Enhanced imaging fails to shine
Colonoscopy remains the gold standard test for reducing the risk of colorectal cancer, but its effectiveness depends on the early detection and removal of adenomatous polyps. Studies show wide variations in the adenoma detection rate (ADR), which ranges from 7 to 44 percent among experienced endoscopists. Not surprisingly, patients evaluated by endoscopists with higher detection rates — greater than 20 percent — have lower rates of colorectal cancer.
Colonoscopy also has a significant miss rate for flat lesions, which account for 10 to 15 percent of all polyps and are more likely to be malignant.
To improve adenoma detection, endoscopists now use enhanced imaging modalities, including chromoendoscopy, high-definition white light endoscopy (HD-WLE) and high-definition narrow band imaging (HD-NBI).
Compared with standard white light endoscopy (SD-WLE), high-definition colonoscopes greatly enhance image quality; chromoendoscopy and narrow band imaging improve visualization of surface details and capillary patterns. Although chromoendoscopy, a technique that stains the colon with dye, has shown up to a fourfold increase in the detection of colonic neoplasia, studies comparing HD-WLE and HD-NBI have shown conflicting results.
So researchers at Mayo Clinic in Arizona performed a meta-analysis of randomized controlled trials comparing HD-WLE and HD-NBI in patients undergoing surveillance or screening colonoscopy. The aim was to compare the yield and miss rates of the two techniques for the detection of colon polyps and adenomas.
An extensive literature search found nine studies comparing HD-NBI and HD-WLE, with 3,059 patients. Three were randomized controlled studies with tandem colonoscopies — that is, each patient received two colonoscopies, one with white light and one with NBI.
Lead author Shabana F. Pasha, M.D., says the results of the meta-analysis were surprising. "One of the main goals of NBI is to improve ADRs over white light colonoscopy. But we found absolutely no difference between the two techniques for the detection of adenomas," she explains.
She adds that HD-NBI showed no benefit for the detection of polyps, flat adenomas and small adenomas compared with HD-WLE and no benefit for lowering the miss rate of polyps and adenomas.
Several factors may account for these results. Dr. Pasha explains that HD-NBI and HD-WLE appear to have higher adeneoma detection rates than SD-WLE. "We may infer that the high-definition component increases yield of polyps and adenomas and that's why we do not see an added benefit with NBI," she says.
Another finding is the potential learning effect of NBI on WLE. Dr. Pasha cites two studies in which an initial higher yield with NBI compared with HD-WLE significantly decreased with the number of colonoscopies performed. Thus, the overall lack of benefit of NBI in the meta-analysis may have been due to the learning effect over time.
Dr. Pasha concludes, "What we learned from this study is that HD-NBI is not superior to HD-WLE for adenoma detection, but the use of high-resolution colonoscopes may increase the yield and lower miss rate of adenomas compared with standard colonoscopes."
She adds that the meta-analysis eliminates the controversy surrounding enhanced imaging and puts it in the proper perspective. She explains, "The modality you use may not be as important as a careful exam and adequate withdrawal time."