Younger Americans are being exposed to worrisome amounts of radiation from medical scans that increase their risk of cancer, U.S. researchers said on Wednesday.

They said the cumulative risk of repeated exposure to radiation from medical scans is a public health threat that needs to be addressed.

Even though the individual risk for any patient exposed to these kinds of doses may be small, when you add that up over millions of people, that can be a concerning population risk, Dr. Reza Fazel of Emory University in Atlanta and colleagues wrote in the New England Journal of Medicine.

Their three-year study of nearly 1 million Americans aged 18 to 64 suggests that as many as 4 million Americans a year are exposed to what they viewed as high doses of radiation.

The findings heap new pressure on imaging equipment makers such as General Electric Co, Siemens AG and Philips Electronics NV, already facing efforts in Congress to cut payments for imaging procedures as a way to find money to expand U.S. health insurance coverage.

While diagnostic scans can give doctors valuable information, studies suggest too much radiation exposure can cause cancer, especially in younger people.

A report in March by the National Council on Radiation Protection and Measurement found that Americans are exposed to seven times more radiation from diagnostic scans than in 1980.

The new study looks at different ranges of dose and individual procedures that contribute to that exposure. The team used medical claims data from people insured by UnitedHealth Group Inc in five U.S. markets between January 2005 and December 2007.

CAT SCANS

The two biggest contributors to radiation exposure were an advanced kind of X-ray called a computed tomography or CT scan and nuclear medicine scans -- in which a small amount of radioactive material is injected into the bloodstream and read by special cameras.

Those two accounted for about three-quarters of all the radiation exposure from these studies, Fazel said.

One of the nuclear medicine procedures, an advanced heart stress test called a myocardial perfusion scan, was the single-biggest contributor. It was easily the procedure that accounted for the greatest proportion of the overall radiation exposure, Fazel said.

Women were more at risk than men, and while mammograms accounted for some of that, they did not account for all, Fazel said. Most procedures -- nearly 82 percent -- were done in outpatient settings such as doctor's offices instead of a hospital, which Fazel said offered some indication of how sick people were.

Dr. Harlan Krumholz of Yale University in Connecticut, who worked on the study, said many people who are insured look at imaging as a freebie because they often do not have to pay a lot out of pocket for them. The truth is it is not a freebie. It's exposing you to some risk, he said.

Dr. Michael Lauer of the National Heart, Lung, and Blood Institute, one of the National Institutes of Health, said imaging equipment makers are working on new products to reduce radiation risks. But he said the biggest problem is that there is often no evidence that many of the scans save lives.

We may know the costs and we may have a good guess at the risk, but we don't know the value, said Lauer, who wrote a commentary calling for clinical trials that prove the scans are worth the risk.

Medicine