NEW YORK (Reuters Health) - Screening for colorectal cancer by colonoscopy seems to prevent about two-thirds of deaths from colon cancer, rather than 90 percent as has been widely claimed, a study indicates.
In fact, the results show that the benefit of colonoscopy in preventing deaths from colorectal cancer is largely limited to malignancies that occur on the left side of the colon.
"While colonoscopy remains the gold standard for evaluation of the colon, our study sheds light on some of the real-world limitations of this practice for screening and prevention," lead author Dr. Nancy Baxter, from the University of Toronto, said in a statement.
The large bowel extends from the rectum up the left side of the abdomen, across, and down the right side to the cecum, or junction with the small intestine. Although the entire colon should be inspected during a colonoscopy, the cecum and the right side of the colon can be difficult to reach during insertion of the colonoscope.
The current study, based in Ontario, Canada, included more than 10,000 patients who were diagnosed with colorectal cancer between 1996 and 2001 and died from the malignancy by the end of 2003. Each patient was matched to five "controls" who did not have colorectal cancer.
Overall, 9.8 percent of controls had undergone colonoscopy compared with only 7.0 percent of the patients who died of colorectal cancer, the researchers report in the Annals of Internal Medicine.
Complete colonoscopy (that is, to the cecum) was associated with a 67 percent reduced risk of death from left-sided colorectal cancer, but was not associated with a significant reduction in deaths from right-sided disease.
One reason for this may be that the colonoscopies were not as complete as reported, or were hurried, Dr. David F. Ransohoff, from the University of North Carolina, Chapel Hill, suggests in an accompanying editorial.
He comments, "Colonoscopy is an effective intervention, but as Baxter and colleagues suggest, we must realize that current evidence is indirect and does not support a claim of 90 percent effectiveness."
Ransohoff adds that, in advising patients about colonoscopy, "we should be realistic and cautious when talking with them about the magnitude of both benefits and risks."
SOURCE: Annals of Internal Medicine, January 6, 2009, released online December 15, 2008.