At least four million Americans under age 65 are exposed to high doses of radiation each year from medical imaging tests, according to a new study in The New England Journal of Medicine.
About 400,000 of those patients receive very high doses, more than the maximum annual exposure allowed for nuclear power plant employees or anyone else who works with radioactive material.
The paper, being published on Thursday, was based on a survey from 2005 to 2007 covering almost one million patients insured by UnitedHealthcare.
It did not estimate the number of cancer cases that the radiation might cause over the next several decades. But Dr. Rita Redberg, a cardiologist and researcher at the University of California, San Francisco, who has extensively studied the use of medical imaging, said it would probably result in tens of thousands of additional cancers.
Each individual patient is at relatively minor additional risk from the tests, Dr. Redberg said, but because they are given to so many people, the cumulative risk is significant.
"It's certain that there are increased rates of cancer at low levels of radiation, and as you increase the levels of radiation, you increase cancer," said Dr. Redberg, who was not connected with the new study.
The radioactive tests are given for hundreds of purposes. In the last two decades, they have become especially common in cardiology, where physicians use them to check for the buildup of plaque in the arteries and the heart's ability to pump blood.
Some cardiologists now encourage their patients to have routine heart scans even if they do not have clinical symptoms of heart disease, like chest pain or shortness of breath. The study did not examine what percentage of the tests were medically necessary.
The use of the tests has risen sharply in the last two decades, as more and more physicians have bought CT and PET scanners and installed them in or near their offices. In 2007, the Department of Health and Human Services estimated that the number of CT scans given to Medicare patients had almost quadrupled from 1995 to 2005, while the number of PET scans had risen even faster.
The new study's lead author, Dr. Reza Fazel, a cardiologist at Emory University, said the use of scans appeared to have increased even from 2005 to 2007, the period covered by the paper.
"These procedures have a cost, not just in terms of dollars, but in terms of radiation risk," Dr. Fazel said.
The researchers calculated the amount of radiation received by the patients by looking at insurance codes for various kinds of imaging tests. Exposure is measured in millisieverts; the average American receives about three millisieverts a year from all sources.
The paper found that in at least one of the three years, 1.9 percent of the UnitedHealthcare patients received at least 20 millisieverts of radiation, or nearly seven times the average. Of that group, about 10 percent, or 0.2 percent of all patients, received at least 50 millisieverts, more than the annual maximum that nuclear regulators allow.
Those figures suggest that about four million Americans receive cumulative doses exceeding 20 millisieverts a year.
Federal rules allow physicians to profit from the use of machines they own or lease. But Dr. Harlan M. Krumholz, a cardiologist at Yale and an author of the paper, said financial incentives were only part of the reason the number of tests had risen so fast.
"I think the central driver is more about culture than anything else," Dr. Krumholz said. "People use imaging instead of examining the patient; they use imaging instead of talking to the patient.
"Patients should be asking the question: ‘Do I really need this test? Is the information in this test going to help in the decision-making process?' "
In many cases, there is little evidence that the routine use of scans helps physicians make better decisions, especially in cases where the treatments that follow are also of questionable efficacy.
In an editorial accompanying the paper, Dr. Michael S. Lauer, director of prevention and epidemiology at the National Heart, Lung and Blood Institute, called for large clinical trials that would assess whether the scans improve care and lead to better outcomes for patients.
"Were we to insist that all, or nearly all, procedures be studied in well-designed trials, we could answer many critical clinical questions," Dr. Lauer wrote.
Until then, patients and physicians should discuss the risk of the tests and keep close track of the overall radiation dose that patients are receiving, he wrote, adding, "We have to think and talk explicitly about the elements of danger in exposing our patients to radiation."