LOS ANGELES—Computed tomographic (CT) colonography, sometimes referred to as virtual colonoscopy, is as accurate as standard optical colonoscopy for detecting large polyps but is inferior to it in detecting medium-sized and small polyps, said presenters at Digestive Disease Week 2006.
The natural history of medium-sized polyps must be determined before the utility of CT colonography in colon cancer screening can be established. Furthermore, extra-colonic abnormalities are found in nearly 1 in 5 patients who undergo CT colonography, increasing the indirect costs.
Therefore, for the moment, experts are not endorsing the routine use of CT colonography for colorectal cancer screening.
An analysis of 666 asymptomatic patients found that CT colonography was 93.6% accurate in detecting polyps ≥10 mm, said Brooks D. Cash, MD, chief, Gastroenterology Division, and director of clinical research, National Naval Center, Bethesda, Md.
The accuracy dropped to 84.1% for detecting polyps 6 to 9 mm, although this accuracy was better than that found in previous studies of CT colonography, he said. The lower limit of polyp detectability was 6 mm.
Another study presented at the meeting found that most colonic polyps detected by optical colonoscopy were <5 mm in size.
“The bulk of polyps in the colon are quite small,” said Douglas K. Rex, MD, lead investigator of this study and professor of medicine, Indiana University, Indianapolis. Of 10,780 polyps removed during consecutive colonoscopies, 78% were <5 mm, and about 90% were <10 mm. The significance of polyps <10 mm has yet to be determined.
“If we made the cutoff 5 mm [for removal], we would be leaving 80% of precancerous polyps in place, and if 6 mm, we would be leaving 90% of precancerous ones in place,” he said. “CT colonography depends on leaving the small polyps in place. To be cost-effective, you have to limit the number of patients who go to colonoscopy. This might be okay, and it might not, because we don’t know the natural history of small polyps.”
The current strategy for colorectal cancer prevention is to resect all neoplasms detected on optical colonoscopy, he noted.
Cost comparisons between CT colonography and optical colonoscopy are difficult, because if CT colonography detects a polyp that needs to be removed, the patient will still need a colonoscopy, said Jacques Van Dam, MD, PhD, professor of medicine, Stanford University Medical Center in California.
In addition, follow-up screening in 5 years is recommended after a normal exam on CT colonography, whereas a 10-year interval is recommended for patients with negative findings on optical colonoscopy, further clouding cost comparisons, said Darren Schwartz, MD, of the University of Wisconsin in Madison.
When 6- to 9-mm polyps are found on CT colonography, patients are usually asked to come back in 2 to 3 years for follow-up, said Dr Rex.
The CT colonography screening program in place in Madison, Wis—the only region in the United States where colorectal cancer screening with CT colonography for average-risk patients is reimbursed by third-party payers—has not reduced the number of optical colonoscopies with polypectomy over 14 months, reported Dr Schwartz.
The cost of additional evaluation for extra-colonic findings on CT colonography must also be factored into cost comparisons. In a study by Dr Cash, 16% of 979 asymptomatic, average-risk patients had extra-colonic findings on CT colonography, most commonly noncalcified pulmonary nodules and coronary artery calcifications (Table).
“The additional cost from these extracolonic findings was $33 per exam,” he said.