2018 ACC/HRS/NASCI/SCAI/SCCT Expert Consensus Document on Optimal Use of Ionizing Radiation in Cardiovascular Imaging: Best Practices for Safety and Effectiveness.

Abstract

A Report of the American College of Cardiology Task Force on Expert Consensus Decision Pathways Developed in Collaboration With Mended Hearts This document has been developed as an Expert Consensus Document by the American College of Cardiology (ACC) in collaboration with the American Society of Nuclear Cardiology, Heart Rhythm Society, Mended Hearts, North American Society for Cardiovascular Imaging, Society for Cardiovascular Angiography and Interventions, Society for Cardiovascular Computed Tomography, and Society of Nuclear Medicine and Molecular Imaging. Expert Consensus Documents are intended to inform practitioners, payers, and other interested parties of the opinion of ACC and document cosponsors concerning evolving areas of clinical practice and/or technologies that are widely available or new to the practice community. Expert Consensus Documents are intended to provide guidance for clinicians in areas where evidence may be limited or new and evolving, or insufficient data exist to fully inform clinical decision making. These documents therefore serve to complement clinical practice guidelines, providing practical guidance for transforming guideline recommendations into clinically actionable information. The stimulus to create this document was the recognition that ionizing radiation-based cardiovascular procedures are being performed with increasing frequency. This leads to greater patient radiation exposure and, potentially, to greater exposure for clinical personnel. Although the clinical benefit of these procedures is substantial, there is concern about the implications of medical radiation exposure both to patients and to medical personnel. The ACC leadership concluded that it is important to provide practitioners with an educational resource that assembles and interprets the current radiation knowledge base relevant to cardiovascular imaging procedures that employ ionizing radiation. By applying this knowledge base, cardiovascular practitioners will be able to select and perform procedures optimally, and, accordingly, minimize radiation exposure to patients and to personnel. This online published document is a more comprehensive treatment of the knowledge base covered in 2 print published documents published under this document's title with subtitles “Part 1: Radiation Physics and Radiation Biology” and “Part 2: Radiological Equipment Operation, Dose-Sparing Methodologies, Patient and Medical Personnel Protection.” In addition, this online document contains 3 sections that are not included in the print-published documents: Modality-Specific Operator Education and Certification, Quality Assurance, and Patient and Medical Personnel Radiation Dose Monitoring and Tracking: Programmatic and Individual Considerations. To avoid actual, potential, or perceived conflicts of interest that may arise as a result of industry relationships or personal interests among the writing committee, all members of the writing committee, as well as peer reviewers of the document, are asked to disclose all current healthcare-related relationships, including those existing 12 months before initiation of the writing effort. The ACC Task Force on Expert Consensus Decision Pathways (formerly the ACC Task Force on Clinical Expert Consensus Documents) reviews these disclosures to determine which companies make products (on the market or in development) that pertain to the document under development. Based on this information, a writing committee is formed to include a majority of members with no relevant relationships with industry (RWI), led by a chair with no relevant RWI. Authors with relevant RWI are not permitted to draft or vote on text or recommendations pertaining to their RWI. RWI is reviewed on all conference calls and updated as changes occur. Author and peer reviewer RWI pertinent to this document are disclosed in Appendixes 1 and 2, respectively. Additionally, to ensure complete transparency, authors’ comprehensive disclosure information—including RWI not pertinent to this document—is available online (see Online Appendix). Disclosure information for the ACC Task Force on Expert Consensus Decision Pathways is also available online, as well as the ACC disclosure policy for document development. The work of the writing committee was supported exclusively by the ACC without commercial support. Writing committee members volunteered their time to this effort. Conference calls of the writing committee were confidential and were attended only by committee members and ACC staff. James L. Januzzi, Jr., MD, FACC Chair, ACC Task Force on Expert Consensus Decision Pathways The writing committee consisted of a broad range of members representing 9 societies and the following areas of expertise: interventional cardiology, general cardiology, pediatric cardiology, nuclear cardiology, nuclear medicine, clinical electrophysiology, cardiovascular computed tomography (CT), cardiovascular imaging, and the consumer patient perspective. Both a radiation safety biologist and a physicist were included on the writing committee. This writing committee met the College's disclosure requirements for relationships with industry (RWI) as described in the Preamble. The writing committee convened by conference call and e-mail to finalize the document outline, develop the initial draft, revise the draft per committee feedback, and ultimately approved the document for external peer review. All participating organizations participated in peer review, resulting in 21 reviewers representing 299 comments. Comments were reviewed and addressed by the writing committee. A member of the ACC Task Force on Expert Consensus Decision Pathways served as lead reviewer to ensure that all comments were addressed adequately. Both the writing committee and the task force approved the final document to be sent for ACC Clinical Policy Approval Committee. This Committee reviewed the document, including all peer review comments and writing committee responses, and approved the document in November 2017. The Heart Rhythm Society (HRS), North American Society for Cardiovascular Imaging (NASCI), Society for Cardiovascular Angiography and Interventions (SCAI), and Society of Cardiovascular Computed Tomography (SCCT) endorsed the document in January 2018. This document is considered current until the Task Force on Expert Consensus Decision Pathways revises or withdraws it from publication. This document's purpose is to assist cardiovascular practitioners to provide optimal cardiovascular care when employing ionizing radiation in diagnostic and therapeutic procedures. It is written to serve as an accessible resource that compiles the current radiation biology and safety knowledge base applicable to cardiovascular imaging. The document covers both patient and medical personnel safety issues for the 3 cardiovascular procedure classes that employ ionizing radiation: x-ray fluoroscopy, x-ray CT, and radionuclide scintigraphy. It includes discussions of radiation dosimetry and its determinants, radiation harm, basics of equipment operation, strategies to minimize dose, and issues of radiation monitoring and tracking. The document's goal is to enable cardiovascular practitioners to select the optimal imaging technique for a given clinical circumstance while balancing a technique's risk and benefits, and to apply that technique optimally to generate high-quality diagnostic images that deliver the greatest clinical value with minimal radiation exposure. Cardiovascular procedures that employ ionizing radiation have transformed the practice of cardiovascular medicine. These procedures have great value for diagnosis and treatment of appropriately selected patients with known or suspected cardiovascular disease. In addition, they enable more refined recognition and characterization of cardiovascular disease. These procedures are also integral to either planning or executing numerous treatment modalities, which can have profound impacts on the outcomes of cardiovascular disorders. However, ionizing radiation has molecular-level detrimental effects on exposed human tissue, with potential for injury both to patients and to exposed medical personnel. Consequently, it is desirable to minimize radiation exposure both to patients and to medical personnel in a manner consistent with achieving optimal health benefits. This principle requires that clinicians employ judicious selection of and conduct of radiation-employing procedures to achieve an optimal balance of a procedure's therapeutic benefit against the incremental risk conferred by the radiation exposure. Currently, cardiovascular diagnostic and therapeutic procedures are a major source of patient exposure to medical ionizing radiation, accounting for approximately 40% of total medical radiation exposure (exclusive of radiation oncology) 1, 2. Among occupationally exposed healthcare workers, interventional cardiologists and clinical electrophysiologists are among the most highly exposed, and there is potential for exposure to support personnel as well (in particular, nonphysician staff who work in x-ray fluoroscopy and nuclear cardiology environments) 3, 4. There is evidence that many cardiovascular specialists who order and conduct radiation-employing procedures are not fully informed about the radiation doses that accompany the procedure or the associated health implications for their patients and for themselves 5, 6. Consequently, there is a need to augment and standardize the level of knowledge and competence that cardiovascular specialists should hold in radiation safety and management. This knowledge base should be incorporated into training curricula and in physician board certification procedures. Cardiovascular specialists fall into 2 categories requiring different levels of knowledge: those who order cardiac imaging procedures and those who perform them. Training curricula should furnish the level of knowledge appropriate for a particular physician's practice activity. Achieving this goal requires collaboration between various stakeholders in graduate and postgraduate education. The blueprints of certification and recertification examinations should include specifications of radiation safety subject matter. Training programs should configure their teaching curricula to prepare their trainees appropriately. The balance between a procedure's risk and benefit determines its appropriateness. Although the technical hazards that accompany a procedure are well known, the hazard associated with attendant exposure to ionizing radiation should also be considered a potentially important determinant of a procedure's risk-benefit relationship. To assess the risk-benefit relationship for a given patient, the cardiovascular specialist who orders or performs the procedure should understand, in the context of that patient's clinical characteristics, how the radiation dose that accompanies the procedure may be detrimental to that patient's health and how the outcome of the procedure may be beneficial. The past 2 decades have seen substantial development and refinement of the 3 cardiovascular imaging techniques that employ ionizing radiation: x-ray fluoroscopy, x-ray CT, and radionuclide scintigraphy. Engineering advances have improved image quality while in many cases reducing the radiation doses employed for image acquisition. These advances have greatly enhanced cardiovascular diagnostic and therapeutic capabilities, thereby improving both diagnosis and therapy. Despite these engineering refinements, the patient radiation doses that accompany these procedures remain substantial and, for the most part, are at the upper range of radiation-based diagnostic studies. Medical professionals should be aware of the radiation dose that these studies deliver to patients. In addition, within a particular type of study, the radiation dose can vary substantially depending on image acquisition protocol and patient characteristics. For reference, the commonly performed cardiovascular diagnostic studies and their radiation dose ranges are listed in Table 1. Note that the doses delivered by x-ray CT and nuclear cardiology can vary substantially depending on particulars of image acquisition protocols. Patient radiation dose ranges (in millisieverts) for the 3 principal radiation-based cardiovascular imaging studies: x-ray fluoroscopy, x-ray CT, and nuclear cardiology. Individual procedure categories are further subdivided according to types of image acquisition protocols. Note that for a particular procedure category, the dose can vary considerably depending on image acquisition protocol and, within a given image acquisition protocol, procedure conduct and patient characteristics. However, augmented capabilities have led to increased utilization levels, resulting in greater radiation exposure both at the individual and at the population levels. In addition, refinement of x-ray fluoroscopic systems, yielding greatly improved image quality, has facilitated the development of increasingly complex cardiovascular interventional procedures. These procedures often require longer fluoroscopic times, resulting in larger radiation exposures than more basic procedures. Increasing radiation exposure has the potential to increase the risk of adverse effects such as radiation-induced cancer. It is uncertain, however, whether medical radiation is in actuality increasing cancer incidence in the population, because a small increase would be difficult to detect against the large background incidence of cancer. During the 2014 calendar year, the U.S. healthcare system performed, on Medicare beneficiaries, an estimated 925,848 diagnostic cardiac catheterization procedures, 342,675 percutaneous coronary interventions, 248,234 clinical electrophysiologic procedures, 61,207 cardiovascular x-ray CT scans, and 2,111,558 nuclear cardiology examinations, for a total of 3,689,522 cardiovascular procedures that use ionizing radiation in Medicare beneficiaries 8. Medicare beneficiaries are estimated to consume 30% to 40% of all cardiovascular procedures. Natural background radiation averages 3.0 millisieverts (mSv) (see Section 4 for a discussion of the Sievert unit of radiation exposure) per person/year in the United States—equivalent to 150 posteroanterior chest radiographs (a posteroanterior chest-x-ray dose is 0.02 mSv; combined posteroanterior and lateral is 0.06 mSv) 9. At the population level, between 1987 and 2006, estimated per person total medical radiation exposure grew from 0.6 mSv/year (0.2 × background) to 3.2 mSv/year (1.07 × background) 10. Consequently, patients are currently receiving, on average, more radiation from medical sources than from natural background sources. 2006 is the latest year for which compiled data are available. (The National Council on Radiation Protection is currently compiling contemporary data—expected availability 2019—and it is likely that current average medical exposure will be found to have increased further). The 2006 medical exposure is equivalent to 160 posteroanterior chest x-rays per person/year. Risks associated with this exposure must be weighed in relation to the health status benefits achieved by these procedures. Physicians who are invasive cardiovascular procedure operators are among the most highly exposed of the occupationally exposed healthcare workers. Measurements of interventional cardiologist operator exposure using current equipment and protection practices demonstrate an exposure range of 0.2 to >100 microsieverts (μSv) per procedure with a per-procedure average of 8 to 10 μSv 11. Thus, an active interventional cardiologist performing 500 procedures/year employing current technology may be expected to receive, in addition to background exposure, a dose of as much as 10 mSv/year or, in a most extreme scenario, 300 mSv over a 30-year active professional career. Nonphysician clinical personnel working in an x-ray environment should receive substantially smaller doses than tableside operators, although nuclear cardiology technologists who handle radioactive materials tend to be more highly exposed. Determinants of nonphysician exposure include time spent in an active procedure room, location in the procedure room during active procedures, and exposure handling radioactive materials. There are no data that characterize the total number of exposed workers or their exposure values. There are 3 important potential consequences of the growing use of ionizing radiation in cardiovascular medicine (see Table 2). The ongoing magnitude of exposure to the general population and to occupationally exposed healthcare workers has health implications at the population level and for individual patients and healthcare workers. It is important that physicians and healthcare workers understand the ionizing radiation knowledge base and apply it to protect patients, themselves, and their colleagues through judicious case selection and appropriate conduct of radiation-assisted procedures. A comprehensive assessment of radiation effects requires consideration of all 5 parameters. The relationships between these metrics are complex and are determined by the properties of both the radiation and the exposed tissue. For clarity in this document, the interaction of radiation with tissue will be characterized from the perspective of 4 of the previously mentioned inter-related frames of reference: exposure, absorbed dose, equivalent dose, and effective dose. It should be noted that in the literature, the terms “exposure” and “dose” are often used with less specific meanings than those used in this document. For this document's purpose, these metrics have specific meanings as defined in the following text. Exposure Radiation exposure refers to the presence of ionizing radiation at the location of the exposed tissue. This is quantified by standardized measures of a physical quantity that represent the amount of radiation present at that location. The typically used measure of radiation quantity is air kerma (Section 4.4.1), which is the amount of energy released by the interaction of the radiation with a unit mass of air. Its unit of measure is the gray (Gy). Its units are joules (J)/kg. One Gy is the quantity of radiation that when interacting with 1 kg of air releases 1 joule of energy. It should be noted that this is a measure of cumulative energy intensity as the energy deposition is normalized to a quantity of the absorbing material. Absorbed Dose Absorbed radiation dose is a measure of the energy that radiation deposits in an exposed tissue through interactions with its molecular constituents. It differs from exposure in that the radiation present at a given location does not deposit all of its energy there. The fraction of its energy that a given radiation exposure will deposit in the exposed tissue varies with the type and energy of the radiation and the tissue composition. Absorbed dose is also a measure of the intensity of cumulative energy deposition (energy deposited per unit mass of tissue) and is expressed in Gy—joules of energy deposited per kilogram of tissue. In exposure by external radiation beams, dose is not uniform throughout the exposed volume, but varies, typically as a function of depth from the beam entrance port. Equivalent Dose Different types of ionizing radiation cause varying degrees of tissue injury for a given absorbed dose. Equivalent dose is a construct used to account for differences in tissue injury caused by different radiation types. X-rays and gamma rays are the benchmarks against which particle radiation types such as protons, neutrons, and beta particles are compared. Some particles, in particular, protons, neutrons and alpha particles, cause greater tissue injury at a given dose than do x-rays, gamma rays, and electron particles. To adjust for this variability, each radiation type is assigned a radiation weighting factor by which the absorbed dose (in Gy) is multiplied to yield a measure of the expected tissue injury caused by that dose. The unit of measure is the sievert (Sv) which is the absorbed dose in Gy multiplied by the radiation weighting factor. Of the different radiation types, x-rays, gamma rays, and electron particles (electrons and positrons) are assigned a radiation weighting factor of 1. Other particle radiation types have weighting factors ranging between 2 and 20. For medical imaging, which employs x-rays and gamma rays, absorbed dose and equivalent dose take the same value, that is, an exposure with an absorbed dose of 20 mGy has an equivalent dose of 20 mSv. Effective Dose Effective dose is a measure of the estimated potential for a biological effect on the complete organism caused by a particular absorbed radiation dose. The effective dose construct has been developed as a measure of the estimated potential for a stochastic effect (such as cancer induction) that would be caused by a particular (nonuniform) absorbed radiation dose. It is the sum of the equivalent doses received by each organ with each organ equivalent dose multiplied by a coefficient that reflects that organ's sensitivity to a stochastic effect. The unit of effective dose is also the Sv, as discussed in greater depth in Section 4.5. The Sv, like the Gy of the absorbed dose's unit, is specific to its particular context and is equal to 1 joule/kg. The connection between effective dose and absorbed dose is that an effective dose of 1 Sv is associated with the same estimated stochastic risk that accompanies a uniform total body exposure with an absorbed dose of 1 Gy of radiation that has a radiation weighting factor of 1. In medical radiation exposures, absorbed dose is typically not uniform throughout all tissues. For x-ray imaging, dose is concentrated in the body region being examined and varies with depth from the beam entrance port. For nuclear imaging, dose is concentrated in the tissues that most avidly take up the tracer or are involved in its elimination. Different tissues have different sensitivities to radiation-induced effects. In the effective dose construct, each tissue is assigned a tissue-weighting factor that specifies its sensitivity to radiation effects. To calculate the effective dose in Sv, each exposed tissue's equivalent dose is multiplied by its tissue-weighting factor yielding that tissue's contribution to the overall risk. The contributions to risk from all exposed tissues are summed, yielding total risk, expressed as the effective dose in Sv. (How the effective dose is calculated is discussed in greater depth in Section 4.5). It is important to note with regard to childhood and teenage radiation exposure that tissue weighting factors do not take into account the increased sensitivity of the tissue of the pediatric population. Thus, for children and adolescents, a given radiation exposure confers a greater risk than the same exposure would confer to an adult population. In addition, children who do not have life threatening disorders have a long life expectancy, which provides a longer period for radiation-induced illness to present 13. Detrimental effects of radiation exposure typically present weeks to years following exposure. In addition, many detrimental effects, principally cancer, have a large background frequency that complicates the attribution of an effect in a particular subject to prior radiation exposure. Radiation in cardiovascular imaging consists of photons with energy >10 kiloelectron volts (keV) (x-rays and gamma rays) and positrons. The physical effect of such radiation is to eject electrons from atoms that comprise tissue molecules forming ions and free radicals. This causes molecular damage, potentially destroying a molecule or altering its function. This is the basis for the term “ionizing radiation” (discussed in detail in Section 5). X-rays and gamma rays are in a class of ionizing radiations, which is transmitted by photons. Photons travel at the speed of light, and have no mass and no charge. Their electromagnetic energy ranges from a few electron volts (eV) to millions of electron volts (MeV). The energies commonly employed in cardiovascular imaging are tens to hundreds of keV. X-ray or gamma photons cause ionization by colliding with and ejecting electrons from atoms of constituent tissue molecules. Energy is exchanged in the process, with the ejected electron gaining energy of motion and the photon losing energy. The incident photon may or may not be extinguished by the interaction. After an initial interaction with an atom, photons that were not extinguished continue to travel through the exposed medium at a degraded energy. The weakened (scattered) photon can collide with additional atoms (further exposing the subject), potentially ionizing them as well, until either all of its energy is dissipated and the photon ceases to exist, or it escapes from the subject (exposing the environment). X-rays used in x-ray fluoroscopy and x-ray CT have a of photon energy spectrum between 30 and 140 keV (the energy spectrum of x-rays generated in typical diagnostic x-ray tubes includes photon energies <30 keV, but the majority of these lower-energy photons are filtered out in the x-ray tube and do not expose the subject). Thallium-201 and Technetium-99m (Tc-99m) are the principal radionuclides used in cardiovascular nuclear scintigraphy studies. Thallium-201 releases photons primarily in the 68 to 80 keV range, similar to diagnostic x-rays. Tc-99m releases photons primarily in the 140 keV range. Positrons are positively charged electrons. They have mass and charge. When positrons travel through a medium, their electrostatic charge causes them to interact readily with electrons in the medium, leaving a trail of ionization. Consequently, they have a very short mean free path in tissue of 6 to 7 mm with a maximum of 15.2 mm. Positrons continue to cause ionization until their energy decreases to a critical level, at which point they are annihilated by colliding with an electron of a constituent atom. This annihilation process releases 2 511-keV gamma ray photons that travel in opposite directions. Because the emitted photons have such high energy, they are minimally attenuated in tissue, and the majority reach the imaging detector. Rubidium-82 is the most commonly used positron emitter for myocardial perfusion imaging; nitrogen-13 ammonia is used less frequently for this purpose. Fluorine-18 deoxyglucose is used in cardiology for metabolic imaging and to detect myocardial sarcoid and other inflammatory conditions. For external radiation beams, the absorbed dose is determined by the total incident exposure, the properties of the incident radiation, and the volume of tissue exposed. Exposure from an external beam is measured with the parameter air kerma. Air kerma is the standard unit of measure for x-ray beam exposure. Kerma is an acronym for “kinetic energy released in material.” Kerma is an energy intensity measured in units of joules of energy released per kilogram of absorbing material (J/kg). The kerma unit of measure is the gray (Gy), which represents 1 joule of energy released per kilogram of absorbing material. The metric “air kerma” is used in medical x-ray fluoroscopic applications because the measurement is made using air as the absorbing material that is ionized by the incident radiation beam. As described in Section 4.1, radiation absorbed dose, as distinguished from exposure, is an energy intensity, the concentration of radiation energy actually deposited in the exposed tissue. Not all radiation energy that impinges on a tissue is absorbed. Some radiation (a variable quantity depending on both radiation and tissue characteristics) passes through the tissue without interacting with it, depositing no energy. (This fraction of the radiation is what generates the radiological image). Absorbed dose is also an intensity measured in gray (Gy), which represents deposition of 1 joule of energy per kilogram of irradiated tissue. External beam energy deposition in tissue is not uniform. X-ray radiation is attenuated as it passes through tissue. For diagnostic x-rays, in most tissues, x-ray intensity decreases by approximately a factor of 2 for each 5 cm of tissue that it traverses. Thus, tissue exposed to an external x-ray beam, as occurs in x-ray fluoroscopy and x-ray CT, is not exposed uniformly—the dose decreases exponentially with depth from the beam entrance port. The incident beam air kerma is a good measure of dose at the body surface, but structures deeper than the body surface receive smaller doses. Thus, to estimate the dose to a particular body structure within the path of an x-ray beam but remote from the beam entrance site, adjustments have to be made to account for beam absorbance. Kerma (measured in Gy) is a measure of dose intensity (joules of energy deposited per kg of tissue). The risk of radiation harm is related both to the intensity of the radiation dose and to the quantity of tissue that receives the dose. (The greater the quantity of tissue that receives a given dose, the greater the risk.) Kerma-area product (KAP) is the product of the beam's kerma and its cross-sectional area. Thus, this parameter also incorporates the volume of tissue irradiated. This concept is particularly important in x-ray fluoroscopy, as imaging field sizes can vary considerably leading to very different KAPs from one examination to another. CT delivers radiation to a patient in a manner quite different from that of projectional imaging or fluoroscopy. Typically, a narrow x-ray beam with a rectangular cross section is used to collect images from multiple angles as it rotates around the patient. This distributes the dose much more uniformly around the patient compared with projectional imaging. Instead of measuring an entrance air kerma to the patient, “dose” is measured by convention as an air kerma inside of an