For about a decade, public health officials have been aggressively pushing colonoscopy, a procedure that inspects the colon for growths using a tube and camera, as the gold standard for detecting tumors while they're still treatable. But controversial new research indicates that the method only reduces deaths from tumors closer to the rectum, where they are more common and easily visible.

Colonoscopy cut deaths from tumors in the large intestine, or left side, by 67 percent, but made no difference for people whose cancers were on the right, deeper inside the colon, according to a new study published online today in the Annals of Internal Medicine.

The researchers St. Michael's Hospital in Toronto compared the claims records of more than 10,000 Canadians who died of the disease with those of 51,000 who did not. They speculate that imperfect screening by surgeons and internists, who aren’t as skilled as gastroenterologists at performing the procedure, and differences in the appearance of tumors on the left and right sides, may be to blame.

And because the right side is closer to the start of the digestive system, it may not be as thoroughly cleansed when doctors perform the colonoscopy, says Jack Soterakis, VP of medical affairs and a gastroenterologist at St. Francis Hospital in Roslyn, N.Y. Patients may not finish drinking the special cocktail required to flush their colon of waste, potentially obscuring tumors farther up, and tumors on the right may be more aggressive than those on the left, he says.

Previous research has inferred that colonoscopy reduces deaths from colon cancer by an estimated 76 to 90 percent. Unlike earlier detection methods such as sigmoidoscopy, which checks the lower left colon, and barium enema, in which liquid barium sulfate illuminates the large intestine on X-ray, colonoscopy can reach the right section.

But some doctors are skeptical of the finding, noting that the study doesn’t distinguish between colonoscopies performed for diagnostic purposes versus prevention. People who undergo colonoscopy to diagnose symptoms of colon cancer such as visible bleeding and bloating are more likely to have cancer than people who are just getting a preventive screen, says Thomas Weber, professor of surgery and molecular genetics at Albert Einstein College of Medicine in the Bronx, N.Y. Unlike unexplained anemia, which can be a sign of cancer in the right half of the colon, bleeding and bloating are more indicative of cancers in the left half, which make up roughly 55 to 60 percent of tumors, says Bob Smith, the American Cancer Society's director of cancer screening. (The remainder are on the right, and are becoming more common, he says.)

"It’s a surprising finding because one of the reasons that the colon cancer-prevention community has promoted this is because there is access to right side of the colon," Weber tells us. "It would be very unfortunate to throw the baby out with the bathwater on this one."

Colon cancer is the third most common tumor in the U.S., with 108,000 cases diagnosed and 41,000 deaths this year, according to the American Cancer Society. Just 40 percent of adults have gotten a colonoscopy in the last 10 years, Smith tells us.

(Updated at 3 p.m. Dec. 16 to correct anatomical descriptions.)

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