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A CT scan may be as good as a colonoscopy, but it has drawbacks as well.

Last week I saw a film starring Ricky Gervais portraying a dentist who elected to have a general anaesthetic for a colonoscopy because he wanted to be quite sure that he was fully asleep while they "messed down there." Clearly, he is talking about a subject that all adults can relate to.

The process of having a colonoscopy comes with a good amount of trepidation. You have to receive a sedative to reduce discomfort during the procedure. Some of these medications remove any recollection of the procedure. Unfortunately, the use of sedatives and pain medications can lead to respiratory distress, nausea and throwing up and potentially fatal infections. Additional side effects can be perforation of the bowel and major bleeding.

Ricky Gervais' character and many others may be thrilled to learn that a CT scan of the colon may be as good a screening test as the colonoscopy, without the need for sedation or risks of perforation or bleeding.

In a paper published in the New England Journal of Medicine, researchers directly compared colonoscopies to a CT scan in 2,600 asymptomatic people age 50 and over. They showed that a virtual colonography using a CT scan is able to detect benign polyps and malignant tumors greater than 1 centimeter in diameter in 90 percent of cases. Based on this information, the authors say that this test provides a viable alternative to colonoscopy.

With this new information, don't be surprised if many physicians and their patients now elect the CT scan approach to screen for colon cancer. No more worries of a large scope snaking through your body, no more risk of adverse effects from sedatives and no risk of perforation.

However, there are some serious concerns. In a companion editorial published in the Journal, Dr. Robert H. Fletcher pointed out that there were a large number of false positives. While 17 percent of patients had polyps larger than 1 centimeter, only one in four were found to have a polyp when a follow-up colonoscopy was done. And if the CT scan is positive, the patient will not be able to avoid the follow-up colonoscopy to ensure an accurate diagnosis. Fletcher also raised concerns that the CT scan may miss a number of smaller and equally serious polyps.

Additionally, CT scans may detect an incidental abnormality outside the colon, leading to anxiety that an undetected malignancy or other serious problem may be present. A total of 16 percent of patients in the study had a problem outside the colon that required further investigation.

Fletcher points out that with the exception of identifying a large aortic aneurysm, which should ideally be screened for in other ways, there is no evidence that early detection of an asymptomatic lesion by CT scan is of any benefit.

Finally, there is risk of radiation. If a CT scan is done every five years to screen for colon cancer, that radiation risk is totally unacceptable and may be unwarranted.

In my view, using a CT scan to detect colon cancer should be limited to those individuals who, for whatever reason, cannot tolerate a colonoscopy. But remember, colorectal cancer is the third most common cancer in the United States, with more than 154,000 new cases and 52,000 deaths annually. Almost all of these deaths could be prevented if screening was done more compulsively.

So, whether you elect to do a colonoscopy or CT colonography, you must be screened for the first time at age 50. If you are perfectly normal, some think a repeat colonoscopy should be done every five years, whereas the American Cancer Society and others recommend every 10 years.

Screening should be more frequent if you have had polyps or cancer diagnosed in the past, have a strong family history of colon cancer, particularly if it occurs at a younger age, or if you or someone in your family has been diagnosed with chronic inflammatory bowel disease (ulcerative colitis or Crohn's disease).

In addition everyone should have their stool screened for microscopic amounts of blood (fecal occult blood test) every year.

Follow this approach and you will virtually eliminate your risk of dying from this cancer.

Geriatrician David Lipschitz holds both a medical degree and a Ph.D. and is the author of "Breaking the Rules of Aging." He can be reached at His Web site is