Although current US guidelines recommend that adults aged more than 50 years undergo sigmoidoscopy screening for colon cancer every five years, that may be unnecessarily frequent, a new study says.
Screening methods for colorectal cancer include fecal occult blood testing—to identify blood in the stool—and colonoscopy and sigmoidoscopy, in which a flexible, lighted tube is inserted into the rectum and passed through the colon to identify any polyps, or small growths, that can develop into cancer. Colonoscopy examines the entire colon, while sigmoidoscopy looks only at the lower part.
Guidelines from the American Cancer Society currently say that adults older than 50 should undergo sigmoidoscopy screening every five years, along with a yearly fecal occult blood test. However, although sigmoidoscopy is known to reduce the risk of colon cancer, it is unclear how long this benefit lasts after screening, the authors of the new study note (Journal of the National Cancer Institute 2003;95:622-5).
Some experts have argued that for people at average risk sigmoidoscopy screening once in a lifetime could be enough, Dr Polly Newcomb and colleagues write. They say that since it can take as long as 15 years for polyps to progress to cancer, sigmoidoscopy screening every five years may be “overly aggressive.”
Dr Newcomb and her colleagues at the Fred Hutchinson Cancer Research Center in Seattle collected information on screening history and risk factors for colorectal cancer from a population based sample of 1668 patients with either distal or proximal colorectal cancer and on 1294 healthy individuals.
The risk of distal colorectal cancers in people who had had a screening sigmoidoscopy was a quarter the risk in people who had never been screened (odds ratio 0.24 (95% confidence interval 0.17 to 0.33)). This reduction in risk seemed to be sustained for more than 15 years—a finding that supports a longer screening interval than is currently recommended, the authors say.
In an accompanying editorial Dr Jack Mandel of Emory University, Atlanta, cautions that it may be premature to recommend lengthening the screening interval. “A more precise estimate of the benefit from sigmoidoscopy screening will have to await the results from the ongoing [randomised controlled] trials,” he says.
“At that time, we will be in a better position to more accurately evaluate the risks, benefits, and cost-effectiveness of screening. Those results will provide a better basis on which to develop screening policy.”