How Dangerous Are CT Scans?

A friend who is a former smoker has a CT of her lungs every year.  Fortunately, nothing bad has shown up, but I worry that all that radiation is dangerous. She’s convinced it’s perfectly safe. Is she right?

– August 20, 2007

A "CT" scan or "CAT" scan are terms used to describe a diagnostic technology known as computerized tomography, or computed axial tomography. The CT scanner uses x-rays to take pictures of cross-sections of the body, which are then digitally assembled to give physicians detailed, cross-sectional views of internal organs and tissues. This makes it possible to diagnose lung, liver and pancreatic cancers, measure tumors, and determine their precise location. They have also proved invaluable in diagnosing skeletal injuries and vascular diseases.

The downside is that they expose patients to radiation, often as much as 10 to 25 millisieverts (mSv) per test. We know from studies of survivors of the atomic bomb attacks in Japan that cancer incidence increases at radiation doses of 50 mSv. However, experts say that lower levels may also increase cancer risks. And clearly patients who have multiple CT scans can get cumulative radiation exposures that exceed 50 mSv.

The latest scientific word on this subject comes from a U.S. government study scheduled to be released in 2008. According to an article published in the New York Times in June 2007, advance word on the findings suggests that per capita dose of ionizing radiation from imaging exams increased by 600 percent from 1980 to 2006. CT scans are responsible for almost half the estimated collective dose of radiation exposure in the United States. Compared to CT scans, radiation exposure from mammograms amounts to only 0.7 mSv, while bone density tests deliver even less, only 0.01 mSv. a chest x-ray would give only 0.1 mSv. (To find out the amount of radiation exposure you would get from specific tests, log onto www. and click on "Safety.")

You can protect yourself against unnecessary exposure by making sure that recommended CT scans and other tests that involve radiation are really necessary. Ask how much exposure is involved and how appropriate the test is considered for your medical problem. You’re also better off if you have imaging tests at facilities accredited by the American College of Radiation. Accreditation means that the machines have been surveyed and calibrated to make sure that the levels of radiation they emit are correct and that the technicians who run the tests are certified to be competent in operating them.

Andrew Weil, M.D.

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