Accuracy of CT Colonography (Virtual Colonoscopy) for Detecting Polyps FREE

Colorectal cancer affects the large intestine or rectum. It is the second leading cause of death from cancer in the United States. Polyps (outgrowths of the wall of the colon) precede most colorectal cancers. Screening procedures can detect and remove polyps before they become cancer and can detect early cancers that are curable with surgery. Doctors often use a long flexible tube with a tiny camera on its tip (optical colonoscopy or OC) to look for and remove polyps and early tumors. The OC procedure requires patient sedation and may miss growths hidden behind folds in the colon. Doctors might also use a newer procedure called CT colonography (CTC, also called virtual colonoscopy) to find polyps and early cancer. CTC uses x-rays and computers to produce multiple thin-sliced images of the inside of the colon. It takes about 10 minutes and does not require sedation. Several recent studies show conflicting results about the ability of CTC to find polyps.

To summarize published reports about the ability of CTC to detect polyps.

6393 adults included in 33 studies that compared CTC with either OC or surgery. Their average age was 62 years, 64% were men, and 74% were at high risk for colorectal cancer.

The authors looked for studies published between 1975 and February 2005. They selected studies that reported CTC findings interpreted without knowledge of OC or surgery results. All patients in the studies had CTC after a full cleansing of the bowel, followed by either OC or surgery. Studies used a variety of techniques for CTC, including different types of scanners, different sizes of x-ray slices, and different computer software. The authors compared the sensitivity of CTC for detecting polyps of different sizes using OC or surgery as the reference standard.

The summary data showed that CTC detected only 48% of the small polyps (<6 mm in diameter) found by OC or surgery. CTC detected 70% of the intermediate-size polyps (6 to 9 mm) and 85% of the large polyps (>9 mm). These estimates varied widely among individual studies. A few studies that used thinner slices for x-ray pictures and more sophisticated technology (such as multidetector scanners, 3-dimensional imaging, and “fly-through” techniques) suggested that CTC detected 90% or more of all polyps.

Few studies involved adults with average risk for colorectal cancer. Most studies used OC as the standard for detecting polyps. OC may miss up to 10% of polyps, particularly smaller polyps. The small number of studies limited the authors' ability to explore reasons for the wide variations in individual study results.

CTC is a promising but variable technology for detecting polyps. Before recommending its general use, the authors urge researchers to identify techniques that lead to consistent, reliably sensitive results.