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.

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

'CT scans produce beautiful pictures, but they generate a huge amount of radiation compared with a standard X-ray.'— Dr. Michael Lauer

Though the annual average radiation exposure from X-rays, computed tomography, or CT, scans and other tests was low, researchers found about 20 per cent of the people studied were exposed to moderate radiation doses and two per cent 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, procedures like magnetic resonance imaging, or MRI, which does not involve radiation, could be used instead.

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 in the U.S. Doctors use them to get detailed views of the brain, chest, abdomen and pelvis. The radiation risk from a single CT scan 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.

2% of study patients had high exposure

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 what the estimated radiation dose was. All were covered by UnitedHealthcare in five regions: Arizona, Dallas, Wisconsin and two areas of Florida.

Nearly 70 per cent 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 per cent in the study had moderate exposure (3 to 20 millisieverts) and 2 per cent had high exposure (20 to 50 millisieverts).

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

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

Tests medically necessary?

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 per cent of the procedures but only 11 per cent of the radiation exposure. By contrast, CT and nuclear imaging — which uses a small amount of radioactive materials — accounted for 21 per cent of total procedures and 75 per cent of the radiation exposure.

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

Far more women had imaging procedures than men — 79 per cent versus 58 per cent. 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 tests were 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.

In April, a report by the Canadian Center for Policy Alternatives questioned the marketing of CT scans at private clinics for screening purposes — looking for suspicious signs in people who aren't showing any symptoms, and pointed to the serious downsides, including radiation exposure.