Should computed tomographic colonography replace optical colonoscopy in screening for colorectal cancer?

Clinical evidence amassed over the last several decades indicates that routine colorectal cancer (CRC) screening, compared to no screening, detects CRC at an earlier stage, reduces the incidence of CRC or the progression early CRC through polypectomy, and reduces CRC mortality. Computed tomographic colonography (CTC) is a minimally invasive, structural evaluation of the entire colorectum that has recently been advocated by multiple American professional medical societies as an effective alternative for CRC screening. The potential advantages of CTC, including rapid image acquisition and processing, non-invasiveness, and decreased procedural risks of perforation, bleeding, and sedation complications may serve to improve the low rates of colorectal cancer screening that are currently observed in our society. Several large studies of CTC as a CRC screening test have reported excellent results but have been criticized because of the expertise of CTC interpreters participating in those trials. As a response to these criticisms, the long-awaited results of the American College of Radiology Imaging Network (ACRIN) National CT Colonography Trial were recently published. The purpose of this study was to assess the accuracy of CTC in a "community based" environment to determine if previous results obtained at expert sites could be replicated. All CTC were confirmed and compared to conventional colonoscopy, the gold-standard colorectal cancer screening test. For polyps >10 mm, the results obtained in the ACRIN trial were comparable to previous studies with a mean CTC sensitivity of 90% and a mean CTC specificity of 86%. The sensitivity of CTC fell to 78% for lesions >6 mm, a value that some studies have suggested is comparable to the detection rate of conventional colonoscopy. This study adds to the body of literature regarding the efficacy of CTC and will likely be cited by many as evidence supporting CTC as an acceptable CRC screening test, in the same league as colonoscopy. Issues remain, however, regarding the extension and reproducibility of these results in the true community setting. There are concerns regarding thresholds for referrals, appropriate intervals between studies, the optimal management of extracolonic findings, and radiation exposure with CTC that remain unanswered by these data.