LOS ANGELES — Referral of patients with adenomas 6–9 mm in diameter for optical colonoscopy remains an area of contention, Dr. David A. Lieberman said at the annual Digestive Disease Week.
Computed tomography colonoscopy (CTC) may miss 40% of lesions of that size, and 7%–20% of the lesions will be at an advanced stage when eventually identified with optical colonoscopy (OC), said Dr. Lieberman, chief of gastroenterology at the Veterans Affairs Medical Center in Portland, Ore.
CTC and OC have comparable rates of identification of adenomas larger than 9 mm in diameter. Using CTC in tandem with OC, some investigators have shown that CTC can identify adenomas measuring 6–9 mm with a sensitivity of 94% and specificity of 96%. Other researchers using CTC, however, have found similar sensitivities, but specificities in the 55%–60% range. The differences may be accounted for by interobserver variability and the difficulty of detecting flat adenomas.
Dr. Lieberman reviewed the natural history of adenomas, 70% of which are tubular and less than 1 cm in diameter when found at screening. There is some epidemiologic evidence that patients with one or two tubular adenomas have a lower risk of progressing to cancer than patients with more adenomas. Polyps less than 9 mm in diameter that are left in place remain stable in 25% of cases, regress in 35% of cases, but progress in 40% of cases, particularly if they are smaller than 5 mm. Polyps 3–9 mm in diameter grow approximately 0.58 mm/year, and the average polyp grows from 6.4 mm to 7.3 mm within 2 years (Am. J. Gastroenterol. 1997;92:1117–20).
Little is known of the natural history of diminutive polyps (those less than 5 mm in diameter), and the availability of CTC offers researchers a chance to study them. On the basis of recent research, diminutive polyps will develop high-grade dysplasia in up to 2.3% of cases and become cancerous in up to 1.5% of cases. Given this prevalence, Dr. Lieberman asked meeting attendees to consider whether it is justifiable to ignore diminutive polyps and to defer colonoscopy when patients are known to have 6- to 9-mm polyps.
Because the risk of conversion into high-grade dysplasia is directly associated with polyp size, many physicians and patients demand colonoscopy for polyps larger than 5 mm in diameter. The clinician's dilemma is whether to suggest immediate OC to patients with diminutive polyps or to repeat CTC—and, if the latter, at what interval. Cost issues for clinicians providing CTC on demand include establishing a threshold for what constitutes a positive test, who gets referred for OC, and how to handle the expense of evaluating extracolonic findings.
If a large proportion of CTC-evaluated patients get referrals to optical colonoscopy, CTC will not be cost-effective. “We in the GI community are going to have to wrestle with the issue of who gets colonoscopy,” Dr. Lieberman told meeting attendees.
The management of diminutive polyps is controversial because the risk of advanced neoplasia is low, and the guidelines developed by the U.S. Multi-Society Task Force on Colorectal Cancer and the American Cancer Society recommend ignoring diminutive polyps. However, “the strategy of not reporting adenomas smaller than 6 mm may lead to medicolegal problems,” Dr. Lieberman said.