A new study finds that colonoscopy is strongly associated with fewer deaths from colorectal cancer. However, the risk reduction appears to be entirely due to a reduction in deaths from left-sided cancers. According to the study, colonoscopy shows almost no mortality prevention benefit for cancer that develops in the right side of the colon.

The study appears online and in the January 6, 2009, issue of the Annals of Internal Medicine.

"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," said Nancy Baxter, MD, PhD, Colorectal Surgeon and a Researcher at St. Michael's Hospital, who is lead author on the study.

Researchers reviewed health records for persons aged 52 to 90 who received a colorectal cancer diagnosis between 1996 and 2001 and died of colorectal cancer by 2003. These patients were compared to a control group who were selected from the population of Ontario and had not died of colorectal cancer.

According to the researchers, complete colonoscopy was strongly associated with fewer deaths from left-sided colorectal cancer. Conversely, the data showed that colonoscopy seemed to have almost no mortality prevention benefit for right-sided colorectal cancer.

"Colonoscopy is an effective intervention," said David F. Ransohoff, MD, author of an accompanying editorial. "The study results, however, should caution physicians about saying that colonoscopy will reduce the risk of dying from colorectal cancer by 90 percent. A 60 to 70 percent risk-reduction rate seems more reasonable."

The researchers suggest several reasons why colonoscopy may be less effective in preventing death from right-sided colorectal cancer. First, some colonoscopies considered "complete" may not evaluate the entire right colon. Second, bowel preparation may be worse in the right colon. Third, right and left colonic cancers and polyps may differ biologically. Right-sided growths may be less likely to have a fleshy stalk and are occasionally flat, which makes them harder to identify and remove, or they may grow more rapidly.

"Although improvements in the quality of screening colonoscopy may improve detection at the right side, differences in tumor biology may limit the potential to prevent right-sided colorectal cancer deaths with current endoscopic technology. Nevertheless, this study clearly demonstrates that colonoscopy is an effective procedure for the prevention of death from colorectal cancer, it just may not be quite as effective as we've thought in the past." said Dr. Baxter.