Colonoscopies less accurate than expected, may miss cancers

For years, many doctors and patients thought colonoscopies, the popular screening test for colorectal cancer, were all but infallible. Have a colonoscopy, get any precancerous polyps removed, and you should almost never get colon cancer.

Then, last spring, researchers reported the test may miss a type of polyp, a flat lesion or an indented one that nestles against the colon wall. And a Canadian study, published today in the journal Annals of Internal Medicine, found the test, while still widely recommended, was much less accurate than anyone expected.

In the new study, the test missed just about every cancer in the right side of the colon, where cancers are harder to detect but about 40 percent arise. And it also missed roughly a third of cancers in the left side of the colon.

Instead of preventing 90 percent of cancers, as some doctors have told patients, colonoscopies might actually prevent more like 60 to 70 percent.

"This is a really dramatic result," said Dr. David Ransohoff, a gastroenterologist at the University of North Carolina. "It makes you step back and worry, 'What do we really know?' "

He and other screening experts say patients should continue to have the test, adding that it still is highly effective. But they also recommend patients seek the best colonoscopists by, for example, asking pointed questions about how many polyps they find and remove.

They also say that patients should be scrupulous in the unpleasant bowel cleansing that precedes the test, and promptly report symptoms such as bleeding even if they occur soon after a colonoscopy.

The American Cancer Society says that even if the test is less effective than many had believed, it has no plans to change its recommended intervals between screenings — the test still prevents most cancers, and the expense and risk of the test argue against doing it more often.

The cancer society and the Centers for Disease Control and Prevention also are focusing on developing measurements of quality so that doctors who do colonoscopies can assess themselves and improve.

But gastroenterologists say that, if nothing else, the study points up the paucity of evidence for the common suggestion that anyone who had a clean bill of health from a colonoscopy is almost totally protected for at least a decade.

"We have to not overpromise," said Ransohoff, who wrote an editorial accompanying the colonoscopy paper. "We need to look at the evidence, and we shouldn't go beyond it."

The new study matched each of 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. The researchers asked how many patients and control subjects had had colonoscopies and whether the doctors had removed polyps. Then the researchers compared the groups and asked how much the colon-cancer death rate had declined in people who had had the screening test.

The results were "a shock," said Dr. Nancy Baxter, the lead author of the paper and a colorectal surgeon at the University of Toronto. When she saw them, she said, "I asked my analyst to rerun the data."

Researchers say the challenge is to find out why the test missed so many cancers — in particular those on the right side of the colon — and whether the problem can be fixed.

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

It might be that Canadian doctors were not sufficiently skilled. About a third of the colonoscopies were done by general internists and family practitioners who might not have had the experience to do the test well.

But, said Dr. Douglas Rex, director of endoscopy and professor of medicine at Indiana University, that cannot be the entire explanation because at least one study, as yet unpublished, involving California Medi-Cal patients also found the test missed many cancers on the right side of the colon.

That leaves several other possibilities.

Perhaps patients did not sufficiently cleanse their bowels of fecal material, a particular problem for the right side of the colon.

"After the prep has been completed, mucus and intestinal secretions start rolling out of the small intestine and colon," Rex explained. The secretions, he added, pour from the base of the appendix into the right side of the colon and are "very sticky" and can obscure polyps.

One solution, supported by six studies, is to be sure there is just a short time between when patients finish taking the strong laxative that cleanses their bowel and the colonoscopy, Rex said. That usually means taking half of the laxative the night before the screening test and the rest in the morning, something that often is not done, he added, but that he and others recommend.

Cancer may also be different in the right colon, researchers said.

Flat and indented polyps tend to cluster in the right colon. And so do another kind, serrated lesions, which, some studies indicate, might turn into cancer much more quickly than typical polyps.

"It's possible that we will never get as good a result" in the right colon, said Robert Smith, director of screening for the American Cancer Society.

Still, he said, that does not mean that patients should have more frequent colonoscopies. The tests are "hugely expensive," he said, and insurers may not pay for more frequent colonoscopies. The test also carries a small risk of perforating the bowel.

Even if colonoscopies miss some cancers, colon cancer remains a rare disease and, after a colonoscopy, "the likelihood that you have cancer is very, very low," Smith said.