WEDNESDAY, Sept. 8, 2004 (HealthDayNews) -- Virtual colonoscopy, a less invasive method of examining the colon for signs of cancer, can catch polyps missed by a conventional colonoscopy.

A new study involving 1,233 patients who had conventional and virtual colonoscopy on the same day found that, after comparing one set of test results against the other, the standard scoping method tended to miss polyps on the backside of colonic folds and in the low rectum where the scope is inserted.

Overall, the miss rate for standard colonoscopy was 12 percent for 10 millimeter-and-greater polyps -- double previous estimates.

"Really what this provides is, for the first time we have a method to check the accuracy of conventional colonoscopy," said principal investigator Dr. Perry J. Pickhardt, an associate professor of radiology at the University of Wisconsin Medical School in Madison.

Then again, neither method proved foolproof. Each test missed 10 percent to 14 percent of polyps measuring 6 millimeters or greater, the authors report in the Sept. 7 issue of the Annals of Internal Medicine.

But used in tandem, they could boost the detection rate, the findings suggest.

"These are complementary tests. Anything the virtual test missed the conventional test picked up," Pickhardt said.

Colorectal cancer, the second deadliest cancer in the United States after lung cancer, will claim an estimated 56,730 lives this year, according to the American Cancer Society. The good news is it's highly preventable: 90 percent of cases and deaths could be avoided through the use of screening tests, the society says. When detected early, colorectal cancer has a five-year survival rate of nearly 90 percent.

Standard colonoscopy involves the insertion of a thin, flexible, lighted tube called a colonoscope into the rectum. The scope, which is guided through the entire length of the colon, transmits computer images that let a doctor spot abnormalities, such as polyps or inflamed tissue inside the colon. The procedure is widely regarded as the gold standard for detecting polyps.

Virtual colonoscopy, also known as CT colonography, uses powerful computer-assisted X-ray imaging to detect colorectal lesions. Because it is minimally invasive, squeamish patients may find it less offensive. It also avoids the risk of perforating the bowel, a serious complication of conventional colonoscopy.

As the technology evolves, virtual colonoscopy is likely to emerge as a standard screening test, a task force of the American Gastroenterological Association (AGA) concluded in a report last week. But the panel's report in the journal Gastroenterology also cited a number of issues that need to be worked out before the test becomes widely accepted.

"What we've concluded is that virtual colonoscopy has tremendous promise, the technology is evolving and it's getting close to the point that it may be a valid option for screening for colorectal cancer," said Dr. Emmet B. Keeffe, AGA president and a professor of medicine at Stanford University.

To date, studies of virtual colonoscopy have yielded widely varying results, the task force noted. These discrepancies reflect differences in patients' risk for colorectal cancer, variations in equipment, patient preparation and study methods and inconsistencies in the way test results are interpreted.

As recently as April, for example, a study in the Journal of the American Medical Association said the accuracy of virtual colonoscopy in detecting cancers was substantially lower than that of conventional colonoscopy. Researchers used two-dimensional imaging, which may explain the disappointing results, while Pickhardt and his colleagues used state-of-the-art 3-D imaging to detect polyps.

"What we need is to see that this is generalizable," Keeffe said of the latest study. "Will there be other centers that will duplicate the results of Pickhardt?"

Although it's been around for a decade now, with many refinements along the way, virtual colonoscopy is still considered experimental. The American Cancer Society currently does not recommend it for early detection of colorectal cancer, citing a lack of scientific evidence that it's effective, or more effective, than currently recommended screening tests.

It's also not yet widely available, nor do many health insurers pay for it, according to the AGA.

Based in part on Pickhardt's results, Physicians Plus Insurance Corp. in Madison, Wis., announced in June that it would begin covering virtual colonoscopy. Others insurers in that market quickly followed, fueling demand for the procedure.

Pickhardt, a member of the American College of Radiology, is busier than ever and believes it's only a matter of time before virtual colonoscopy catches on nationally.

"Pretty soon," he said, "people are going to realize this makes sense."

More information

Visit the American Cancer Society to learn more about colorectal cancer screening.

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