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- Matthew J. Budoff, MD⁎ ( and )
- Mohit Gupta, MD
- ↵⁎Reprint requests and correspondence:
Dr. Matthew J. Budoff, Harbor-UCLA Medical Center, 1124 West Carson Street, Bldg. RB-2, Torrance, California 90502-2064
Much has been written recently regarding the radiation doses and theoretical risks of cardiac imaging. Several principles must always drive medical decision making regarding ordering diagnostic tests. First and foremost, the benefits must outweigh the harms. Cardiovascular disease (CVD), being the number one killer in the world, requires appropriate and timely primary and secondary prevention measures. In the U.S. population, the risk of CVD increases with age and is estimated to affect >40% of the middle-aged population, with lifetime risk as high as 70% for those with multiple risk factors (1). Regarding harms, one study suggested computed tomography (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 (2). 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. Given robust event reduction associated with preventive and treatment strategies for coronary artery disease (CAD) (1), the benefits of imaging the middle-aged and older population to detect CAD outweigh the cancer risk.
Despite the clear risk/benefit ratio being in favor of diagnostic CVD imaging, the findings from Chen et al. (3) in this issue of the Journalmust give us pause. They demonstrate the cumulative radiation dose from cardiac imaging procedures in a large cohort (n = 952,420) of nonelderly (age 18 to 64 years, mean age 35.6 ± 23 years) insured adults in 5 major health care markets. They estimated 3-year cumulative effective doses of radiation from these procedures and then calculated population-based annual rates of radiation exposure to effective doses ≤3, >3 to 20, and >20 mSv/year. The annual population-based rate of receiving an effective dose of >3 to 20 mSv/year was 8.9% and 0.3% for cumulative doses >20 mSv/year. Annual effective doses increased with age and were generally higher among men, which exactly tracks the prevalence of CAD in the population. In the current study, myocardial perfusion imaging (MPI) accounted for the majority (74.2% of the cumulative dose), diagnostic catheterization and percutaneous coronary intervention (PCI) procedures contributed to 21.4%, whereas multigated acquisition scan and cardiac CT contributed equally to 1.9%.
Clearly, increased radiation will be a significant side effect of CT, nuclear, and fluoroscopic imaging. As the recent U.S. Food and Drug Administration statement (4) has concluded:
Managing the risks of CT, fluoroscopy, and nuclear medicine imaging procedures depends on two principles of radiation protection: appropriate justificationfor ordering and performing each procedure, and careful optimizationof the radiation dose used during each procedure. These types of imaging exams should be conducted only when medically justified. 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.
At the same time, it is also well recognized that these cardiac imaging procedures facilitate early and accurate diagnosis of the disease, improve treatment planning, and guide therapeutic interventions necessary to save one's life.
However, the entire premise that radiation doses from medical testing causes cancers remains hypothetical. There is no doubt that high levels of ionizing radiation (i.e., atomic bomb exposure) can cause cancers and death; however, the relationship between low-dose medical imaging and harm has never been established. Because the authors rely on multiple assumptions to make this association, great care must be taken when estimating radiation doses. Over the past few years, there has been a big emphasis on dose reduction with both nuclear imaging and cardiac CT, leading to dramatically lower doses than reported by the authors. The authors repeated use outdated estimates that are significantly higher than current clinical practices, and therefore their estimates of theoretical harm are consistently overstated (2,3). An example is the estimates used for cardiac CT. Gerber et al. (5) provided a comprehensive scientific statement on radiation doses from the American Heart Association, which noted that prospectively triggered coronary artery calcium scans are 1.5 mSv, and retrospectively acquired studies are 3 mSv. Because >90% of all coronary artery calcium scans are prospectively obtained, the mean dose estimates should be <2 mSv, not 3 mSv, as estimated in the article (3). Gerber et al. (5) estimated the doses for retrospective cardiac CT angiography at 15 mSv without tube modulation, 9 mSv with dose modulation, and 3 mSv for prospectively acquired studies. As a vast majority of cardiac CT angiography performed today use either dose modulation or prospective gating (6,7), the mean dose is estimated to be <10 mSv, not 16 mSv modeled by Chen et al. (3). These 2 examples lead to an overcalculation of harm by >33%. Accurate estimates of radiation exposure will take away 1 major flaw in the model and move us closer to real-world estimates of the potential implications of medical imaging.
Although not uniformly practiced, cardiologists and radiologists are guided by the “as low as reasonably achievable” concept, which urges providers to use the minimum level of radiation needed in imaging examinations to achieve the necessary results. The actual risk of malignancies induced by different levels of radiation exposure in this study was based on the BEIR (Biologic Effects of Ionizing Radiation) VII report from the National Research Council of the National Academies (8), a scientific summary of the relationship between exposure to radiation and human health. The principal source for the development of these risk estimates was the life span study of malignancies associated with radiation exposure from the atomic bomb explosions in Japan in 1945. However, it is still not clear whether the effects of whole-body “acute” exposures to “significantly high levels” of radiation can be extrapolated to the “partial-body” exposure at much “lower levels” of radiation. It is very difficult to estimate the risk of malignancy associated with low effective radiation doses (<100 mSv). In the study by Chen et al. (3), the annual mean effective dose was 0.73 mSv per person per year. This is substantially lower than the average annual background radiation that patients are exposed to from natural sources, estimate to range from approximately 3 mSv/year at sea level and increasing to 7 mSv/year with elevation. There is no increase in cancer in the U.S. going from sea level to cities at elevation, suggesting the simple linear no-threshold model of radiation exposure may be flawed. Further complicating this simple model is the fact that airline pilots, although estimated to be occupationally exposed to 150 mSv during their career, have never been demonstrated to have an increased prevalence or incidence of cancer, despite careful occupational health observations (9). Interventional cardiologists and radiologists have exposure estimates of 16 to 18 mSv/year, again without documented increases in cancer rates (10). This further confounds the risk attributable to exposure to medical radiation and requires outcome studies rather than modeling.
Radiation-induced malignancies have a biological latency of approximately 10 to 40 years and are far less likely to manifest in older individuals (2,3). The multisociety appropriateness criteria (11,12) for both cardiac CT and MPI suggest that these imaging studies are most indicated in a symptomatic population with intermediate or high risk of CAD. Clearly, the benefit of these tests and interventions are significant, and the availability of effective CVD treatments makes imaging an important component of current preventive cardiology. Chen et al. (3) state that “[e]ducating cardiologists on more effective methods to inform patients regarding the risks of radiation from cardiac imaging procedures and potential alternatives can help to reduce lifetime radiation exposure.” This suggests that cardiologists do not understand the radiation doses of medical testing. This supposition is incorrect, as competency and board eligibility for both cardiac CT and nuclear imaging specifically require physics education, and cardiovascular fellowship training mandates this education (13).
It should be re-emphasized that cardiac imaging using ionizing radiation was fairly uncommon in this population. Many subgroups had prevalence rates of <2%, and the overall cohort had a use of only 9.5%. Because CVD is expected to be the cause of death in more than one third of this population, imaging in <10% appears reasonable. It is interesting that the overall radiation exposure was so strongly driven by nuclear imaging (76%). Although the use of nuclear imaging far outweighs cardiac CT and cardiac catheterization procedures, the quantification of that relative risk is important and requires us to take pause when ordering tests, especially among young patients. Importantly, among women and men age 18 to 34 years, MPI was the largest contributor to radiation dose (81.3% and 78.4%, respectively). This subgroup, although very rarely imaged in general, clearly demonstrates inappropriate test selection by the physician. The young are theoretically most susceptible to radiation exposure, and use of alternative testing (such as stress echocardiography and treadmill testing) is most important in this subset. In contrast, only 75 patients out of nearly 1 million received very high radiation doses (>50 mSv), and 97% of these had ≥5 catheterization/percutaneous revascularization procedures. Chen et al. (3) estimated that of these 75, 1 would be expected to experience an additional lifetime malignancy. It is clear that the benefits of treatment of advanced CAD far outweigh the induced risk of cancer after imaging in this very high risk cohort.
Further research is clearly needed to define the most appropriate risk stratification algorithm that would provide more optimal use of these imaging modalities. It would have been useful if the present study could determine the appropriateness of these tests and the actual doses used. 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.
Dr. Budoff is on the Speakers' Bureau of GE Healthcare. Dr. Gupta reports that he has no relationships to disclose.
↵⁎ Editorials published in the Journal of the American College of Cardiologyreflect the views of the authors and do not necessarily represent the views of JACCor the American College of Cardiology.
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