CARDIAC IMAGING AND RADIOATION EXPOSURE

July 9, 2010 (New Haven, Connecticut) — A new study of administrative claims data confirms that patients undergoing cardiac imaging are exposed to relatively high doses of radiation, raising the urgency of efforts to define the risks of this exposure [1].

Dr Jersey Chen (Yale University, New Haven, CT) and colleagues analyzed data from about 950 000 nonelderly insured adults in five US healthcare markets, including about 90 000 patients who underwent cardiac imaging, to measure the effective radiation dose to the general population from cardiac imaging. Their results are published online July 7, 2010 in the Journal of the American College of Cardiology.

Radiation from cardiac imaging is an increasingly hot topic of discussion among imaging experts. For example, a recent Duke University study showed that acute MI patients receive over 17 mSv of ionizing radiation per hospital admission from imaging. The new study is the first to document the radiation exposure from cardiac procedures at the population level; previously, "it wasn't--from an epidemiological perspective--really known," Chen told heartwire .

Chen et al estimated the three-year cumulative effective doses of radiation in mSv from these procedures to see how often cardiac imaging exposes people to an effective dose greater than 3 mSv/year, which is the background level of radiation from natural sources, and how many cardiac imaging patients are getting more than 20 mSv/year, the standard upper annual limit for occupational exposure averaged over five years.

A total of 90 121 (9.5%) individuals in the five studied markets underwent at least one cardiac imaging procedure using radiation during the study period, and their cumulative effective dose over three years was 16.4 mSv, ranging from 1.5 to 189.5 mSv.

The annual population-based rate of receiving an effective dose of between 3 and 20 mSv/year from cardiac imaging was 89.0 per 1000 people, and 3.3 people per 1000 received a cumulative dose over 20 mSv/year from cardiac imaging. The annual effective doses increased with age and were generally higher among men. "So now we know that 10% of the population gets some kind of cardiac imaging, and of those, some get a fairly substantial dose."

These findings should prompt "doctors to talk to their patients and get them to think about when is the right time to use it." Since the risk of cancer increases with exposure over time, "for younger people, maybe alternatives that don't use radiation [should be considered], but for older patients or people with known multivessel disease or known large coronary atherosclerotic burden, the decision that the doctor makes in conjunction with the patient is that they'll accept that risk for the benefits," Chen said.

There were nearly 35 myocardial perfusion imaging (MPI) procedures per 1000 study enrollees annually, and MPI accounted for 74.2% of the cumulative effective dose. There were nearly 22 diagnostic cardiac catheterization and PCI procedures per 1000 people per year, and those imaging modalities contributed 21.4% of the total radiation dose for the population. Based on the published literature, Chen et al estimate that the effective dose of MPI with ejection fraction measurement creates an effective dose of 15.6 mSv per procedure, and diagnostic catheterization and PCI have effective doses of 7 mSv and 15 mSv, respectively.

CT Is a Relatively Minor Contributor to Radiation Exposure

Many of the recent efforts to reduce the radiation dose from cardiac imaging are focused on cardiac computed tomography (CT). Chen et al estimate that cardiac computed tomography for the assessment of the coronaries without assessment of coronary calcium has an effective dose of 16 mSv, and noncontrast CT for assessment of coronary calcium has an effective dose of 3 mSv. However, they found that cardiac CT was a very small contributor to the effective radiation dose for the population overall, because there was only one cardiac CT per 1000 enrollees per year. Electrophysiology procedures and equilibrium radionuclide angiograms were also relatively small contributors to the population's radiation exposure from cardiac procedures.

Chen said that the next step in understanding the impact of radiation from cardiac imaging will be to study clinical data rather than administrative data. "We used administrative data because it gave us a wide swath. Getting clinical data for such a large population is very difficult, but it can be done." He also suggested that future research should include more geographic areas and look for variations between geographies and perhaps devise methods for reducing radiation doses in populations that appear to be receiving more than average effective doses.

What Is All This Radiation Doing to Patients?

In an accompanying editorial [2], Drs Matthew Budoff and Mohit Gupta (Harbor UCLA Medical Center, Torrance, CA) write that population rates of high radiation exposure from cardiac imaging "must give us pause" but stress that the reason anyone is concerned about radiation exposure is that the radiation increases the risk of cancer, and yet "the entire premise that radiation doses from medical testing cause cancers remains hypothetical."

Budoff and Gupta argue that "there is no doubt that high levels of ionizing radiation (ie, atomic-bomb exposure) can cause cancers and death; however, the relationship between low-dose medical imaging and harm has never been established." They claim that Chen et al "repeatedly use outdated estimates that are significantly higher than current clinical practices, and therefore their estimates of theoretical harm are consistently overstated." For example, Chen et al estimate that the effective dose for cardiac CT can be as high as 16 mSv per procedure. But Budoff and Gupta point out that a 2009 recent scientific statement from the American Heart Association found that prospectively triggered coronary artery calcium scans produce 1.5 mSv, and retrospectively acquired studies expose the patient to 3 mSv.

"Measuring the subsequent cancer rates among this large cohort would go a long way in establishing whether low-dose medical imaging is actually associated with increased subsequent cancer," Budoff and Gupta conclude. "We need to move beyond radiation models, with so many assumptions, to studies documenting the real risk (if any) to the cardiac patient."