Don’t be overconfident of that recent colonoscopy’s clean bill of health.

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.


The actual exam is performed as the doctor withdraws the scope and looks for polyps. A variety of tiny tools -- loops, snares, jaws with teeth -- can be passed through the scope and used to take out any growths the doctor finds.

On average, a colon is about a yard long, with several curves and sharp turns along the way. It takes a good deal of skill and practice not only to reach the end without bumping into the walls (which hurts the patient), but then to withdraw the scope slowly and not miss any polyps.

Physiology also complicates the right-side picture. “The stool is more liquid on the right side,” Baxter says. “So it coats the lining and makes polyps harder to see.”

But the right side of the colon isn’t just harder to examine. Cancer on the right side is actually different from cancer on the left side. “They’re like two different diseases,” says Dr. Heinz Lenz, professor of medicine and preventive medicine at the Keck School of Medicine and Norris Comprehensive Cancer Center at USC.


Polyps on the left are generally mushroom-shaped, projecting out on a stalk, while polyps on the right side are generally flatter, with no stalk. So polyps on the left are easier to see and to remove.

Besides, while most colorectal cancers develop from polyps, some develop directly from the lining of the colon. This may happen more often on the right side, Baxter says.

In an additional complication, the most characteristic symptom of colorectal cancer -- blood in the stool -- doesn’t occur with cancer in the right side, where symptoms may just be abdominal discomfort or pain, Lenz says. For patients with these complaints, doctors are less likely to recommend a colonoscopy.

Right-side detection can probably be improved by improving colonoscopy technique and pre-colonoscopy preparation. But because of the differences between left- and right-side cancers, it may be very hard, even impossible, to get right-side detection up to left-side standards.


The Canadian study was limited to some extent by its design and its data. It was not a randomized, controlled trial, so it couldn’t prove that colonoscopy causes a reduction in cancer deaths. Also, the researchers didn’t have enough information to know for sure whether colonoscopies were for screening or diagnostic purposes. Because they’re performed when doctors already suspect disease, diagnostic colonoscopies are more likely to be followed by death than screening ones, which are performed on apparently healthy people.

The quality of colonoscopies keeps improving, Baxter says, so current colonoscopies may well be more accurate than the ones in the study, which were mostly done in the 1990s.

In any case, colonoscopy is a highly effective weapon against colorectal cancer -- the third most common cancer in the United States, and the second most deadly, after lung cancer.

“A 60% to 70% mortality reduction . . . should not be considered disappointing,” Dr. David Ransohoff, a professor of medicine at the University of North Carolina, wrote in an editorial with the study.


By comparison, mammograms are only associated with a 25% reduction in deaths from breast cancer, and PSA testing has not been shown to reduce deaths from prostate cancer.

“It would be dangerous if the study led doctors to recommend fewer colonoscopies,” Lenz says.

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