MONDAY, Sept. 12 (HealthDay News) -- Whether the most technologically advanced way to check for colon cancer will become the standard screening method of the future does not appear to be a slam-dunk.

The method, known as virtual colonoscopy, combines X-ray and computer technology to create three-dimensional views of the full length of the colon, the large intestine. It allows doctors to look for polyps, or pre-cancerous growths, or other signs of cancer or other intestinal disease. According to the U.S. National Cancer Institute, virtual colonoscopy can be done with computed tomography (called a CT or CAT scan) or with magnetic resonance imaging (MRI).

Colon cancer is one of the few types of preventable cancer, with doctors able to find and remove pre-cancerous polyps in the colon before cancer can develop. The current "gold standard" procedure for colon cancer screening, however, is colonoscopy, a time-consuming procedure for which preparation is unpleasant and sedation is necessary.

Perhaps because of this, only half of all people older than 50 have gotten this potentially life-saving test for colon cancer, according to the U.S. Centers for Disease Control and Prevention.

Doctors who tout the virtual form of colonoscopy argue that it takes less time and does not require sedation and is a more comfortable procedure for those having it.

Yet others contend that its drawbacks far outweigh its benefits.

"It's a test that has a tremendous number of questions still yet to be answered," said Dr. David A. Johnson, chief of gastroenterology at Eastern Virginia Medical School, past president of the American College of Gastroenterology and co-author of the group's guidelines for colon cancer screening.

Virtual colonoscopy, however, has advanced far enough that it's now recommended as a frontline screening test by the American Cancer Society and as an alternative to regular colonoscopy by the American College of Gastroenterology.

Some major health insurers, including United Healthcare, CIGNA and BlueCross BlueShield, have begun covering virtual colonoscopy, said Dr. Judy Yee, an associate professor and vice chairwoman of radiology and biomedical imaging at the University of California, San Francisco, and chief of radiology at the San Francisco VA Medical Center.

In fact, when President Obama underwent his first-ever colon cancer screening last year, he chose virtual colonoscopy, Yee said.

People who have a virtual colonoscopy still have to undergo the same preparation that they would for a normal colonoscopy, in which powerful laxatives are used to clear out the colon -- a process that many, if not most, describe as unpleasant at best.

But anesthesia is not needed for the procedure, which means they can be back to their regular routine immediately afterward.

"It's a less invasive test," Yee said. "You don't have to introduce a 6-foot-long probe into the colon through the rectum."

Virtual colonoscopy does have its drawbacks, however.

For one thing, tissue samples cannot be taken, nor can a polyp be removed, during a virtual colonoscopy. If doctors believe they've located a polyp, then the patient has to have a second procedure -- a normal colonoscopy -- to confirm the diagnosis and have the polyp removed. That could be done on the spot, thus requiring only one procedure, if the person were having a regular colonoscopy.

Also, virtual colonoscopy may be a less accurate test. It produces very clear images, but experts say they're not as detailed as what can be seen in a conventional procedure.

Studies have found that virtual colonoscopy is not a reliable tool for locating polyps less than 5 millimeters in size or smaller, which constitute about 80 percent of pre-cancerous polyps in the colon, according to the American College of Gastroenterology.

The test also produces a considerable number of false positives, suggesting a problem that turns out to not be there. But, determining that requires people to undergo a normal colonoscopy.

"They not only are not detecting a sizable number of polyps, they are calling polyps that aren't there," Johnson said.

Then there's the question of exposure to radiation, which he described as "not inconsequential."

Radiologists and advocates of virtual colonoscopy acknowledge the concerns but argue that the test has proven its value.

For example, though virtual colonoscopy might not be as accurate, it is effective in finding the polyps that most often lead to colon cancer, Yee said.

"Virtual colonoscopy has been shown through multiple studies to be as sensitive as normal colonoscopy to detect clinically significant lesions of 10 millimeters or larger," she said.

Yee added that radiation concerns are overblown. "It's been shown that virtual colonoscopy is a low-dose examination," she said.

Researchers also are working to make virtual colonoscopy even better, Yee said. New computerized, post-processing techniques are being developed to improve the test's accuracy and lower the radiation dose, and doctors are working to develop a form of the test for which the patient would not need to take laxatives in advance.

At the moment, though, the American College of Gastroenterology still would prefer that people have a conventional colonoscopy done every 10 years because, as Johnson said, it's a better test that allows doctors to immediately remove any polyps that are found.

However, if the drawbacks of a normal colonoscopy are enough to dissuade a person from undergoing colon cancer screening, then they should consider virtual colonoscopy as an alternative test that they should have every five years, according to ACG guidelines.

The hope is that virtual colonoscopy "will bring in individuals who aren't ever considering being screened, who need to be screened," Yee said. "We'd like to bring patients in before the cancer ever develops."

More information

The U.S. National Digestive Diseases Information Clearinghouse has more on virtual colonoscopy.