Author + information
- Received December 10, 2013
- Accepted December 23, 2013
- Published online April 22, 2014.
- Lewis Wexler, MD∗ ()
- ↵∗Reprint requests and correspondence:
Dr. Lewis Wexler, Stanford University School of Medicine, 805 Tolman Drive, Stanford, California 94305-1025.
Three patients consult with their primary care physician because of chest pain. After taking careful histories and doing physical examinations, the physician is concerned about the possibility of cardiac causes for the chest pain, but the level of concern is different in each case. After performing office electrocardiography, she wishes to refer the patients for appropriate further testing, taking into consideration the expected benefits and risks of the test as applied to each patient's particular situation.
She knows that diagnostic imaging studies are available that provide, with varying degrees of resolution, cardiac morphology and function. Some tests can image the coronary arteries but provide uncertain evidence of the significance of a demonstrated area of plaque formation and stenosis. Others might indicate an area of myocardial malfunction during a stress test but provide only speculative information about the specific location of the causative coronary lesion. Some tests are easily available but have high monetary costs; others take time to schedule and perform; still others may involve exposure to ionizing radiation.
The first patient is a 48-year-old woman who has been seen on previous occasions with similar symptoms; the second is a 58-year-old executive who presents with new onset of chest pain noted during his weekly 2-mile jog; the third gentleman is a 68-year-old retired laborer with type 2 diabetes who had a stent placed in a coronary artery 6 months previously. In what ways do age, sex, education level, race or ethnicity, and financial status influence a physician's decisions about appropriate further diagnostic testing?
How should the responsible physician proceed in evaluating and recommending an appropriate test? What information should be provided to the patient about the potential benefits and risks of each test and how should that information be delivered to the patient? In this era of readily available medical information through Internet searching, patients may have questions about the appropriateness of performing any given test, and in particular, they may have concerns about exposure to radiation.
At this point, the primary care physician may opt to refer the patient to a cardiologist, or she might consult various guidelines that have been developed by cardiology and radiology professional societies to aid in the selection of a diagnostic strategy (1–4). Cardiologists should also be guided by these recommendations, which are generally broad enough to allow individualization on the basis of a given patient's clinical situation.
Once a test is chosen on the basis of its appropriateness and likelihood of providing information that will affect treatment, the physician needs to have a conversation with the patient to explain what the test is designed to reveal, how likely it is to provide useful information, whether there are alternative tests that might provide equivalent information, and what risks the patient might face as a result of the proposed test. Thought should be given to anticipating whether additional testing will be required depending on the result of the first test. The quality of the laboratory performing the test, the cost of the test, and the reliability of the test results are also factors to consider.
There are basically 5 imaging procedures available that provide information on cardiac morphology and function: echocardiography, magnetic resonance imaging, radionuclide imaging, computed tomographic (CT) imaging, and invasive coronary or cardiac angiography. Each of these tests has its own strengths and weaknesses, but only the last 3 involve the use of ionizing radiation. How can a physician have a productive conversation about the risks of ionizing radiation with a patient?
This question is addressed in the guidelines proposed by Einstein et al. (5), in this issue of the Journal, the product of a 3-day symposium sponsored by the National Heart, Lung, and Blood Institute and the National Cancer Institute, which brought together an international group of cardiologists, radiologists, and radiation physicists to consider the sometimes controversial and contentious science of the risks of exposure to low-level ionizing radiation. In their discussion of the potential risks from diagnostic tests that use ionizing radiation, the contributors were forced to confront the lack of scientific evidence that has demonstrated significant risk from any of these diagnostic procedures in adult patients. Patients, by contrast, may have an unrealistic fear of radiation without any rational basis, fueled by articles in the press and given voice in the media (6).
What are the potential risks associated with ionizing radiation? Radiation in massive doses can cause radiation sickness and death. This type of exposure results from atomic bombs and nuclear reactor accidents but is not achieved during diagnostic imaging examinations. High-dose radiation localized to a particular area of the body can cause skin inflammation and/or depilation. This type of injury may be seen with radiation therapy, unusually long fluoroscopy for an interventional procedure, or a malfunctioning CT scanner resulting from machine or operator error (7).
Although we know that there are potential genetic effects from radiation, these are likely to be minimal considering that there has been no evidence of excess genetic defects in the first generation born to children who had survived the atomic bomb attacks on Japan.
The cancer-causing effects of radiation account for the greatest fear among patients. What are these risks, and how do they relate to radiation exposures that are natural to the environment? The best estimates of future cancer risk use data from atomic bomb survivors and other high-dose procedures, such as radiation treatment for Hodgkin disease and ankylosing spondylitis. These data are then extrapolated downward, assuming a linear no-threshold relationship between low-dose and very-low-dose radiation and cancer risk (8). This relationship has never been proved scientifically but remains the accepted method for calculating risk and is often quoted in the literature and the popular media.
It is estimated that individuals living in the United States are subjected to an average background radiation dose of 3 mSv/year. By far, the highest source of radiation to which the public is exposed comes from naturally occurring radon, which accounts for half the average yearly radiation exposure. Other terrestrial sources found in the soil, water, and vegetation account for an additional 8%. Internal radiation derived from isotopes within their bodies, such as potassium-40, carbon-14, and lead-210, account for about 10%. Cosmic radiation provides an additional 8%. This increases at altitude, such that residents of Denver receive almost twice as much cosmic radiation as those living in San Francisco, and a cross-country airplane flight is equivalent to an exposure of 0.02 to 0.05 mSv (9).
Manmade sources include radiation from consumer products (3%), for example, tobacco (thorium), building materials, televisions, tritium from luminous watches and dials, airport x-ray systems, and smoke detectors (americium). Medically related sources, including x-rays and nuclear medicine, account for an estimated 48% of the radiation to which the average U.S. citizen is exposed (10).
For comparison purposes, radiation exposure from a typical chest x-ray is 0.04 mSv, only 1/12 the 0.5-mSv yearly radiation dose received from cosmic radiation for a Denver resident. Radon exposure, which varies by location, averages 2 mSv/year. Coronary CT angiography performed using the best available equipment and dose-reducing protocols exposes the patient to <0.5 to 3 mSv, whereas older equipment might result in exposures ranging from 6 to 20 mSv; a typical single-photon emission CT study performed at rest and with stress using tetrofosmin might result in exposures from 9 to 14 mSv but might reach 18 mSv if sestamibi is used.
How much of this information is useful to patients, and how might it be put into context? Can a meaningful comparison be made with other kinds of risks to their lives that patients assume each day? Would it make any sense to compare the risk of CT angiography to crossing the street 2,500 times or driving a distance of 2,500 miles? If the estimated population risk for developing cancer is 40% and the incremental risk from cumulative very-high- dose multiple CT examinations is 0.7%, then it might be anticipated that instead of 400 cases of cancer per 1,000 persons, there would be 407 cases. This number is too small to detect.
Another issue of concern is whether there is a cumulative effect from repeated radiation exposures and whether it matters if multiple exposures occur over a short time period or over the life of an individual. There are very few data to support the small estimated increase in risk accrued by patients who undergo repeated CT examinations over time, either during a short period of an acute illness or over a lifetime (11). Most patients who undergo multiple examinations already have life-threatening illnesses such as cancer and have shortened lifespans. It is difficult to determine whether their outcomes have been influenced negatively or positively by repeated CT examinations. For patients undergoing diagnostic cardiac studies, the added risk is extremely small. Most patients presenting with symptoms of coronary ischemia are in the older age groups, and data from Japanese atomic bomb survivors suggest that there is at least a 20-year lead time before new cancers might become manifest. To date, no study has established an increase in actual cases of cancer in patients who have undergone diagnostic cardiac imaging compared with a cohort of matched patients who have not.
To avoid overly scientific terminology in their recommendations, the conference participants decided to “avoid statistical terms and constructs” and to “use analogies” with “simple comparisons” that would be more easily understood by patients. They suggest specific language that provides an informed way for physicians to discuss the potential incremental risk for cancer after radiation exposure from an imaging procedure. They propose introductory conversations based on the anticipated level of radiation exposure incurred during a given test. If radiation to the patient will be ≤3 mSv, the punch line is that the risk of the procedure is minimal because the “amount of radiation…is less than what most Americans are exposed to from their surroundings during 1 year of their life.” Appropriate comparisons with radiation from natural sources are made for exposures to higher doses, >3 to 20 mSv and >20 to 50 mSv, with the added caveat that, for the highest dose level, “Although experts are not certain, some evidence suggests that there may be a very small increase in your risk of developing cancer at a later age.” The patient is also told, “Your healthcare provider believes that the benefits of this test outweigh this small potential risk.” It is further stated, regarding prior exposure to radiation, “To the best of our current knowledge, your risk from today's test is not affected by how much radiation you have received from previous tests.”
This language is simple and supported by the available evidence. It could be a useful tool as a starting point for a discussion about the risk-benefit relationship of a proposed imaging study. It also provides an opportunity to discuss alternative tests that might not expose patients to ionizing radiation.
The investigators indicate that this conversation should begin in the referring physician's office but that the imaging center has a shared responsibility to provide more specific data to the patient. They suggest several mechanisms to achieve this, including providing educational material to both referring physicians and patients. The procedural information available in the imaging laboratory could serve as a type of informed consent that would indicate expected radiation exposure, a justification for the procedure, and the steps the laboratory has taken to optimize radiation dose. These steps are intended to promote an experience with patients that would offer adequate opportunity to explore their concerns and help them understand the rationale for proposing a given test, including the expected benefits and potential risks.
The report makes a number of other recommendations that need attention from professional societies and imaging laboratories that could have an important impact on medical practice, including laboratory procedures to report and track radiation doses and to improve radiological protection practices (12). The investigators additionally recommend that payer groups, including insurance carriers and the government, should become involved so that diagnostic testing using radiation is performed optimally when indicated. Manufacturers and laboratories have already responded to initiatives such as the Image Wisely (13) effort to reduce exposures in adults without significant loss of diagnostic capability and Image Gently (14), a parallel effort directed toward the pediatric population. A creative and very important recommendation in the report concerns widening the scope of the medical school curriculum to include study of the benefits and risks of radiation.
Taken together, these suggestions are vital to the patient-centered health care the investigators favor and will prepare physicians to have the necessary conversations with patients as diverse as the 3 described in the introduction to this commentary. Patients deserve a conversation that honors their concerns and presents them with evidence in a manner that is understandable to help them share in the decision to undergo a cardiac imaging test that is best suited to elucidate the nature of their problems and that puts their concerns in perspective.
Dr. Wexler has reported that he has no relationships relevant to the contents of this paper to disclose.
- Received December 10, 2013.
- Accepted December 23, 2013.
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