By Amanda Gardner
WEDNESDAY, Aug. 26 (HealthDay News) -- Medical imaging tests nearly double the amount of radiation Americans would otherwise be exposed to, new research shows.
CT scans and nuclear imaging contributed to more than three-quarters of the exposure, and more than 80 percent of the procedures were performed on an outpatient, said Dr. Reza Fazel, senior author of a study appearing in the Aug. 27 issue of the New England Journal of Medicine.
"We don't want to scare people and have them refuse procedures. The individual risk in any patient is very small. If it's going to benefit the patient, it's well worth the risk," said Fazel, who is an assistant professor of medicine in the division of cardiology at Emory University School of Medicine in Atlanta.
But one expert called for further research on the subject.
"It's striking that such a large proportion of the non-elderly population is being exposed to at least a moderate amount of radiation," said Dr. Michael S. Lauer, who wrote an accompanying perspective in the same issue of the journal. "This is an opportunity to stimulate us to do the trials we need to do to figure out the value of all these tests."
In the meantime, he added, "physicians need to know about the risks and communicate them, and patients need to talk with their doctor and understand why they're getting the procedure."
Although there are masses of trials demonstrating the benefit of mammographies, the values of other types of tests, most notably cardiovascular ones, are much less clear.
"We're actually working in a knowledge vacuum," said Lauer, who is director of the Divisions of Prevention and Population Sciences and of Cardiovascular Diseases at the U.S. National Heart, Lung, and Blood Institute. "What we know is that the radiation exposure that people are getting now might entail a risk but we don't know the benefit."
Up to 2 percent of all cancers in the United States may be due to CT scans, Lauer added.
And Harvard researchers reported in March that cumulative exposure to radiation from CT scans alone can increase the risk for cancer by as much as 12 percent.
According to this latest study, the number of CT scans performed since 1992 has quadrupled. The test used most frequently, and with the highest amount of radiation, is a myocardial perfusion scan to assess blood flow through the heart or trace damaged heart muscle. Use of that scan increased more than 6 percent each year between 1993 and 2001, the researchers found.
The authors looked at records on nearly 1 million adults aged 18 to 64 in five regions across the United States: Arizona, Texas, Wisconsin, South Florida and North Florida. All participants were enrolled in the same insurance plan.
During the study period, which lasted from the beginning of 2005 to the end of 2007, almost 70 percent of the participants had at least one imaging procedure that exposed them to radiation.
"That's a huge proportion," Fazel said.
Although the mean cumulative exposure was 2.4 millisieverts (mSv) per year, considered a low dose, the design of the study allowed the researchers to find wide variations.
Cumulative doses tended to be higher in women and in older people.
A "sizable minority" received moderate-, high- and very high-intensity doses, defined as 3-20 mSv per year, 20-50 mSv per year and more than 50 mSv a year, respectively.
By way of comparison, health-care workers and those who work in the nuclear industry are only allowed to be exposed to 20 mSv per year.
"More than 2 percent had very high doses, higher levels than radiation workers," said study author Dr. Andrew J. Einstein, director of cardiac computed tomography research and an assistant professor of clinical medicine at Columbia University Medical Center and New York-Presbyterian Hospital in New York City.
The typical American gets about 3 mSv per year, considered a low dose, Einstein added.
For more on CT scans and other imaging techniques, visit the American College of Radiology.