CT Colonography: Where Is It Now?

Because of their specialized training, gastroenterologists are experts in colorectal cancer screening (CRC) and colorectal disease. Gastroenterologists should be able to translate their knowledge of the endoscopic appearance of colorectal disease to CT colonography (aka virtual colonoscopy), an emerging and increasingly important CRC screening modality and this requirement is now included in the GI Core Curriculum.

The practice of CT colonography has evolved over the last 5 years with recent large, methodologically rigorous clinical trials demonstrating that CT colonography has accuracy (for polyps ≥ 6 mm) comparable to that of colonoscopy. 1-4 By extension, it is reasonable to expect the results of CRC screening with CT colonography are superior to non-colonoscopic methods of CRC screening and several of the CT colonography trials have actually shown that to be the case in their selected populations.

However, the question of whether or not CT colonography is ready for “prime time” as a screening test remains unanswered. While the accuracy appears to be in place, there are multiple issues that have proven contentious regarding CT colonography, such as the unknown significance of missed diminutive polyps, the unknown risk of small doses of radiation from the CT scan, and the management and costs associated with extracolonic findings from the CT scan.

In May 2009, the Centers for Medicare and Medicaid (CMS) issued a non-coverage Decision Memo for Screening CT colonography for Colorectal Cancer, stating that the evidence is inadequate to conclude that CT colonography is an appropriate CRC screening test under §1861(pp)1 of the Social Security Act. 5 CMS cited several reasons for their non-coverage decision. Since CT colonography cannot reliably detect polyps < 6 mm, the impact of these polyps in the intervening screening interval of CT colonography is unknown at this point. As the percentage of participants with extracolonic findings ranged from 58% to 66% and since individuals undergoing screening are asymptomatic by definition, the potential impact of extracolonic findings on health outcomes needs to be determined prior to general use of this modality, according to CMS.

A contrasting opinion was offered in August 2009, when the Blue Cross/Blue Shield Technology Evaluation Center (TEC) released an analysis of CT colonography 6 in which they concluded that CT colonography meets the criteria to be considered an effective CRC screening test. A 90% sensitivity of CT colonography for detection of polyps 10 mm or larger is consistent with an improvement in health outcomes due to detection and removal of precancerous lesions. The 86% specificity of CT colonography would result in some false-positive tests which, in turn, would result in some unnecessary follow-up colonoscopies. However, compared with optical colonoscopy, there are several other types of health outcomes that may differ in terms of convenience, cost, detection of unrelated health problems, and radiation exposure. These are difficult to quantify and are probably small in magnitude compared to the health benefit of identifying and removing cancer precursors.

The future of CT colonography remains uncertain. There currently are centers where this modality has been deployed with great success. Whether or not these models can be replicated in private practice gastroenterology settings remains to be seen, but the concept of gastroenterologists offering CT colonography will likely be increasingly common over the next several years.

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