Cardiac CT

Exposure to ionizing radiation from cardiac imaging procedures following heart attack is associated with significantly increased rates of cancer, with each 10 mSv of cumulative radiation correlating with a 3 percent increased risk of developing cancer, a study published Feb. 7 in the Canadian Medical Association Journal reported.

As scrutiny over exposure to low-dose ionizing radiation from imaging continues to surge, the authors performed what they believed to be the first study to directly link cumulative exposure to cancer risk, while expressing their objection that the improved predictive capacity of imaging does not necessarily translate into better clinical outcomes, according to Mark J. Eisenberg, MD, MPH, of the department of cardiology and clinical epidemiology at Jewish General Hospital in Montreal, and colleagues.

Investigating the records of all first-time hospital admissions for acute myocardial infarction (MI) in the province of Quebec between 1996 and 2006, Eisenberg and co-authors identified 82,861 patients, of which 77 percent underwent at least one low-dose ionizing cardiac imaging or therapeutic procedure. The median age of patients was 63.2 years, with 31.7 percent of the sample consisting of females.

The procedures of interest (and their average effective radiation dose in mSv) were myocardial perfusion imaging (15.6 mSv), diagnostic cardiac catheterization (7 mSv), percutaneous coronary intervention (15 mSv) and cardiac resting ventriculography (7.8 mSV). The cumulative exposure to low-dose radiation from cardiac imaging was 5.3 mSv per patient-year, with 84 percent of this dose occurring within the first year of heart attack. The authors found that patients’ exposure to radiation was higher when treated by a cardiologist as opposed to a general practitioner.

The 82,861-size population experienced 12,020 cancers (following a one-year post-MI latency period). Two-thirds of these cancers occurred in the abdomen, pelvis or thorax.

Adjusted for age and sex, cumulative exposure to radiation served as an independent predictor of incident cancer, with a hazard ratio of 1.003 per mSv. For every 10 mSv of radiation, patients’ risks of cancer increased by 3 percent within a mean follow-up of five years.

“Our results suggest that exposure to low-dose ionizing radiation directly affects the likelihood of cancer,” Eisenberg and colleagues stated. “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.”

The authors yoked the cumulatively higher doses administered to younger patients and more remissive ordering for older patients in terms of overly permissive cardiac exam ordering. Because cancer risk appears to accumulate linearly with exposure, these factors diminish the fact that “For the average patient surviving an acute myocardial infarction [63.2 years in this sample], life expectancy is substantial.”

An accompanying editorial extrapolated on the authors’ calls for tempered enthusiasm and more attentive documentation of patients’ exposure to radiation, calling it “imperative that Canadian healthcare providers adopt … [such a] strategy.”

The authors pointed to several limitations to their study, including their reliance on well-documented, though not directly measured, dosimetry. Eisenberg and co-authors also argued that they likely underestimated radiation exposure because Canadians undergo fewer ionizing radiation procedures than American patients.

Moreover, “only a few of the cancers diagnosed in their study were likely a direct result of exposure to radiation from the medical imaging,” noted Mathew Mercuri, MSc, and co-authors from the division of cardiology in the department of medicine at Hamilton Health Sciences and McMaster University in Hamilton, Ontario, in their commentary.

Still, Mercuri and colleagues continued by arguing that the “study by Eisenberg and colleagues shows a positive dose-response relation for radiation exposure and cancer risk at cumulative doses for which there is little previous epidemiologic evidence of increased cancer risk (i.e., cumulative dose < 50 mSv).”