Cancer risk increases slightly for heart patients who get multiple imaging scans with low-dose radiation, say Canadian doctors who call for the doses to be tracked.
The conclusions of the study in Monday's issue of the Canadian Medical Association Journal are aimed at the medical community more than at heart patients, who experts urge not to be alarmed.
In Canada and the U.S., physicians increasingly order imaging tests and treatments using low-dose ionizing radiation in people with heart disease or suspected of having it, several studies show.
Newer tests such as CT angiography and isotope scans have replaced earlier exams that traditionally did not use radiation, such as exercise treadmill tests, researchers say.
Now that the association of cancer and radiation exposure is coming to light, cardiologists caution these procedures need to be carefully considered. That's particularly true when patients have multiple tests that help doctors diagnose a condition but might not actually improve prognosis, they say.
"We found that a substantial proportion of patients were exposed to high levels of low-dose ionizing radiation from cardiac imaging and therapeutic procedures after acute myocardial infarction," Dr. Mark Eisenberg of the division of cardiology and clinical epidemiology at Jewish General Hospital in Montreal and his co-authors concluded.
"We were able to show that exposure to radiation from cardiac imaging was associated with an increased risk of cancer in this patient population."
The study looked at hospital data on 82,861 patients who had a heart attack in Quebec between April 1996 and March 2006, but no history of cancer. Of this number, 77 per cent had at least one cardiac procedure with low-dose ionizing radiation within one year of the attack.
During the three-year study period, there were 12,020 cancers detected, with two-thirds of the cancers affecting the abdomen or pelvis and chest areas, the researchers found.
Document radiation doses
"These results call into question whether our current enthusiasm for imaging and therapeutic procedures after acute myocardial infarction should be tempered. We should at least consider putting into place a system of prospectively documenting the imaging tests and procedures that each patient undergoes and estimating his or her cumulative exposure to low-dose ionizing radiation."
The federal government is researching such a tracking plan.
In the study, most patients were exposed to low or moderate amounts of radiation, but a "substantial group" were exposed to high levels. Most tended to be "younger male patients" with few other health problems.
About a decade ago, most heart attack patients exposed to radiation were likely in their 60s or 70s with little opportunity to develop cancer in their remaining lifetime. But if someone starts getting imaging in their 30s or 40s, there is more time for them to develop cancer, Eisenberg noted.
Brian Campkin of Whitby, Ont., was 46 when he had a triple bypass. After the surgery, Campkin had X-rays and CT scans to monitor his progress.
"Knowing about this risk factor is very important going forward," the 50-year-old said. "I'm dealing with heart disease, I certainly do not want to deal with cancer."
Campkin, who coaches other cardiac survivors, calls the findings valuable information for patients having more tests.
Source: Radiological Society of North America
Overall, the research team found a three per cent increase in cancer risk over an average of five years for every 10 milliSievert of low-dose ionizing radiation.
To put that into perspective, a regular angiogram image delivers about seven mSv, and an angiogram with angioplasty to open up blood vessels in the heart narrowed by plaque gives about 15 mSV, Eisenberg said.
Mathew Mercuri, an epidemiologist with Hamilton Health Sciences co-authored a journal commentary on the research that raises the question of whether radiation exposure from medical imaging is a "silent harm."
Patients shouldn't worry, Mercuri stressed, since the risk of not having a procedure is far worse than its potential radiation risk. He suggested patients talk over the risks and benefits with doctors.
For doctors considering ordering procedures, the commentary urges consideration of avoiding the test altogether.
For physicians, "if you're in a situation where you're not sure if should go ahead with a test, then maybe you might actually want to switch to a different test that doesn't expose you to as high a dose of radiation," Mercuri said in an interview.
The study was funded in part by a grant from the Canadian Institutes of Health Research.