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Radiation from Medical Imaging Tests Increases Cancer Risk, Study

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According to a new Canadian study, radiation from widely-used medical tests can increase the risk of developing cancer. Montreal doctors who followed nearly 83,000 patients suffering from heart attack for five years found that exposure to low-dose ionizing radiation from common tests used to diagnose and treat heart problems are associated with an increased risk of cancer.

Dr. Louise Pilote, director of the division of internal medicine at the McGill University Health Centre, said: “The risk is small, but it's definitely there and the higher the level of exposure, the higher the risk. These are not benign tests." For every 10 millisieverts of radiation, there was a three percent increase in the risk of developing cancer over the next five years. An angiogram with angioplasty, a procedure to open blocked coronary arteries being the most reliable treatment for heart attacks and angina, exposes patients to about 15 millisieverts of radiation. A mammogram, by contrast, exposes women to radiation doses of less than one millisievert.

Although most of participants in the study were exposed to low or moderate radiation doses, a substantial number were exposed to high levels. Younger patients, not older ones, were more likely to be exposed to higher doses of radiation. The authors write in the Canadian Medical Association Journal: "For the average patient surviving an acute myocardial infarction, life expectancy is substantial. Our results suggest that exposure to low-dose ionizing radiation directly affects the likelihood of cancer. Although these patients most likely will die of cardiac-related causes, the increased exposure to low-dose ionizing radiation increases their risk of cancer and perhaps mortality." While an individual person's risk appears small, from a population perspective it could be important, McMaster University researchers noted in a related commentary. "The real trouble arises when an individual undergoes multiple tests or procedures," they said.

It's impossible to identify how many of the cancers developed in the study were the direct result of radiation exposure, said Mathew Mercuri, a graduate student at McMaster and a project manager at Hamilton Health Sciences. The McMaster group estimated that there would be one patient developing cancer for every 2,000 patients receiving a 20-millisieverts dose of radiation from medical imaging. Many of the patients in the Quebec study received a dose in that range.

The results raise questions about whether it's time to reconsider our enthusiasm for CT scans and other procedures that rely on ionizing radiation. The researchers recommended that the number of radiation-based tests and procedures each patient undergoes should be tracked in order to estimate their cumulative radiation dose. "Nobody is keeping track of the level of radiation that patients are exposed to," Pilote said.

However, scientists stressed that the benefit of tests such as CT angiograms and nuclear scans, where patients are injected with radioactive dyes, still exceed its risks. "What I'm concerned about is that patients are going to second-guess their physicians and refuse to have tests they need, and that physicians are going to be angry with us for scaring patients. The vast majority of tests that we're doing are appropriate. But routine imaging of patients who are not having any symptoms on a regular basis is probably something we should not be doing.", said co-author Dr. Mark Eisenberg, a professor of medicine at McGill University and staff cardiologist at Montreal's Jewish General Hospital.

The use of CT scans and other tests that use radiation is increasing in Canada and exploding in the United States. One in 10 Canadians surveyed by the Canadian Centre for Policy Alternatives in 2009 said they had paid out-of-pocket (or their employer had) for a CT scan of their lungs, heart or entire body, even though they had no symptoms of a health problem; 61 percent of Canadians surveyed said they would take a free full-body CT over $1,000 cash. Several billion imaging studies are performed annually worldwide; about one-third of them in cardiovascular patients, according to the American Heart Association.

The new study involved 82,861 heart attack patients in Quebec; 77 percent underwent at least one test or procedure involving ionizing radiation in the first year after suffering their heart attack. Studies involving atomic-bomb survivors have found that people who lived close to the blast sites had a higher incidence of cancer than those who lived further from the epicenter. “But little is known about the relationship between exposure to low-dose ionizing radiation from medical procedures and the risk of cancer," the researchers write in the Canadian Medical Association Journal.

Using Quebec medicare records, the researchers identified patients who had a heart attack between 1996 and 2006. They excluded patients if they had been diagnosed with cancer in the year before or the year after their heart attack. Patients were followed for five years on average. Cardiac catheterization and angioplasty accounted for about 64 percent of the radiation from cardiac procedures. Angioplasty uses a balloon to open narrowed or blocked arteries, and stents, tiny, cage-like tubes or scaffolds, to keep arteries propped open. A total of 12,020 cancers were diagnosed during follow up. "We found a dose-dependent risk associated with the radiation," Eisenberg said. Cancers of the abdomen, pelvis and thorax (including breast cancer) accounted for about two-thirds of the cancers detected. Eighty-four percent of the radiation the patients were exposed to occurred in the first year after their heart attack, and it was almost all related to studies looking at their heart.

"We don't want people to worry," Eisenberg said. "What we've shown here is an association, but it's very small."What's more, for heart attack survivors, the likelihood of having a repeat heart attack or dying from a heart condition "is much, much higher than the concern about developing a cancer down the line," he said. Still, there are other tests that can be done that involve lower, or no radiation exposure such as stress tests on treadmills, MRIs and echocardiography.

Doctors sometimes order routine tests on patients who have had bypass surgery or angioplasty simply for reassurance, the researchers said. "There are some doctors who are ordering nuclear tests on these patients every couple of years to look for blockages coming back, even if the patient is not experiencing chest pain and otherwise seems healthy. That's the kind of situation I think we should maybe think twice about."  Eisenberg said.

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