Accuracy of CT Colonography (Virtual Colonoscopy) for Detecting Polyps

Summaries for Patients are a service provided by Annals to help patients better understand the complicated and often mystifying language of modern medicine.

Summaries for Patients are presented for informational purposes only. These summaries are not a substitute for advice from your own medical provider. If you have questions about this material, or need medical advice about your own health or situation, please contact your physician. The summaries may be reproduced for not-for-profit educational purposes only. Any other uses must be approved by the American College of Physicians.

The summary below is from the full report titled "Meta-Analysis: Computed Tomographic Colonography." It is in the 19 April 2005 issue of Annals of Internal Medicine (volume 142, pages 635-650). The authors are B.P. Mulhall, G.R. Veerappan, and J.L. Jackson.

What is the problem and what is known about it so far?

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.

Why did the researchers do this particular study?

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

Who was studied?

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.

How was the study done?

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.

What did the researchers find?

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.

What were the limitations of the study?

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.

What are the implications of the study?

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.