By Robert J. Mayer, MD

Q. I read that President Obama had a virtual colonoscopy. Is this the colon cancer screening test that everybody should be getting instead of a regular colonoscopy? I thought it was kind of experimental.

A. You're right about the president having a virtual colonoscopy (which is a nickname; the more formal term is computed tomography colonography). Virtual colonoscopy isn't experimental, but it hasn't quite entered the mainstream, either. Experts have differing opinions about when and how it should be used. The American Cancer Society and other groups have endorsed it as an option for colon cancer screening, but the U.S. Preventive Services Task Force decided in 2008 that there wasn't enough evidence to recommend it. In 2009, the federal Medicare program followed the lead of the task force and decided not to cover virtual colonoscopy for routine screening.

A regular colonoscopy is done with a colonoscope, a flexible fiberoptic cable with a video camera on its tip. A cathartic -- the dreaded "bowel prep" -- is necessary before a regular colonoscopy because the colon needs to be free of any fecal matter for the polyps to be reliably seen. Patients need to be sedated during the exam.

Virtual colonoscopy is also a visual inspection, but the images come from a computerized tomography (CT) scanner. The colon has lots of folds, so to produce clear CT images, the colon must be inflated with a little bit of carbon dioxide. The gas is pumped into the colon through a tube inserted briefly into the rectum. Occasionally people find this part of the test, called insufflation, uncomfortable. Usually two scans are done: one with you lying on your back and the other lying on your front. The scan itself takes just a few minutes.

Like a regular colonoscopy, a virtual one requires an empty colon, so the bowel prep is, unfortunately, not virtual. But you don't need to be sedated during the test, which may be the reason the president opted for virtual colonoscopy.

The main advantages of virtual colonoscopy include its brevity, the lack of sedation, and it being less invasive than regular colonoscopy. Virtual colonoscopy eliminates any chance of bowel perforation, a rare but very real risk of regular colonoscopy. It also costs less than a regular colonoscopy.

The drawbacks? If a polyp is found and needs to be removed, the patient has to get a regular colonoscopy. Virtual colonoscopy misses small polyps (those smaller than 1 centimeter) that a regular colonoscopy will typically find, although there's a lot of debate about whether small polyps are worth finding, because they're less likely to become cancerous. Partly because it misses those smaller polyps, virtual colonoscopy needs to be done more often than regular colonoscopy -- every five years instead of every 10.

CT scanning uses x-rays to create images, so virtual colonoscopy does result in radiation exposure of about 6 to 10 millisieverts per exam, which is roughly 100 times the radiation exposure you'd get from a standard chest x-ray. That sounds like a lot, and radiation exposure from medical imaging has become an issue. But I think the health risk from virtual colonoscopy is probably quite small, given the number of exams and the age of the population (radiation is inherently less risky for older people because their cells aren't dividing as much).

Virtual colonoscopy sometimes finds abnormalities outside the colon. These "extracolonic" findings can lead to early detection and treatment of important, unrelated medical problems -- a plus, not a minus. But there's also a chance that an incidental finding will unleash unnecessary testing and treatment.

But let's not lose sight of the big picture. The absolute number of deaths from colon cancer has dropped by 13 percent in the U.S. since 1990. That largely reflects the growing number of Americans getting screened for the disease. I don't think virtual colonoscopy is for everybody, but it's a good test and is bound to get better. And if having virtual colonoscopy as a choice means more Americans get screened for colon cancer, more lives will be saved.

-- Robert J. Mayer, M.D., Dana-Farber Cancer Institute, Boston, MA


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Health - Colonoscopy: Is It Time to Go Virtual Colonoscopy