Physicians should advise patients of colonoscopy limitations
Colonoscopy is 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, with almost no mortality prevention benefit for cancer that develops in the right side of the colon, according to a study published Dec. 16 online before print in 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 in Toronto, who is lead author on the study.

Researchers reviewed health records for 10,292 case patients, 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 of 51,460 who were selected from the population of Ontario and were colorectal cancer survivors.

“Compared with controls, case patients were less likely to have undergone any attempted colonoscopy or complete colonoscopy. Complete colonoscopy was strongly associated with fewer deaths from left-sided colorectal cancer but not from right-sided colorectal cancer,” the authors wrote.

David F. Ransohoff, MD, author of an accompanying editorial, said that while colonoscopy is an effective intervention, these study results 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,” he wrote.

The researchers suggested 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 Baxter.