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updated 8/27/2009 9:43:56 AM ET 2009-08-27T13:43:56

As many as two-thirds of adults underwent a medical test in the last few years that exposed them to radiation and in some cases, a potentially higher risk of cancer, a study in five areas of the U.S. suggests.

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It is the latest big attempt to measure how much radiation Americans are getting from sometimes unnecessary medical imaging.

Though the annual average radiation exposure from X-rays, CT scans and other tests was low, researchers found about 20 percent were exposed to moderate radiation doses and 2 percent were exposed to high levels. "Super X-rays" to check for heart problems accounted for nearly a quarter of the radiation people received.

"Given the growing use of medical imaging procedures, our findings have important implications for the health of the general population," the researchers reported in Thursday's New England Journal of Medicine.

The study did not directly address whether medical imaging is being overused, but some doctors are concerned that advanced tests like CT scans are being over-prescribed, and that evidence of their value in certain situations is lacking. In some cases, tests like MRI scans, which do not involve radiation, could be used instead.

Popular 3-D peeks
In the last three decades, CT scans have emerged as a popular way to get a 3-D peek inside the body. Some 83 million CT scans were performed in 2007. Doctors use them to get detailed views of the brain, chest, abdomen and pelvis. The radiation risk from a single CT, or computed tomography, to an individual is small, but some doctors are worried about the buildup over time.

"CT scans produce beautiful pictures, but they generate a huge amount of radiation compared with a standard X-ray," said Dr. Michael Lauer of the National Heart, Lung and Blood Institute, who was not part of the research.

Some insurers, citing spiraling costs and safety concerns, are requiring pre-authorizations and setting other limits before patients can receive these scans.

For their study, researchers led by Emory University analyzed insurance claims from 952,420 people between ages 18 and 64 to determine how many had an imaging test and the estimated radiation dose. All were covered by UnitedHealthcare in five regions: Arizona, Dallas, Wisconsin and two areas of Florida.

Nearly 70 percent had at least one medical test between 2005 and 2007 that exposed them to radiation doses double than what would be expected from natural sources in the environment such as radon in soil and cosmic energy from the sun, the researchers said.

The annual average radiation exposure was small — less than 3 millisieverts, a measure of dose. However, about 20 percent in the study had moderate exposure (3 to 20 millisieverts) and 2 percent had high exposure (20 to 50 millisieverts).

Given these findings, the researchers estimated that medical imaging exposes 4 million nonelderly adults to radiation doses greater than 20 millisieverts a year. The annual safe limit is 50 millisieverts.

Cancer risk
High radiation exposure is a known risk factor for cancer. Many years usually pass between radiation exposure and the appearance of cancer.

In the research, most of the radiation punch did not come from routine X-rays, but from fancy heart scans that have not been medically proven to improve health. Regular X-rays for the chest and ankle made up 71 percent of the procedures, but only 11 percent of the radiation exposure. By contrast, CT and nuclear imaging — which uses a small amount of radioactive materials — accounted for 21 percent of total procedures and 75 percent of the radiation exposure.

Nuclear tests to detect heart problems made up 22 percent of the radiation exposure while CT scans of the abdomen, pelvis and chest accounted for nearly 38 percent.

Far more women had imaging procedures than men — 79 percent versus 58 percent. This is a concern because women live longer and face a higher risk of developing radiation-induced cancer.

Dr. James Thrall, chief radiologist at Massachusetts General Hospital, said a big limitation of the study was the lack of information about why the tests were done. Without it, he said, it's impossible to know whether the test was medically necessary.

"There's a risk that people who need a lifesaving or life-improving imaging procedure might not get one" because of radiation worries, said Thrall, who is also chairman of the American College of Radiology's board of chancellors.

The research was funded by the National Institute on Aging, American Federation of Aging Research and National Institutes of Health. Some of the authors have ties to medical imaging or drug companies.

The study did not look at very low radiation exposure from dental X-rays, which are generally not of concern. It also did not include the elderly; imaging is not as controversial in that age group.

Though it included nearly a million people, the researchers said it was unclear to what extent the findings can be applied to the general population, including uninsured people who likely would get fewer imaging tests.

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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