(USA TODAY) -- A new study suggests CT heart scans may help millions of seemingly healthy people get a better fix on their heart risk, but critics say the price may be a higher cancer risk.

Research shows that scanning the coronary arteries for calcium deposits offers a revealing window into how many blockages exist in vessels supplying the heart. But CT scans -- sequential, rapid-fire X-rays -- also deliver radiation to vital organs and, over time, boost a person's risk of cancer.

Doctors are becoming increasingly concerned about the amount of radiation people absorb in the 70 million scans done annually in the USA.

"The question is, how many cancers are you willing to take, 10 years down the road, to prevent (an unknown number) of heart attacks and deaths?" says Andrew Einstein of Columbia University, who has studied radiation risk posed by coronary calcium CT scans.

The study assessed the potential benefit of adding CT scans to a roster of other tests used to predict heart risk in 5,878 people without apparent heart disease in the Multi-Ethnic Study of Atherosclerosis. Testing for coronary calcium allowed researchers to reclassify 26% of participants; 728 bumped up into a high-risk category and 814 into a lower-risk category.

"There's a very substantial improvement in risk prediction," says lead author Philip Greenland of Northwestern University. "It works best in patients whom physicians are perplexed by, those with intermediate risk." Roughly 150 million people in the USA fall into the moderate-risk category. Twenty million of them could be shifted into a higher-risk group based on the results, he says.

"Those are pretty impressive numbers," says Reza Fazal of Emory University, who has studied CT scanning in medicine. But he notes that the study does not answer the key question: whether patients who were reclassified into the higher-risk groups gained any benefit from additional tests or treatment.

Greenland says the results, published in today's Journal of the American Medical Association, justify more study to answer that question.

Einstein notes that scanning 20 million people is likely to produce an additional 4,200 cancers in men and 6,200 cancers in women over their lifetimes, depending on how often they're scanned.

Rita Redberg, a cardiologist at the University of California-San Francisco, challenges whether the test is needed at all, because it hasn't been shown to help people.

"There's no known benefit," Redberg says. "Any risk isn't worth it."

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