Introduction

Ischemic heart disease (IHD) is the leading cause of death in adult men and women worldwide.1 This is not different in Latin America.2 Changes in lifestyle, nutrition habits, and obesity are contributing to an increased prevalence of type 2 diabetes mellitus and consequently IHD in the region.3,4 While mortality rates have progressively decreased over the past 4 decades in the developed world, the same phenomenon is not observed in low to middle income countries, many of them in Latin America.5 Trying to reduce mortality rates is particularly challenging considering social structure and lack of appropriate financial resources in most of the developing world. Two Latin American countries, Brazil and Cuba, are among the 10 nations in the world with the highest mortality rates due to IHD and the top 2 nations in Latin America considering age 35-74 years old.6

Myocardial perfusion imaging (MPI) is an important non-invasive diagnostic test to stratify risk and guide management, widely used in many Latin American countries.7 In fact, a published registry including patients from Brazil8 demonstrates a 3 times higher abnormality rate for MPI compared to data from one leading center in the United States.9 This suggests a “sicker”, higher probability of IHD population evaluated by nuclear cardiology in Latin America. This finding is consistent with the high mortality rate due to IHD observed in this region.6

Although MPI has several demonstrated advantages to help face the challenge of IHD mortality, significant concerns have been raised regarding its associated radiation exposure to patients10-12 and, in particular, the radiation burden from MPI, that in some settings is the medical test with the highest per capita radiation dose.13,14 A variety of protocols can be used to perform MPI15,16 and several approaches and “best practices” have been developed to lower radiation exposures to patients, in accordance with international standards including International Atomic Energy Agency (IAEA) recommendations and the well-known ‘As Low As Reasonably Achievable’ (ALARA) principle.

We have recently shown, in the International Atomic Energy Agency Nuclear Cardiology Protocols Study (INCAPS) study, the current patterns of nuclear cardiology practices, worldwide. These findings have provided us an overview of potential opportunities for reduction of radiation exposure to patients in many parts of the world.17 A better understanding of current practice of nuclear cardiology in Latin America offers the opportunity to identify areas to improve quality of care and to reduce disparities. The present study compares Latin American to rest-of-the-world (RoW) nuclear cardiology practices, and evaluates their impact on radiation exposure to patients, with the objective of decreasing radiation burden from MPI to patients and optimizing protocols in this region.

Methods

Study Design and Conduct

Details have been published as part of the INCAPS study for the entire 65 countries.17 Briefly, the International Atomic Energy Agency (IAEA) organized a needs assessment meeting in 2012, where experts identified knowledge of worldwide nuclear cardiology protocols and practices as a priority. A global study in centers performing nuclear cardiology procedures was performed to identify what laboratories “around the world [are] doing in terms of tracer utilization, doses used and technology that is available”. Information about all SPECT and PET cardiac imaging procedures over a 1 week period between March 18 and April 22, 2013 was provided by participating laboratories. Approval of this study was provided by the Columbia University Institutional Review Board. As no individually identifiable health information was collected, the study was deemed exempt from the requirements of US federal regulations for the protection of human subjects (45 CFR 46).

Data Collection Instrument

Each site provided information on laboratory demographics. For each MPI study completed, the site also provided patient demographics and clinical characteristics, including age, gender, and weight, and study parameters including radiopharmaceuticals used and injected activities, camera type, patient positioning, additional scanning (CT or nuclear) performed for attenuation correction, and any camera efficiency improving hardware and software.

Radiation Dose Estimation

The patient effective dose (ED) was used to quantify radiation exposure. This is a whole-body measure reflecting organ doses and their relative sensitivity to the deleterious effects of radiation. The radiopharmaceutical(s) and their activities (MBq) administered to each patient were used to estimate the individual patient EDs. The estimation of ED was based on methods provided by the International Commission on Radiological Protection, as described in the INCAPS study.17

Best Practices Quality Index

Prior to data analysis, an expert committee of physicians and medical physicists was convened by the IAEA in order to determine practices that can be implemented by operators to optimize radiation dose from MPI. Using current clinical practice guidelines,15,18 the committee identified eight measurable laboratory “best practices”, such as avoiding administering too much isotope, avoiding higher-dose isotopes, and application of dose lowering technologies and protocols. The full list of best practices is detailed in Table 1. A best practices quality index (QI) was developed, a priori, by the committee. The QI score was defined as the number (0-8) of best practices a specified laboratory adhered to during the specified week. A QI score of 6 or greater was determined as a desirable level by the committee. We used data extracted from the INCAPS study to determine each laboratory’s adherence to these practices

Table 1 Definitions of the 8 best practices17
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Statistical Methods

Patient EDs and laboratory QI scores were calculated, as above. We describe continuous variables using means (±standard deviation) and medians (interquartile range; IQR), and categorical variables as proportions. Continuous variables were compared using either analysis of variance or Kruskal-Wallis tests. Chi squared tests were used to compare categorical variables. The primary comparison was between Latin American and RoW laboratories. We also compared laboratories within Latin America. A linear regression model was developed to examine the relationship between laboratory volume and mean laboratory ED. All statistical analyses were performed using Stata/SE 13.1 (StataCorp, College Station, TX, USA).

Results

The INCAPS study acquired data on 7911 patients undergoing MPI in 308 laboratories in 65 countries, including 1139 patients (14.4%) from 10 Latin American countries (Argentina, Bolivia, Brazil, Chile, Colombia, Costa Rica, Cuba, Mexico, Peru, Uruguay). Patients undergoing MPI in Latin America were younger (62.4 ± 11.5 vs 64.1 ± 12.0 years, p < 0.001). Mean ED for all patients in Latin America was 11.8 ± 4.1 mSv (median 12.1, IQR 8.5-14.6 mSv) compared to 9.1 ± 4.5 mSv (median 10, IQR 6.2-12.4 mSv) in the RoW. The distribution of patient ED is shown on Figure 1. The mean and median EDs both differed significantly between laboratories (p < 0.001) (Table 2) and countries (p < 0.001). In Latin America, 11% of laboratories had median ED ≤ 9 mSv, which is lower compared to 32% in RoW (Table 3). Only 8 of 36 laboratories adhered to six or more best practices. The proportion of laboratories in Latin America and RoW adhering to each best practice is presented in Table 4. On linear regression analysis, laboratory volume was associated with reduced ED (beta: −0.03, p = 0.016), explaining 15.9% of the variance in mean laboratory ED (Figure 2).

Figure 1
figure 1

Distribution of patient effective doses for MPI studies in Latin America

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Table 2 Latin America laboratory volume, radiation dose, and quality index score
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Table 3 Patient and laboratory demographics and clinical characteristics
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Table 4 Number and proportion of laboratories adhering to each best practice
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Figure 2
figure 2

Distribution of mean laboratory ED by patient volume

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Discussion

Our current study indicates that, compared to the RoW, on average, the current practice of nuclear cardiology in Latin America is associated with higher radiation exposure to patients, although there is great heterogeneity in the region. There are certainly regional differences in terms of regulation, organized scientific groups, level of knowledge regarding current guideline recommendations, and reimbursement issues which may influence the practice of nuclear cardiology in the region and in consequence the ED delivered to patients.

Brazil is the largest, most populous country in Latin America, where IHD is the number one killer of men and women. This country has contributed the largest number of patients from Latin America in the study (535 of 1139) which in some way reflects the higher utilization of nuclear cardiology compared to some other countries in the region, as previously shown.19 Among those studied we could find one laboratory delivering a mean ED of 8.4 mSv to 127 patients, which meets the current guidelines recommended goal of ≤9 mSv. Nevertheless, another laboratory in the same country delivered an ED of 17.8 mSv to seven patients (Table 2). It is interesting to note the difference in volume of patients seen within that week, comparing these two laboratories. Evaluating the correlation between mean ED and volume (Figure 2) there is a trend for a higher volume laboratory to deliver lower radiation doses. Considering that these patients are from the same country, working under the same regulations and a population of similar characteristics, it appears that protocol adjustments could be implemented, towards a more homogeneous practice of nuclear cardiology in the region, in order to achieve the guideline recommended goal.

We see opportunities for improvement in the practice of nuclear cardiology in many of these 36 laboratories in these 10 Latin America countries involved in the study and also potentially to other laboratories in the region which were not part of the study. Quite simple adjustments, which are costless to implement, could lead to lower ED and/or improved QI. These include measures such as: (1) implementing stress only imaging in at least some patients, when the stress “first” imaging is completely normal, therefore avoiding an unnecessary second injection, which is feasible as previously shown;19,20 (2) performing prone imaging as well as supine imaging in some patients, both to improve diagnostic accuracy, preventing potentially unnecessary additional downstream testing which often involves additional radiation exposure, and also to increase the normalcy rate of stress imaging, thereby enabling increasing the rate of stress-only imaging and lowering radiation dose for the particular studies in which rest imaging can be omitted;21 and (3) adjusting the injected activity based on patient´s weight instead of using a fixed dose to patient of all weights. One barrier to performing stress-only imaging in Latin America is the low rate at which stress imaging is performed first (38% vs. 49% in the RoW). Additional dose-reduction approaches are available for those Latin American laboratories able to afford software- and hardware-based dose-reduction techniques. For example, high-efficiency cameras with cadmium-zinc-telluride detectors,22,23 as well as advanced reconstruction software incorporating resolution recovery and noise reduction algorithms,24 both enable obtaining comparable diagnostic information to scans using conventional equipment while reducing administered activity and thus radiation dose. However, none of the Latin American laboratories reported using high-efficiency cameras or PET, and the rate of advanced reconstruction algorithm use was 20% lower in Latin America than in RoW. This technological difference accounts in part for the higher radiation doses observed in Latin America.

The appropriate use of nuclear technology can potentially help to control costs, important for any country but particularly relevant to those developing nations more financially challenged. Utilization of MPI is heterogeneous worldwide, being underused in many countries.25 In fact, MPI utilization should still grow in a region like Latin America, helping to reduce high rates of IHD mortality and prevent potential unnecessary invasive procedures in some of these nations. The IAEA remains committed to support the implementation and appropriate use of MPI worldwide as a way to help member states to face these challenges. At the same time, it is committed to quality and optimal use of radiation. This study demonstrated opportunities for improvement and optimization of clinical practice of nuclear cardiology in Latin America and will help to design educational strategies to reduce ED delivered to patients.

Study Limitations

Despite our efforts, we could not reach all laboratories performing nuclear cardiology in Latin America and our conclusions are based on those laboratories replying to our contact. In addition, data reported during a single week may not reflect precisely the entire practice of nuclear cardiology nor all the protocols potentially used in that laboratory. Nevertheless we believe that it reflects general patterns of practice in the region and it serves the purpose of pointing to where adjustments can be made.

New Knowledge Gained

The current practice pattern of nuclear cardiology in Latin America delivers higher than optimal ED to patients. Adjustments can be made to reduce significantly radiation exposure in the region complying with international accepted standards, guideline recommendations, and the principle of ALARA.

Conclusion

Marked variation exists in radiation exposure to patients undergoing MPI in Latin America. On average Latin American countries have higher ED compared to the RoW. Opportunities to reduce radiation exposure can be targeted at expanding the practice of stress-only imaging, using camera-based dose reduction strategies, and reducing injected activity to reflect a patient’s habitus.