A study of nearly 100,000 U.S. patients who underwent cardiac imaging found that the procedures were associated with "substantial" doses of ionizing radiation in potentially large numbers of patients, according to results published online Thursday in the Journal of the American College of Cardiology.
For now, the largest source of radiation by far was SPECT myocardial perfusion imaging (MPI), but cardiac CT is also expected to add significantly to the dose total in coming years, the study showed.
The research team led by Jersey Chen, MD, from Yale University School of Medicine in New Haven, CT, also found that the average dose per cardiac imaging patient -- including many who were quite young -- was about 16 mSv.
But an editorial accompanying the article emphasized the lifesaving utility of cardiac imaging scans, along with the exponentially greater risk of dying from an undiagnosed myocardial infarction than from an imaging study. The editorial authors called for careful dose optimization, and more research into imaging optimization algorithms for choosing the most appropriate exams.
The payment-based analysis in the Yale study relied on estimated rather than real radiation doses, but it does raises questions as to whether eliminating some exams, or substituting radiation-free alternatives, might better protect patients without sacrificing diagnostic efficacy.
"Although advances in cardiac imaging procedures have improved the ability to assess and treat cardiovascular conditions, there is a dearth of patient-level data on radiation exposure overtime from cardiac imaging procedures despite a rapid increase in their use," wrote Chen and colleagues (JACC, July 7, 2010).
There is reason to pay attention. In the U.S. alone, the number of cardiac SPECT exams rose from fewer than 3 million in 1990 to 10 million by 2002, they wrote. Between 2002 and 2006, cardiac catheterization procedures grew by 9%, and cardiac CT use is expected to grow dramatically over the next decade.
The authors cited a 2009 study by Fazel and colleagues that showed substantial cumulative radiation exposure from general medical imaging in many individuals, with 22% of the total effective radiation dose resulting from SPECT MPI. But the study didn't look at other cardiac imaging exams.
Cumulative effective dose
In their study, Chen and colleagues looked at the proportion of individuals who received cumulative effective doses of radiation above thresholds, including more than 3 mSv per year (the background radiation level in the U.S.) and more than 20 mSv per year (representing the upper annual limit for occupational exposure for at-risk workers averaged over five years).
"Our overall goals were to describe radiation exposure that accumulates over time in a general population from cardiac imaging procedures and to identify patient groups at risk of high radiation exposure," they wrote, along with co-investigators from Columbia University College of Physicians and Surgeons in New York City, Mount Sinai School of Medicine in New York City, and Emory University School of Medicine in Atlanta.
The authors used administrative claims to identify cardiac imaging procedures performed from 2005 to 2007 in 952,420 nonelderly insured adults in five U.S. healthcare markets. All adults ages 18 to 65 who were covered by insurance between 2005 and 2007 were included.
"We estimated three-year cumulative effective doses of radiation in millisieverts from these procedures," then calculated population-based annual radiation exposure rates to the two dose levels, the authors wrote.
The results, from a study population of 952,420 continuously insured individuals, showed a total of 90,121 (9.5%) who had at least one cardiac imaging procedure using radiation during the three-year period, Chen and colleagues reported. Among patients who underwent more than one exam, the mean cumulative effective dose over three years was 16.4 mSv (range, 1.5 to 189.5 mSv).
SPECT MPI
A single test, cardiac SPECT MPI, accounted for 74% of the cumulative effective dose, the authors wrote. Cardiac catheterization and percutaneous coronary intervention (PCI) accounted for approximately 11%, and less than 2% of the total overall radiation dose was attributed to equilibrium radionuclide angiography (ERNA/MUGA), cardiac CT, and electrophysiology procedures, the group reported. MPI was the most common cardiac imaging procedure performed; the second most commonly performed procedure was cardiac catheterization and/or PCI.
In all, 89 per 1,000 patients received an effective dose of more than 3 to 20 mSv per year, and 3.3 per 1,000 had cumulative doses of more than 20 mSv per year.
"Although the overall proportion of enrollees experiencing high cumulative effective doses was relatively small in our study population, the absolute number individuals at risk may be considerable when our findings are extended to the U.S. population," Chen and colleagues wrote. "With approximately 191 million adults ages 18 to 64 years in the U.S., annual effective doses of > 20 mSv/year from cardiac imaging procedures would occur in ∼636,000 Americans if cardiac testing patterns were similar to those in our [United Healthcare (UHC)] population."
The annual effective doses increased with age and were generally higher among men, and annual rates for all cardiac imaging procedures were generally higher in men, with the exception of equilibrium radionuclide angiograms/multigated acquisition scans.
Overall, about half of all the cardiac imaging procedures took place in physician offices (47.8%), with the remainder divided between hospital inpatients (26.4%) and nonhospitalized patients who underwent imaging in hospital facilities (25.5%). Most MPI (74.8%) and cardiac CT (76.5%) studies were performed in physician offices.
The authors acknowledged that the public health and clinical implications of radiation exposure from cardiac imaging are not easily determined. They advised that clinicians andpatients must consider tradeoffs between the benefits and risks of procedures, mainly those of malignancy, as cited in the Biological Effects of Ionizing Radiation (BEIR VII) report, which estimates that a 100-mSv radiation dose would lead to one additional cancer per 100 individuals over alifetime.
"Of course, radiation exposure from cardiac imaging procedures may be worth the risk for many patients," the authors wrote. "Certain procedures, such as PCI or device implantation, are performed to mitigate the risk of near-term and even life-threatening cardiac events, whereas the potential malignancy risk from radiation exposure is a long-term stochastic consideration that may occur typically decades after exposure. Yet our study demonstrates that there are sizable rates of radiation exposure for patients ages 35 to 54 years, many of whom will likely live long enough to for such long-term complications to potentially develop."
Alternative imaging
Many of the younger patients may be candidates for alternative imaging modalities that avoid ionizing radiation but offer similar clinical information, according to the researchers. They also suggested the use of stress echocardiography or exercise testing alone as a potential alternative to SPECT MPI scans in some patients.
"Similarly, although cardiac blood pool studies contributed a small amount to overall radiation exposure, judicious use of echocardiography or cardiac MRI may lead to lower lifetime doses," they wrote.
Even in cases where imaging using ionizing radiation is clearly indicated, use of the lowest possible dose, such as in cardiac CT, may reduce the risks even further.
"As frequent providers of record for many cardiac imaging services and procedures, cardiologists bear a particular responsibility for minimizing radiation risk to patients while achieving clinical goals, balancing potential benefits from reducing the burden of cardiac disease in the near term with potential harmof malignancy in the long term," they wrote.
However, many cardiologists aren't optimally trained to make these decisions, Chen and colleagues wrote; further training in imaging appropriateness could be beneficial.
Although the number of imaging procedures and the dose tended to rise with age, the study showed that many young and middle-aged patients also had some radiation exposurefrom cardiac testing. Some 56 per 1,000 individuals ages 35 to 44 years old in the study population had a cumulative effective dose greater than 3 mSv/year from cardiac imaging procedures.
Because younger patients are likely to continue to be exposed to radiation over time from both cardiac and noncardiac testing, they are more susceptible to higher risks of malignancy.
"Our finding that MPI studies contributed to approximately 80% of all radiation dose for patients ages 18 to 34 years suggests that alternative modalities to evaluateischemia (such as stress echocardiography) would decrease the radiation dose substantially for these patients," they wrote. "Among adultswith the highest doses of radiation, cardiac catheterization and PCI procedures were important contributors to overall radiation dose."
Cardiologists should take great care to use established techniques for minimizing dose in the cardiac catheterization laboratory, particularly for patients undergoing repeat procedures, they added.
Cardiac imaging specialists comment
In an editorial accompanying the JACC study, Matthew Budoff, MD, and Mohit Gupta, MD, from Harbor-UCLA Medical Center in Torrance, CA, noted that in the U.S., the risk of cardiovascular disease is estimated to affect more than 40% of the middle-aged population, with lifetime risk as high as 70% for individuals with multiple risk factors.
"Regarding harms, one study suggested ... CT angiography in a 60-year-old man had a lifetime attributable risk of 1 in 1,911 (0.05%) for the development of cancer," they wrote. "Thus, the 60-year-old individual is 800 times more likely to die of a cardiovascular event than to have attributable cancer from the test."
Because treatment and prevention have such a strong effect on reducing the incidence of cardiac events, the benefits of cardiac imaging tend to outweigh the risks by far, they wrote (JACC, July 7, 2010).
Nevertheless, the frequency of scans and the high estimated radiation doses described by Chen and colleagues "must give us pause," they wrote. The annual population-based rate of receiving an effective dose of more than 3 to 20 mSv per year was 8.9% -- and 0.3% for cumulative doses > 20 mSv per year.
Citing the U.S. Food and Drug Administration's (FDA) statement, Budoff and Gupta noted that managing the risks of CT, fluoroscopy, and nuclear medicine imaging depends on justification for ordering and performing each procedure, as well as optimization of the dose used for each procedure. Exams that use ionizing radiation should only be used when they are medically justified, the team wrote.
"When such exams are conducted, patients should be exposed to an optimal radiation dose -- no more or less than what is necessary to produce a high-quality image. In other words, each patient should get the right imaging exam, at the right time, with the right radiation dose," they wrote.
It's also important to keep in mind that cardiac imaging procedures serve to facilitate early and accurate diagnosis, improve treatment planning, and guide the interventions necessary to save lives, they added.
More research is needed to optimize a risk-stratification algorithm that would optimize the use of imaging modalities, Budoff and Gupta concluded.
"It would have been useful if the present study could determine the appropriateness of these tests and the actual doses used," they wrote. "Finally, 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. We need to move beyond radiation models, with so many assumptions, to studies documenting the real risk (if any) to the cardiac patient."
By Eric Barnes
AuntMinnie.com staff writer
July 9, 2010
Related Reading
JACC special issue tackles radiation dose in cardiac imaging, May 14, 2010
Young adults face higher CT radiation risk than seniors, May 4, 2010
Studies spotlight high CT radiation dose, increased cancer risk, December 14, 2009
NEJM study: Imaging procedures, radiation growing, August 26, 2009
BEIR VII and separating fact from fear, November 18, 2005
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