Virtual Colonoscopy, Real Accuracy

Sept. 17, 2008 -- Virtual colonoscopy -- colon cancer screening using CT scans -- finds 90% of large, precancerous polyps.

The finding comes from 15 academic and community medical centers that performed both virtual colonoscopy (CT colonography) and traditional colonoscopy on 2,600 patients aged 50 and older.

The study "validates" new guidelines endorsing virtual colonoscopy as a colon cancer screening option, says study leader C. Daniel Johnson, MD, professor of radiology at the Mayo Clinic in Scottsdale, Ariz.

"Up to now, there has been conflicting data on how good this test would be," Johnson tells WebMD. "This is the largest study to date on CT colonography. The performance data shows the sensitivity is very similar to that quoted for colonoscopy."

The study showed that virtual colonoscopy was able to detect 90% of polyps 10 millimeters or more in diameter. That's the same accuracy reported for colonoscopy itself in other studies.

Colonoscopy is, however, able to detect much smaller polyps. As the size of polyps gets smaller, so does the ability of virtual colonoscopy to detect them.

Johnson and colleagues found that CT colonography could detect 78% of polyps as small as 6 millimeters in diameter. Yet the scans caught only 65% of polyps 5 millimeters or smaller.

"It is not clear that leaving small polyps is safe," notes a news release from the American Gastroenterological Association.

Polyps of 10 millimeters or more progress to cancer at a rate of 1% per year. Polyps half that size should not progress to cancer any faster, but the rate at which such polyps progress is not known.

Johnson and colleagues note that fewer than 2% of polyps smaller than 5 millimeters have features that suggest they are precancerous.

Virtual Colonoscopy, Real Bowel Prep

A downside of virtual colonoscopy is that it requires the same bowel prep (cleansing of the bowels) as a real colonoscopy.

Patients' biggest complaint about colonoscopy is the prep, not the procedure itself, says internal medicine expert Robert Fletcher, MD, professor emeritus at Harvard University.

"Nobody ever said the prep was easy, and it is just the same for the CT colonography as it is for colonoscopy," Fletcher tells WebMD. "And if the scan finds something in the colon, which it does about 17% of the time, you have to turn right around and get prepped again and have a colonoscopy, unless it can be arranged the same day."

Johnson says that centers performing CT colonography should be equipped to offer same-day colonoscopy for patients who need it. Fletcher says it's important for patients to ask about this before choosing virtual colonoscopy as a screening option.

Virtual colonoscopy has a 14% false-positive rate. That's not bad, considering it's a screening test designed to catch as many polyps as possible. But patients who get a false alarm will have to undergo an unnecessary colonoscopy.