CT Colonography Trial's Findings Raise Questions for Patients

Results of a U.S. National Computed Tomography (CT) Colonography Trial revealed improvements in the technology's ability to diagnose intermediate- to large-sized polyps in the colon, but this method of testing is not as effective in diagnosing small polyps.

CT colonography is one of several colorectal cancer screening options. The American Society for Gastrointestinal Endoscopy (ASGE; Oak Brook, IL, USA) encourages patients to speak to their doctor about the screening method that is best for them. "Colorectal cancer is largely preventable and curable when diagnosed in its early stages. Screening saves lives. Unfortunately, far too few people undergo screening for colorectal cancer. Any advances that result in increased screening of the population are encouraging,” said ASGE president John L. Petrini, M.D., FASGE. "The results of this trial may prompt those who otherwise would have avoided being tested, to get screened for colorectal cancer. With so many different options to screen for colorectal cancer, it is important to talk to your doctor and discuss which screening method is best for you. Each screening option has appropriate applications and limitations.”

CT colonography, also referred to as virtual colonoscopy, relies on a computer program to generate a three-dimensional (3D) image of the large intestine (colon) using X-ray images. This differs from colonoscopy, which provides actual images of the colon and rectum via a camera attached to a thin, flexible scope that is physically guided through the large intestine.

The results of the National CT Colonography Trial, published in the September 18, 2008, issue of the New England Journal of Medicine (NEJM), have shown some improvements in the ability of CT colonography to diagnose intermediate- to large-sized polyps. Complete data for 2,531 participants at 15 U.S. centers showed that CT colonography identified large colorectal adenomas (polyps most likely to become cancerous) or cancers 10 mm or larger that were detected by colonoscopy in nine out of 10 asymptomatic patients. The study also demonstrated that CT colonography had a lower sensitivity for smaller colorectal lesions of 6 to 9 mm in size.

Researchers also reported that the sensitivity for the detection of adenomas or tumors greater than or equal to 5 mm, 6 mm, 7 mm, 8 mm, and 9 mm was 65%, 78%, 84%, 87%, 90%, respectively. If all patients with a lesion measuring 5 mm or more on CT colonography were to be referred for colonoscopy, the colonoscopy-referral rate based on these study results would be 17%. Study participants were asymptomatic, 50 years of age or older, and were scheduled to undergo routine colonoscopy. Patients were excluded if they had lower abdominal pain, inflammatory bowel disease, or familial polyposis syndrome among other exclusionary criteria.

There are, however, concerns about CT colonography's failure to detect small lesions and inability to remove lesions in the colon. Although small polyps often are not cancerous, they can be adenomatous polyps and have a risk of developing into cancer. If a polyp is detected through CT colonography, the patient must subsequently undergo a separate colonoscopy to remove the polyp. Most likely, this would happen on another day and the patient would require a second bowel prep before undergoing the colonoscopy.

The primary end point of this study was detection by CT colonography of large adenomas or cancers 10 mm or larger. The radiologists making the interpretations were instructed to record only lesions measuring 5 mm or more in diameter. It has been suggested that patients with small polyps may be followed by CT colonography at shorter intervals, without referral for colonoscopic removal. This approach of leaving polyps behind has not been tested and it is important to recognize that even small polyps can infrequently harbor early or advanced cancer. Follow-up CT examinations will also expose patients to cumulative doses of radiation. Data published in the New England Journal of Medicine (NEJM) on November 29, 2007, pointed to the potential dangers of radiation exposure over an individual's lifetime from diagnostic medical testing using CT scans. Articles in Time (June 27, 2008) and the New York Times (June 29, 2008) also noted the radiation risks of CT scans. CT colonography could negatively affect the cost-effectiveness of colorectal cancer screening, as each positive CT colonography will require a subsequent colonoscopy.

"ASGE supports colonoscopy for colorectal cancer screening because it offers the advantage of allowing for detection and prevention through the removal of polyps during the same procedure, without unnecessary radiation exposure. Colonoscopy also has a high detection rate for polyps of all sizes and is the only method that allows us to remove polyps before they turn into cancer,” said Dr. Petrini.

According to a study released in October 2007 from the U.S. Centers for Disease Control and Prevention (Atlanta, GA, USA) and the American Cancer Society (Atlanta, GA, USA), colorectal cancer deaths decreased nearly 5% between 2002 and 2004, more than the other major cancer killers (prostate, breast, lung). Among the key factors responsible for the decline was prevention through screening and the removal of precancerous polyps. This is excellent news and reinforces the importance of colorectal cancer screening beginning at age 50, or even younger if there is a family history of colorectal cancer or polyps.

"CT colonography may be best in low risk patients who cannot undergo colonoscopy or who have had an incomplete colonoscopy due to various factors,” said Dr. Petrini. "Colorectal cancer prevention is a major priority for ASGE, by whichever form it is achieved. Not enough people are getting screened for colorectal cancer and opening a dialogue about screening between the physician and patient is crucial in preventing this disease. Therefore, we will continue to follow developments in all modes of colorectal cancer screening and to critically evaluate which methods are best for patients.”

Founded in 1941, the mission of the American Society for Gastrointestinal Endoscopy is focused advancing patient care and digestive health by promoting excellence in gastrointestinal endoscopy.

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