3D Virtual Colonoscopy as Sensitive as Conventional Test

CHICAGO—An advanced three-dimensional (3D) fly-through-reality form of virtual colonoscopy is an effective frontline screening tool for an average-risk, asymptomatic population, according to a prospective, multicenter trial. "It is accurate for finding clinically important polyps, and it is comparable in sensitivity to the accepted gold standard of conventional colonoscopy," Perry Pickhardt, MD, associate professor of radiology, University of Wisconsin, Madison, said at a press conference at the 89th Annual Meeting of the Radiological Society of North America (RSNA).

Virtual colonoscopy is generated from regular CT scans, obtained after insertion of a small flexible rectal catheter, with postprocessing done by specialized software. The 3D method of performing virtual colonoscopy in this study differs from techniques employed in previous investigations that involve two-dimensional (2D) or axial slice scanning for polyp detection, explained Dr. Pickhardt, who participated in the study when he was a researcher at the National Naval Medical Center, Bethesda, Maryland.

Virtual colonoscopy is especially useful in an average-risk screening population "because these are the people who are least likely to have a large polyp that would need to be removed," he said.

This study was performed at three hospitals, the National Naval Medical Center, the Naval Medical Center, San Diego, and Walter Reed Army Medical Center, Washington, DC. It was recently published in the New England Journal of Medicine (349:2191-2200, 2003).

In the study, a total of 1,233 asymptomatic patients underwent standard colon preparation and virtual and conventional colonoscopy on the same day. For the virtual colonoscopy, "no intravenous contrast was used and no intravenous sedation or pain control was needed," Dr. Pickhardt said.

The mean age of the patients was 58 years, which is slightly older than the recommended age for an initial screening colonoscopy. The vast majority of the patients in the study were at average risk for colon cancer; only 32 patients had a significant family history of colon cancer. The rate of completion of conventional colonoscopy was high—99.4%—"which means that the endoscopists involved were quite skilled," he said.

A total of 1,310 polyps were found in 622 patients. Most of the polyps (966) were diminutive in size—5 mm or less—and had no clinical significance; 344 polyps were 6 mm or larger, 210 of these were adenomas, and 2 were malignant. Of note, virtual colonoscopy identified both cancers, while conventional colonoscopy missed one.

The sensitivity of virtual and conventional colonoscopy, which was assessed on a per-patient and per-polyp basis, was similar for finding polyps at or beyond a 6-mm threshold. The sensitivity for virtual colonoscopy rose from 88.7% for 6-mm polyps to 93.9% and 93.8% for 8- and 10-mm lesions, respectively. The sensitivity for standard colonoscopy was 92.3%, 91.%, and 87.%, for 6-, 8-, and 10-mm lesions, respectively. The differences between the two examinations was not statistically significant.

Specificity of virtual colonoscopy was 96% for adenomas of at least 10 mm in diameter, 92.2% for those at least 8 mm, and 79.6% for those at least 6 mm.

An interesting sidelight from this study was the rate of false-negative findings for conventional colonoscopy. The previously reported miss rate for conventional colonoscopy, based on comparing results from back-to-back colonoscopies, is about 6%, but in this trial, when conventional colonoscopy was compared with virtual colonoscopy, the false-negative rate for conventional colonoscopy for 10-mm adenomas was 11.8%.

Dr. Pickhardt pointed out that conventional colonoscopy has "blind spots" and may miss polyps located on or behind a colonic fold, since it cannot visualize the entire surface. "With virtual colonoscopy, we’re not restricted by flight direction. We can turn around and find polyps behind the colonic fold," he said. "This illustrates the complementary nature of the two studies."

Virtual colonoscopy also resulted in extracolonic findings that were of potentially high clinical importance in 4.5% of patients, he said. Unsuspected extracolonic cancer was found in five patients: two lung cancers, one kidney cancer, one lymphoma, and one ovarian cancer.

Dr. Pickhardt in the near future hopes to institute a primary virtual colonoscopy screening program that will follow up patients who have clinically insignificant polyps in 5 to 10 years. "The intervals haven’t really been completely worked out, but patients with intermediate-sized polyps could be followed in a shorter time period; any polyp 10 mm or above would require immediate removal," he said. Findings from the study suggest that less than 10% of patients undergoing virtual colonoscopy would need conventional colonoscopy for polyp removal, he said.

Despite the promising results from the study, virtual colonoscopy for widespread screening faces a number of challenges, Dr. Pickhardt said. Radiologists need to be trained to use the new 3D method, and systems that can perform 3D virtual colonoscopy need to be more widely available. Also, radiologists will need to work closely with gastroenterologists in order to offer same-day polyp removal to patients who have large lesions. Finally, third-party reimbursement for the test will be required.