updated 4/20/2004 1:04:17 PM ET 2004-04-20T17:04:17

Virtual colonoscopy — a cancer-detecting procedure that gives doctors a computer-generated 3-D view of the colon — is less reliable than previously thought and not ready for widespread use, researchers say.

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Its accuracy varies considerably, depending on the training and methods of the doctors performing it, according to a study of 600 patients at nine major clinics.

In conventional colonoscopy, a long, flexible viewing tube about a half-inch thick, or the thickness of a finger, is inserted in the rectum and threaded several feet into the colon. A device on the end of the tube is used to remove suspicious growths, which are later tested for cancer.

Virtual colonoscopy is designed to take some of the discomfort out of the examination. A narrower rectal catheter is inserted, and a CT scanner produces images of the colon.

In the latest study, the patients underwent virtual colonoscopy first, then traditional colonoscopy on the same day.

Low detection rates
The virtual method detected 55 percent of patients with at least one suspicious polyp at least 10 millimeters in diameter, compared with a 100 percent success rate for traditional colonoscopy. For smaller tumors, at least 6 millimeters in diameter, the results were worse: 39 percent for virtual colonoscopy versus 99 percent for the traditional method.

Eight patients ultimately were diagnosed with cancer; virtual colonoscopy missed the disease in two of them.

As recently as December, a widely reported study found that virtual colonoscopy was at least as accurate as the conventional variety.

But the authors of the new research said many of the previous studies were largely based on data from a single hospital and did not take real-world conditions into account.

The leader of the research team called the latest findings “a bucket of cold water” thrown on the growing enthusiasm for virtual colonoscopy.

While the technique might be effective “in the hands of experts, it has yet to be proven that this expertise can be taught and disseminated reliably into daily practice,” said the research team, led by Dr. Peter Cotton, a gastroenterologist at Medical University of South Carolina.

“If this technique is worth doing, it’s worth doing by regular folks,” Cotton said.

Supporters still optimistic
Nevertheless, Cotton said doctors should not give up on virtual colonoscopy. “I suspect that in five years’ time, with better software and training and maybe some more intelligent computing ... this will become a useful technique,” he said.

The study appears in Wednesday’s Journal of the American Medical Association.

Participating radiologists were required to have performed at least 10 previous virtual colonoscopies. Only one of the centers had substantial previous experience with the procedures and it had the best results, detecting 82 percent of patients with at least one of the smaller polyps. The eight other centers combined had a detection rate of 24 percent.

“The differences between what virtual colonoscopy can do and what it will do if applied in ordinary practice circumstances are so great that physicians must be cautious,” Dr. David Ransohoff of the University of North Carolina in Chapel Hill said in a JAMA editorial.

The American Cancer Society recommends that people over 50 get a conventional colonoscopy every 10 years, calling it the “gold standard” of colon cancer screening. The society concluded that there is not enough evidence to recommend the virtual test for those with average colon cancer risk.

While some doctors offer virtual colonoscopies, most insurance companies do not cover the method. And when the technique does find suspicious growths, standard colonoscopies are required to remove them.

Colon cancer is the second leading cause of cancer-related deaths in the United States. It accounts for nearly 60,000 deaths each year. The detection and removal of polyps can help prevent cancer from developing.

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