Colorectal cancer (commonly referred to as “colon” cancer) develops in the digestive tract from precancerous growths in the lining of the colon and the rectum. These growths are called polyps . When it comes to diagnosing colorectal cancer, most doctors agree that it is best to identify and remove all polyps six millimeters (mm) or more in diameter. Typically, this is done during a conventional colonoscopy . During this procedure, a doctor slowly inserts a lubricated, pencil-sized colonoscope through the rectum and into the colon. The colonoscope injects air into the colon while a small video camera allows the doctor to view the colon's lining.

At the moment, conventional colonoscopy is the best method for detecting colorectal cancer. However, many people avoid it because preparations for the procedure are unpleasant, it is invasive, and there are certain risks. In recent years, some studies have suggested computerized tomographic colonography (CTC), commonly called “virtual colonoscopy” may be a reasonable alternative to conventional colonoscopy. Virtual colonoscopy is a noninvasive procedure that allows a doctor to examine computer-generated images of the colon.

But, researchers asked, was virtual colonoscopy sensitive and specific enough to replace conventional colonoscopy as the standard of care for diagnosing colorectal cancer? To answer this question a group of researchers designed a study that would allow them to compare the two procedures. The results of this study were published in the April 14, 2004 issue of The Journal of the American Medical Association. The study found that while a step in the right direction, the current methods for virtual colonoscopy are not yet accurate enough to replace its conventional counterpart.

About the Study

The researchers enrolled 615 participants at least 50 years or older who had been referred by their doctors to undergo a colonoscopy. Each participant underwent a virtual colonoscopy and then a conventional colonoscopy two hours later. The researchers then compared the results of the two procedures to determine how closely their results resembled one another.

The goal of the study was to compare the sensitivity (the ability to detect colon polyps) and specificity (the ability to correctly identify only those polyps that may be precancerous or cancerous) of both virtual and standard colonoscopy.

The Findings

The researchers found that the sensitivity of virtual colonoscopy was 39% for detecting precancerous or cancerous polyps at least six mm in size and 55% for detecting precancerous or cancerous polyps at least 10 mm or greater in size. This was significantly lower than the sensitivity of conventional colonoscopy, which was 99% and 100%, respectively.

The specificity of the two procedures was much closer. Virtual colonoscopy correctly identified 90.5% of the participants who did not have polyps at least six mm in size, whereas conventional colonoscopy identified 100% of these participants. These results were similar for polyps greater than 10 mm.

How Does This Affect You?

The researchers concluded that currently available methods of virtual colonoscopy are not yet sensitive or specific enough to replace conventional colonoscopy as the standard of care for diagnosing colorectal cancer. These findings disagree with those of a number of other studies and are disappointing for virtual colonoscopy enthusiasts.

Why the discrepancy? One reason, according to the researchers may be that many of the previous studies were conducted at single research centers where the lead researcher was a highly experienced radiologist. This study was conducted at a number of centers, only one of which was highly experienced in virtual colonoscopy. However, in order for virtual colonoscopy to replace conventional colonoscopy, the technology must be applicable and accurate in multiple centers and with doctors who have varied experience with the technology.

Fortunately, virtual colonoscopy technology is still evolving. Scanners are getting more sophisticated and faster. Patients may one day be able to forgo the unpleasant aspects of colonoscopy without sacrificing its accuracy. There is even the possibility that the recognition of polyps could become automated. Of course, virtual colonoscopy could never completely replace traditional colonoscopy, which is still required to remove any polyps that are discovered. Nevertheless, a reliable and acceptable screening test for colorectal cancer is sorely needed. While virtual colonoscopy may ultimately become a promising candidate, it looks like we’ll need to endure the real thing a bit longer.