Colonoscopy can reduce overall risk of colorectal cancer by 70 percent

Using colonoscopy to screen average-risk people can reduce the overall risk of a late-stage colorectal cancer diagnosis by about 70 percent, according to a new study.

Researchers analyzed the health records of 1,012 average-risk adults ages 55 to 85 to determine the effect of screening colonoscopy or sigmoidoscopy on the risk of being diagnosed with late-stage colorectal cancer. In addition to the 70 percent reduction in overall risk, people who had been screened using colonoscopy had a 74 percent reduction in risk for left-sided colorectal cancer and a 64 percent reduction in risk for right-sided colorectal cancer. (The right side of the colon, also known as the ascending colon, travels up the right side of the abdomen. The left side, or descending colon, travels down the left side of the abdomen.)

Cancer in the right side of the colon accounts for about half of all new cases of colon cancer in the United States.

By contrast, people who had been screened with sigmoidoscopy had a 50 percent reduction in overall risk for late-stage colorectal cancer, a 64 percent reduction in risk for left-sided colorectal cancer and a modest 21 percent reduction in risk for right-sided colon cancer.

Among the 474 people in the study who had advanced colorectal cancer, 251 (53 percent) had advanced cancer on the right side of the colon, which likely would not have been detected by sigmoidoscopy.

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The study is published today (March 4) in the journal Annals of Internal Medicine.

Colonoscopy has been widely recommended as a screening tool for colorectal cancer since it allows a physician to detect and remove polyps that might turn into cancer, and find early-stage cancers that are more likely to respond to treatment. However, the test's ability to detect right-sided colorectal cancer has been questioned, said study leader Dr. Chyke Doubeni, presidential associate professor of family medicine and community health at the Perelman School of Medicine at the University of Pennsylvania.

A 2009 study published in the Annals of Internal Medicine found that colonoscopy did a better job of lowering death rates for left-sided colon cancer than it did for right-sided. A possible reason for this is that it's technically more difficult to reach the right side of the colon, Doubeni said. Also, bowel preparation may not be as effective in the right colon. What's more, there may be biological differences in the tumors that occur in the left versus the right colon.  For example, polyps on the right side may be flatter and, thus, harder to find and remove.

The results of the new study may put to rest any doubts about colonoscopy's value as a screening tool for left- and right-sided cancer. "We knew colonoscopy was quite good as a diagnostic test in high-risk people, but we didn't know if it was effective in people at average risk," Doubeni said. "This study provides the average person without risk factors assurance that this test does work."

During a flexible sigmoidoscopy, the doctor uses a flexible, lighted tube (called a sigmoidoscope) to examine the interior walls of the rectum and part of the colon. During a colonoscopy, the doctor uses a flexible, lighted tube (a colonoscope) to examine the rectum and the entire colon.

Not counting skin cancer, colorectal cancer is the third most common cancer diagnosed in American men and women. Overall, people have a 1 in 20 lifetime risk of developing the disease.

According to the American Cancer Society, an estimated 102,480 new cases of colon cancer and 40,340 cases of rectal cancer will be diagnosed in 2013. About 50,830 people will die from colorectal cancer.

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