Americans may be receiving too much radiation from medical tests whose value has not been proven, researchers reported today in the New England Journal of Medicine. More than two-thirds of Americans underwent at least one such imaging procedure in the three years covered by the study, reported Dr. Reza Fazel of the Emory University School of Medicine and colleagues. The two biggest contributors to the radiation exposure are CT scans, which use a series of X-rays to produce a three-dimensional image of the body, and heart perfusion scanning to measure blood flow through the arteries leading to the heart. In that test, radioactive technetium-99m is injected into blood vessels and its progress through the heart monitored with external radiation detectors.
Radiation is known to cause cancer, typically years after exposure. By some estimates, medical testing radiation contributes 2% of all cancer cases, but experts fear that it may be higher in the future as more and more patients are exposed to these relatively new procedures. They are also concerned because increasing numbers of tests are being performed on younger people, which allows more time for tumors to develop, and on women, who normally live longer than men.
Some studies have suggested that the growing number of CT scans being performed results at least in part from ownership of the machines by physicians, who view them as a new profit source and prescribe unnecessary tests. There is also a growing incidence of whole-body CT scans in which physicians check for any signs of potential disease in healthy individuals. Such scans were not included in the report because they are not covered by insurance.
The researchers studied medical records of 952,420 adutls between the ages of 18 and 64 who were insured by United Healthcare plans in Arizona, Dallas, Orlando, South Florida and Wisconsin. Between 2005 and 2007, 655,613 of them underwent at least one procedure that exposed them to radiation. The mean dose of radiation was 2.6 milliSieverts (mSv), a relatively low dose. A dose of 3 to 20 mSv is considered moderate, from 21 to 50 MSv is considered high and a dose over 50mSv is considered very high. Federal regulations put the maximum annual safe dose at 50 mSv.
Cardiac stress testing was the procedure that exposed patients to the highest radiation levels, an average of 15.6 mSv, and accounted for 22% of all radiation exposure. CT scans of the abdomen, which typically produce about 8 mSv, accounted for more than 18% of exposure. A mammogram -- a single X-ray -- produces about 0.4 mSv.
If the findings are extrapolated to the entire population, more than 4 million Americans are receiving a dose greater than 20 mSv each year, the authors said. "It is important to note that we are talking about radiation doses that are incurred in one year," said Dr. Brahmajee Naliamothu of the University of Michigan, senior author of the study. "Cumulative doses over a lifetime may be much higher."
About 79% of women in the study had at least one exposure to radiation, compared with 58% of men. Mammograms accounted for only a small part of the difference, Fazel said.
Dr. Michael Lauer, director of the division of prevention and population sciences at the National Heart, Lung and Blood Institute, said in an editorial in the same journal that clinical trials of the efficacy of such testing should be conducted before their use is expanded further. Despite the wide use of nuclear perfusion for cardiac imaging, he noted, there is no evidence that it increases survival.
-- Thomas H. Maugh II
Photo credit: Lawrence K. Ho / Los Angeles Times