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For years doctors have believed colonoscopies, the screening test for colorectal cancer, prevented 90 percent of cancers, but a new study shows they might actually only prevent 60 percent to 70 percent.

Last spring a study found that the test may miss a type of polyp, a flat lesion, or an indented one that nestles against the colon wall. And this month, a Canadian study published in the journal Annals of Internal Medicine found the test was much more inaccurate than previously thought.

The new study showed that the test missed almost every cancer in the right side of the colon, where cancers are hardest to detect and where about 40 percent of cancers arise. The test also missed about one-third of cancers on the left side of the colon.

Dr. David F. Ransohoff, a gasteroenterologist at the University of North Carolina, says patients should continue to have the test, but also recommends patients seek the best colonoscopists and be scrupulous in the bowel cleansing before the test.

The American Cancer Society has no plans to change its recommended intervals between screenings because it still prevents most cancers. They and the Centers for Disease Control and Prevention are focusing on developing measurements of quality so doctors can assess themselves and improve.

The study was conducted by matching 10,292 people who died of colon cancer to five people who lived in the same area and were of the same age, sex, and socioeconomic status. They asked how many people had colonoscopies and whether doctors removed polyps, and then compared the groups and asked how much the colon cancer death rate had declined in people who had the screening test. The results were “a shock,” said Dr. Nancy N. Baxter, the lead author of the paper and a colorectal surgeon at the University of Toronto.

Now doctors will try to find out why the test missed so many cancers. One reason might be because one-third of the colonoscopies in the Canadian study were done by general internists and family practitioners who might not have had the experience to do the test well. However, at least one unpublished study involving California Medi-Cal patients also found the test missed many cancers on the right side of the colon.

Another possibility as to why the test was so inaccurate is that patients may not have sufficiently cleansed their bowels of fecal material, a particular problem for the right side of the colon. One solution to this, supported by six studies, is to shorten the time between when the patient cleanses his or her bowels and the colonoscopy, said Dr. Douglas K. Rex, director of endoscopy and professor of medicine at Indiana University. This means taking half of the laxative the night before the test and the rest in the morning.

A third possibility is that cancer may be different in the right colon. Flat and indented polyps tend to cluster in the right colon, along with another kind, serrated lesions that may turn into cancer quicker than typical polyps.

Though the tests are expensive, and there is a small risk of perforating the bowel, gastroenterologists still urge patients to have frequent colonoscopies. Other tests available may be even more inaccurate and may give false positives.

Experts said people should realize that even if colonoscopies prevent just 60 percent of colon cancer deaths, that is still a substantial percentage. Mammograms, for example, prevent 25 percent of breast cancer deaths, and the PSA test for men has not been shown to prevent prostate cancer deaths.

About 148,000 people will learn they have colon cancer this year, and nearly 50,000 people will die from it, the American Cancer Society reports.

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