New insight into effectiveness of colonoscopy

That's the lesson of a new study suggesting that colonoscopies reduce the risk of dying from colorectal cancer not by up to 90%, as believed, but by perhaps 60% to 70%. Further, they're much better at finding problems in one side of the colon than the other.

"Colonoscopy seems to prevent two-thirds of deaths on the left side," says Dr. Nancy Baxter, a colorectal surgeon and researcher at St. Michael's Hospital in Ontario, Canada, and lead author of the study. "But there's not much effect at all on the right."

She led a team of researchers who reviewed public health records in Ontario to identify more than 10,000 people, or "case patients," ages 52 to 90. All had been diagnosed with colorectal cancer between January 1996 and December 2001, and all had died of the cancer by December 2003.

For each case patient, the researchers selected five controls who matched for age, sex, socioeconomic status and residence location but differed in that they were still alive at the time of the case patient's death.

The researchers then compared the percentage in each group that had received a colonoscopy between January 1992 and the date six months before the case patient was diagnosed with colorectal cancer. Essentially, they were looking at a window of at least several years in which case patients and controls might have had colonoscopies. And they tried to maximize the chances that a case patient's colonoscopy was done for simple screening purposes. The study, published online last week in the journal Annals of Internal Medicine, confirmed that controls were more likely to have had a colonoscopy than were case patients. That is, having a colonoscopy was associated with fewer deaths. But the advantage of a colonoscopy was restricted to patients with cancer on the left side of their colon.

Experts suggest several explanations for the left- versus right-side difference. For one thing, it's harder to examine the right side of the colon.

In a colonoscopy, the doctor inserts a scope at the lower left of the inverted-U-shaped colon, then moves it up, across and down to the lower right. The process is a bit like maneuvering a very narrow, floppy garden hose through a long tunnel, says Dr. Jacques Van Dam, professor of medicine and director of endoscopy at the Stanford University Medical Center.