More than 56,000 Americans die from colorectal cancer each year. In fact, colorectal cancer is the second leading cause of cancer-related deaths in the United States. But colorectal cancer is largely preventable. Screening tests, which are recommended for all people age 50 and over, can detect cancer before it develops or in its early stages. Early detection can dramatically increase the chance of survival. The problem is, only a third of Americans age 50 and over report having had a colorectal cancer screening test.

Colonoscopy is the current gold standard for colorectal cancer screening. It detects more than 90% of colorectal cancers. But colonoscopy is invasive, uncomfortable, inconvenient, and not without risks.

Would more people get screened if a less invasive, but equally reliable, test were available? In 1994, researchers introduced virtual colonoscopy, a minimally invasive test that uses a series of x-ray images to construct a virtual video of the colon. Like conventional colonoscopy, virtual colonoscopy requires bowel-cleansing preparation, but it does not require sedation and only takes 10-15 minutes. Since previous research on its effectiveness has produced mixed results, virtual colonoscopy is not yet widely used in clinical practice.

A new study in the December 4, 2003 issue of the New England Journal of Medicine found that state-of-the-art virtual colonoscopy was as accurate as conventional colonoscopy in detecting precancerous polyps . In addition, most people in the study reported that virtual colonoscopy was more convenient than conventional colonoscopy.

About the Study

This study included 1,233 people between the ages of 40 and 79. People with symptoms and signs suggestive of colorectal cancer were excluded from the study.

The participants were given a virtual colonoscopy and a conventional colonoscopy on the same day. Twenty-four hours before the colonoscopies, the participants underwent standard preparation: a bowel-cleansing preparation and a clear liquid diet. In addition, the participants consumed a solution that marked any residual stool so it could be digitally removed in the virtual colonoscopy for better viewing of the colon.

During the virtual colonoscopy, the participants held their breath while a small, flexible catheter was placed into their rectum to take x-ray images. A software program used these images to generate two- and three-dimensional virtual videos of the participants’ colon. A radiologist examined the three-dimensional video for abnormalities, used the two-dimensional video to confirm suspected abnormalities, and measured any polyps that were found.

During the conventional colonoscopy, a colonoscopist (gastroenterologist or colorectal surgeon) inserted a video colonoscope into the colon and directly observed it, segment-by-segment, for abnormalities. If polyps were found, they were photographed and measured. After each segment was examined, the colonoscopist was informed of any polyp that was detected in the virtual colonoscopy but not seen during the conventional colonoscopy. The colonoscopist then re-examined the segment to determine whether the polyp did indeed exist.

After the colonoscopies, the participants completed a questionnaire about their experiences with virtual and conventional colonoscopy.

The researchers compared the results of the virtual and conventional colonoscopies with the final result of the conventional colonoscopy (including any reexamination) to determine the accuracy of the screening procedures in detecting precancerous polyps.

The Findings

On average, the virtual colonoscopy took 14 minutes, while the conventional colonoscopy took 96 minutes (including recovery time after sedation). Although some extra time was required for reexamining segments of the colon during the conventional colonoscopy, the researchers said this was likely a minor contributor to the total time. Virtual colonoscopy required about 20 additional minutes for interpretation by a radiologist.

During the course of the study, 554 precancerous polyps were identified. Only two of these polyps were cancerous. Interestingly, both of these cancerous polyps were identified by virtual colonoscopy, but one was missed during conventional colonoscopy.

The virtual colonoscopy was just as accurate as the conventional colonoscopy in finding precancerous polyps. Virtual colonoscopy detected 94% of polyps at least eight millimeters in size, while conventional colonoscopy detected 92% of these polyps.

Although 54% of the participants reported greater discomfort associated with the virtual colonoscopy, 68% rated the virtual colonoscopy as more acceptable than the conventional colonoscopy.

How Does This Affect You?

These results suggest that virtual colonoscopy is an effective screening test for colorectal cancer. Does this mean virtual colonoscopy will soon be the standard for colorectal cancer screening? Perhaps. Virtual colonoscopy is less invasive, quicker, more convenient, and safer than conventional colonoscopy.

In the future, virtual colonoscopy may even surpass conventional colonoscopy in its accuracy of detecting precancerous colon polyps. Since x-ray images are used and residual stool can be digitally removed in virtual colonoscopy, radiologists can detect “hidden” polyps that may be missed in conventional colonoscopy. In this case, the cancerous polyp that was detected by virtual colonoscopy but initially missed by conventional colonoscopy was located on a fold in the colon.

The major drawback of virtual colonoscopy, however, is that polyps cannot be removed during the procedure. In conventional colonoscopy, the physician can remove polyps during the procedure if necessary. The authors of this study suggest a remedy to this drawback: set up colon-screening centers that offer virtual colonoscopy to patients, with the opportunity of having a conventional colonoscopy with the removal of polyps if needed. This process would save people from having to prepare for colonoscopy more than once.

Before virtual colonoscopy becomes the standard colorectal screening tool, physicians must establish accompanying clinical guidelines. In other words, what size polyp warrants immediate removal? What size polyp should just be monitored closely? Once these questions are answered, virtual colonoscopy may very well become the colorectal cancer screening tool of choice.