon - in which most cancers arise - is another option. Colonoscopy is almost always done while the patient is sedated, but this is not usually necessary with sigmoidoscopy.

“Physicians need to educate their patients about the advantages and disadvantages of the available tests and assist them in selecting the test that is best for them,” study lead author Dr. V. Paul Doria-Rose told Reuters Health. “Sigmoidoscopy remains an important option for colorectal cancer screening in the average-risk patients that were the focus of our study.”

Doria-Rose and colleagues used the Kaiser Permanente of Northern California database that includes over 70,000 participants in a colon cancer screening program to look at the occurrence rate of colorectal cancer during the first five years after a negative sigmoidoscopy.

The investigators documented cancers arising in the proximal section of the colon (that is, furthest from the rectum) and in the distal (closest) section. Proximal cancers would not be spotted by sigmoidoscopy, since the optical tube is relatively short.

“For the entire follow-up period, incidence of distal colorectal cancer remained much lower than age-adjusted rates in the general population,” the team reports in the medical journal Gastroenterology. Rates of proximal tumors were “modestly” lower than in the general population.

“The incidence of colorectal cancer, both distal and proximal, is low for at least five years following a negative screening sigmoidoscopy,” Doria-Rose said. “A screening interval shorter than five years is unlikely to be worth the added risk and inconvenience associated with more frequent examinations.”

Nonetheless, he cautioned, “No screening test can offer one hundred percent efficacy.”

That point is echoed in a related editorial. “Colorectal cancer cannot be completely eliminated as a possibility, even with very intense screening,” writes Dr. Michael Pignone from University of North Carolina, Chapel Hill, North Carolina.

Even though current tests are not perfect, Pignone stresses, “Screening by any of the currently recommended test strategies is effective in reducing colon cancer incidence and mortality and should be encouraged.”

SOURCE: Gastroenterology, September 2004.