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Which Screening Test for Colon Cancer?

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Duke Health News 919-660-1306

The standard screening exam for colon cancer uses a flexible endoscope inserted in the rectum to look for early signs of disease. The newer 'virtual' colonoscopy procedure is faster and less invasive, but not yet as accurate.

Colon cancer is the second-leading cause of cancer deaths in the United States, accounting for almost 60,000 deaths per year. Early detection and removal of polyps (non-cancerous growths) on the inner wall of the colon and rectum can help prevent these growths from becoming aggressive cancers.

Physicians recommend that everyone be screened for colon cancer starting at age 50. There is some difference of opinion about how often screening should be performed, but many experts believe every five years is a good routine interval for individuals with no past history of disease.

There are currently several tests that try to detect polyps. The standard test is a colonoscopy, a screening exam that involves inserting and threading a flexible, lighted tube, an endoscope, through the rectum up to the colon. Images are then displayed and studied on a video monitor. The test is extremely accurate, but most patients find it uncomfortable and unpleasant.

A newer option is "virtual" colonoscopy, which combines a CT (computed tomography) scan and sophisticated computer software to generate high-quality 2-D or 3-D images of the colon. These images are then reviewed by a radiologist to look for abnormalities.

Although virtual colonoscopy is faster, more convenient and non-invasive, most patients choose the standard test. Dr. Don Rockey, a specialist in gastroenterology and hepatology at Duke University Medical center, says conventional screening is probably still the right choice for most patients.

"Conventional colonoscopy has been around for almost 30 years," says Rockey. "Using an endoscope inserted through the rectum, you can see the entire colon. You can identify cancers and polyps and blood vessels. You can take biopsies and remove polyps and even small cancers safely without major surgery."

Setting aside discomfort and potential risks, how do the two procedures compare? Rockey says results to date are somewhat inconclusive.

"There have been a series of studies, including three large studies that have had discrepant results. One of the studies showed that the virtual colonoscopy was just as good, in terms of accuracy, as the standard colonoscopy. The other two studies showed, however, that the virtual colonoscopy failed to achieve that same degree of accuracy. So right now, we're in a bit of a conundrum as to exactly where these stack up."

Another factor that leads many patients to choose the conventional method is that, if abnormalities are detected in a virtual colonoscopy, a second test, a conventional colonoscopy, will then have to be done to perform a biopsy or preventively remove a polyp.

"If you tell a patient, OK, if you have the virtual colonoscopy and we find something, you're going to have to have the other one anyway, a lot of patients will say, 'Well just let me have to colonoscopy to start with.'"

Rockey adds that most medical insurance plans currently do not pay for a virtual colonoscopy, except in certain circumstances.

"There are a few areas where it's covered, but for the most part, it is not covered as a standard screening test. It probably will be covered soon as a diagnostic test, for example for the patient who is too ill to have a conventional colonoscopy and needs to have a diagnostic examination of their colon."

"It's clear that conventional colonoscopy is very, very accurate," says Rockey. "What is not quite so clear is the precise accuracy of the virtual colonoscopy.

"My advice is that, for patients who are comfortable with the potential risks of colonoscopy and understand its capabilities and don't mind having IVs, that right now in 2005, colonoscopy is the more accurate test. For patients who are concerned about potential side effects, don't like to have IVs, don't want to have sedative medications, then a virtual colonoscopy is an option, understanding that it may not be quite as accurate.

"That may change. Virtual colonoscopy is undergoing great changes, and I expect that it will become better."

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