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CAT SCAN  AND CANCER RISK
An ordinary CT scan delivers roughly the same average amount of radiation, 3,000 millirems, as World War II Japanese atom bomb survivors received a mile or two from ground zero. Studies of those survivors showed that a person's individual risk is low, but that excess cancers slowly accumulate in a large population. The following risk estimates are based on scans delivering 1,000 millirems, the lowest likely dose:

Age Gender Lifetime odds of getting cancer 
Infant girl 5 in 1,000
10 girl 2.5 in 1,000
30 girl 1 in 1,000
Infant boy 2.5 in 1,000
10 boy 2 in 1,000
30 boy 7 in 10,000

In his USA Today article Steve Sternberg states, "Overuse of diagnostic CT scans may cause as many as three million excess cancers in the USA over the next two to three decades, doctors report Thursday."

Researchers say they're not trying to discourage all use of CT scans - CT stands for computed tomography - which superimpose multiple X-ray images to make 3D pictures. Rather, they say, CT scanning is an invaluable tool in many cases. The problem, they say, is that doctors too often overlook its risks.

"About one-third of all CT scans that are done right now are medically unnecessary," says David Brenner, of Columbia University, lead author of the study in Thursday's New England Journal of Medicine.

CT scans offer an unparalled window into the human body, and their use has grown dramatically in recent decades as doctors use them to identify ailments in the head, abdomen and heart. Today, about 62 million CT scans are performed nationwide every year, up from 3 million in 1980, the authors say. Medical exposure to radiation, mainly through CT scans, has replaced environmental radon as the dominant source of radiation exposure for the U.S. population, the doctors say.

"On average we now get double the radiation exposure we got in 1980 because of increased CT scans," Brenner says. "Virtually anyone who presents in the emergency room with pain in the belly or a chronic headache will automatically get a CT scan. Is that justified?" Brenner asks.

University of New Mexico radiologist Fred Mettler, who was not part of the study, agrees that CT scans are overused. "We're always behind on CT scans because of demand from clinicians," he says. As many as 5 million scans are now done in children, who are 10 times more sensitive to radiation than adults. The increase was driven by technical advances that allow doctors to capture images in less than a second, eliminating the need for anesthesia to keep a child from moving.

And the use of the scans continues to grow, Brenner says. Doctors are scanning smokers and ex-smokers for early stage lung cancer, a highly controversial practice; they're using non-invasive "virtual" colonoscopies to check for colon cancer, and CT angiography is now being tested as a possible complement to ordinary angiography as way to diagnose blockages in arteries leading to the heart.

Brenner and his coauthor, Eric Hall, also of Columbia, say that many doctors don't realize that just a scan or two can bathes a patient in roughly the same amount of radiation as the atom bomb delivered to the Japanese survivors of Hiroshima and Nagasaki standing a mile or two from ground zero. And many people receive multiple scans over a lifetime.

The amount of radiation delivered during a single CT scan may range from 1,000 to 10,000 millirems, depending on the machine and the protocol. Japanese survivors a mile or two from ground zero received about 3,000 millirems, on average.

The cancer rates in the new study were drawn directly from a joint $1 billion study of the bomb survivors funded by the USA and Japan.

  WHAT TO ASK YOUR DOCTOR

 Some experts recommend that people keep a diary of any tests they or their children get involving radiation, because risk accumulates over a lifetime. Here are questions for any doctor who recommends a computed tomography, or CT scan:
• How will my medical care be affected by the results? Will the scan determine my treatment?
• What about alternatives like ultrasound or MRIs?
• How about just waiting? What are the risks of not doing the test?
• Does any research show this test will help me or lessen my chances of dying?
• Is this being done to diagnose a specific problem? (CT scans for people with no symptoms are not recommended).
• Is more than one scan truly necessary?
• Is the facility accredited by the American College of Radiology?
• What is being done to make sure I receive the lowest possible radiation dose?
• Does this center regularly do scans on children and follow guidelines for adjusting doses to reduce radiation?

Sources: Associated Press interviews with radiation experts