A study in today’s issue of the Annals of Internal Medicine reports a miss rate of approximately 10 percent for large (>5mm) precancerous polyps during conventional colonoscopy.
CT colonography (also called virtual colonoscopy) was used as the reference standard to assess complete colonoscopy in this study.
This 10 percent miss rate is identical to the largest previous study to address this issue using optical colonoscopy as its own reference standard. (Rex et al. Gastroenterology 1997:112:24-8.) According to the author this earlier study, Dr. Douglas Rex, who is currently President of the American College of Gastroenterology, “It has been clear for some time that colonoscopy is not a perfect test. On the other hand, based upon the literature to date, colonoscopy is still the best test and the current gold standard for colorectal cancer screening and prevention.”
Dr. Rex explains, “Three studies have shown that colonoscopy prevents about 80 percent of colorectal cancers from developing by removing precancerous polyps. There is no evidence that any radiographic test, including CT colonography, which is also known as virtual colonoscopy, prevents the development of colorectal cancer.”
According to Dr. Rex, “Colonoscopy remains the most sensitive test available for detection of colorectal polyps and cancer.” The four largest studies of CT colonography have been published in the last year, and three of the four studies showed very poor results for CT colonography compared to colonoscopy. Only the study by Pickhardt et al. showed results for CT colonography comparable to complete colonoscopy, and the reasons for the discrepancies between studies remain uncertain. No guideline group has yet endorsed CT colonography as appropriate for colorectal cancer screening.
There are considerable radiation exposure issues that should be questioned by patients undergoing CT scans, especially for screening. The implications of this exposure over time clearly needs to be better determined, particularly for a screening test which will be repeated.