Computerized tomographic colonography (CTC), so-called virtual colonoscopy, proved accurate in locating colon polyps or cancers 10 millimeters or larger in a large study conducted in a number of community centers across the United States. The x-ray-based test found 90 percent of l0 millimeter polyps identified by traditional colonoscopy, which uses a lighted tube inserted into the rectum to view the colon.
Precancerous polyps as small as 5 millimeters (about the diameter of a pencil) were also found during the CTC exams. While the test was most sensitive for polyps that were 9 mm or larger, finding 90 percent of them, sensitivity for polyps 6 mm or larger was 78 percent.
Twenty-six hundred people over the age of 50 were screened for colorectal cancer during the National CT Colonography Trial. None had symptoms of colon or rectal cancer or risk factors other than their age.
Trial participants had CT colonography followed on the same day by an optical colonoscopy. The doctor performing the colonoscopy did not know the results of the CTC. About 17 percent of patients had a polyp 5 millimeters or larger and would have needed a follow-up colonoscopy to remove and test them.
CTC uses a computer program to assemble a 3-dimensional picture of the inside of the colon using a series of x-ray images. Colonoscopy views the entire colon directly with a small scope at the end of a long, flexible tube inserted into the rectum. Preparation for CTC is the same as for colonoscopy — a clear diet and laxatives to clean the colon the day before the test. Patients receive IV medicine to relax them and relieve any pain during colonoscopy, but sedation is not used for CTC.
If polyps are found during the CTC exam, a colonoscopy has to be scheduled to remove them and test them for cancer cells. Since all most all people in the trial were able to have both the CTC and colonoscopy scheduled for the same day, this was not a difficult problem. However, when polyps are found on CTC and a colonoscopy has to be scheduled on a different day to remove them, a second bowel prep will be necessary. Ideally, radiologists and gastroenterologists can work together so that colonoscopies, when necessary, can be done on the same day as the CTC.
In March of 2008, CT colonography was added to the recommended guidelines for colorectal cancer screening for average risk adults 50 and over to prevent colorectal cancer by finding and removing adenomatous polyps.
The National CT Colonography trial was conducted by the American College of Radiology Imaging Network (ACRIN), a cooperative group sponsored by the National Cancer Institute.
C. Daniel Johnson, MD and his colleagues in the ACRIN the trial concluded,
In this study of asymptomatic adults, CT colonographic screening identified 90% of subjects with adenomas or cancers measuring 10 mm or more in diameter. These findings augment published data on the role of CT colonography in screening patients with an average risk of colorectal cancer.
Research advocates were an important part of planning and carrying out the clinical trial. Pam McAllister, colon cancer survivor and long-time patient advocate with cancer cooperative groups, said,
As the ACRIN patient advocate who worked with the research team and as a research advocate with C3: Colorectal Cancer Coalition, I am pleased that CT colonography can be added to the list of screening options. Having a method that is accurate and comprehensive while being minimally invasive is needed if we are to succeed in substantially reducing deaths from colorectal cancers.
Answers to questions about the study are available from the National Cancer Institute.
SOURCE: Johnson et al., New England Journal of Medicine, Volume 359, Number 2, September 18, 2008.
Image of polyp from CT colonography courtesy of the National Cancer Institute.