Virtual Colonoscopy Identifies Large Polyps
Results from the American College of Radiology Imaging Network (ACRIN) National CT Colonography Trial, published in the September 18 New England Journal of Medicine, show that computed tomography (CT) colonography - also known as virtual colonoscopy - can detect 90 percent of adenomas (noncancerous tumors that can progress to cancer) or colorectal cancers measuring 1 centimeter or more in diameter.
These results compare favorably with standard optical colonoscopy, "which misses roughly 8 to 10 percent" of lesions of this size, says Dr. Carl Jaffe, chief of the NCI Cancer Imaging Program's Diagnostic Imaging Branch.
Both CT colonography and colonoscopy can be employed to screen for precancerous polyps, the removal of which helps prevent the development of colorectal cancer. While colonoscopy uses a thin, tube-like instrument to physically examine the inside of the colon and rectum, CT colonography takes 2- or 3-dimentional pictures of the colon and rectum using a high-powered x-ray machine linked to a computer. The computed tomography technology required for CT colonography is already found in almost all hospitals, explains Dr. Jaffe.
Investigators at 15 hospitals participated in the ACRIN trial and enrolled more than 2,500 participants aged 50 or older who were scheduled for a screening colonoscopy. Participants first underwent CT colonography followed by standard colonoscopy, which was performed on the same day for 99 percent of participants.
The investigators compared results from the CT colonography exam to colonoscopy results for each patient, for the detection of lesions 5 millimeters or more in diameter, and calculated the false negative and false positive rates for CT colonography - the likelihood of missing a lesion (false negative) or falsely identifying a lesion that could not be found on follow-up colonoscopy (false positive).
While CT colonography could correctly identify 90 percent of people who had at least one polyp 10 millimeters in diameter or greater, the ability to correctly identify people who had smaller polyps was lower - down to 65 percent for polyps 5 millimeters in diameter.
Some additional questions about CT colonography remain to be answered, explains Dr. Robert Fletcher, professor emeritus from Harvard Medical School, in an accompanying editorial; in particular, whether CT colongraphy may miss some flat or depressed adenomas and what cumulative radiation dose may result from exposure to regular CT scans for screening.
The difficulty with flat and depressed adenomas, says Dr. Kramer, is that "we don't really yet know their natural history. We don't know how important it is to catch every last one," he explains. Both CT colonography and colonoscopy techniques need to be refined to better detect this type of adenoma.
Dr. Jaffe does not think that the radiation exposure, which is less than a standard CT scan, will be a large concern in this population. "For people over 50, the risk becomes diminutive, and if you use newer CT techniques, you can keep the exposure relatively low," he says.
For him, the most exciting thing about CT colonography is the ability to potentially reserve colonoscopy - which is more expensive and invasive, and currently requires specialists who are in short supply in the United States - for patients who have identified polyps, "so that [gastroenterologists] can concentrate on cases that merit polyp removal," he concludes.