AGA Institute statement: Data support CT colonography as viable colorectal cancer screening option

September 18, 2008

Death from colorectal cancer is highly preventable with effective screening and early detection. Many screening options are available, each with advantages and disadvantages, but half of eligible patients still do not participate in colorectal cancer screening. For that reason, a goal of the American Gastroenterological Association (AGA) Institute is to increase colorectal cancer screening rates and improve public health.

The AGA Institute considers colonoscopy the definitive test for colorectal cancer screening and prevention. Colonoscopy is the only test that can both detect cancer at an early curable stage and prevent cancer by removing pre-cancerous polyps. Data published in the Sept. 18, 2008, issue of the New England Journal of Medicinei suggests that computed tomographic (CT) colonography may be another acceptable technology for colorectal cancer screening.

The availability of CT colonography could increase screening rates in eligible patients (i.e. asymptomatic patients over the age of 50) who have not participated in other colorectal cancer screening procedures. However, a number of important questions need to be addressed:

* Does CT colonography find all possible cancers? These study results showed that CT colonography detected 90 percent of large polyps (>10 mm), a rate on par with traditional colonoscopy. However, CT colonography was less sensitive for small polyps (5 mm to 9 mm), with detection rates as low as 65 percent (5 mm). These small polyps were not removed. It is not clear that leaving small polyps is safe; there are no long-term, adequately controlled studies on the subject. The need to define the natural history and biological significance of small polyps is central to refining colorectal cancer screening, irrespective of modality. This study did not investigate diminutive polyps (<5 mm) and the ability of CT colonography to detect flat lesions remains unanswered.

* Will CT colonography be accurate in all settings? Study investigators were highly trained in CT colonography, perhaps increasing the accuracy rate of this test. Standardized, rigorous training and proper technique are essential to ensuring that CT colonography achieves appropriate sensitivity, specificity and performance.

* Do patients understand the pros and cons of the available colorectal cancer screening tests? Despite the perception that CT colonography is less invasive than traditional colonoscopy, it requires similar bowel preparation as for a colonoscopy. Patients need to understand that a prep is required for CT colonography, and if a polyp is found, it must be removed through a subsequent colonoscopy. Only colonoscopy can prevent colorectal cancer, by removing pre-cancerous polyps.

Patients also need to understand that for most people colorectal cancer screening is not a one-time event and that interval examinations are recommended. According to a study by Thomas F. Imperiale et al.ii, published in the same issue of NEJM, a screening interval of five years or longer is appropriate in patients with a normal colonoscopic exam. Because small (< 5 mm) polyps are not typically reported on CT colonography, a negative exam on CT may not be equivalent to a negative colonoscopy. The optimal interval between CT colonography examinations has yet to be defined.

* What are the radiation risks? The potential for harm from radiation is difficult to assess given the uncertainty of true risks from low levels of radiation exposure. However, the ionizing radiation exposure from a single abdominal or chest CT may be associated with elevated risk for DNA damage and cancer formation. The rate of radiation exposure with CT colonography may depend on the machine used and the type of CT colonography (2-D versus 3-D imaging) performed; therefore the methodology for screening must be standardized. For more information about the risk of radiation exposure, read the NIH fact sheet, "What We Know About Radiation."

Guided by the principle that gastroenterologists are ideally suited to manage patients with gastrointestinal disorders and that they should be able to utilize any technology that can enable them to provide better patient care, the AGA Institute has taken a leadership role with technologies such as CT colonography. With the best interest of patients in mind, the AGA has monitored this technology, created training standards and will continue to educate those gastroenterologists who wish to provide this procedure to patients.

More information about the AGA Institute's work related to CT colonography reimbursement, training and standards can be found at www.gastro.org/ctc.

Resources for patients on CT colonography, colonoscopy and colorectal cancer can be found at www.gastro.org/patient.

American Gastroenterological Association

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