Sources of Radiation Exposure
Radiation used in medical examinations and tests is the largest man-made source of radiation exposure. The biological effects of the radiation dose received is expressed in milliSievert (mSv), with the effective dose of 1 mSv corresponding to the radiation dose of 50 chest X-rays. An average of 2.4 mSv per head per year comes from natural sources (see Figure 1).1 The medical sources of radiation were about one-fifth of the natural radiation in 1987, exceeded natural background in the radiological year 2006 and are still rising at a rate of 10 % per year.2 Computed tomography (CT) and interventional cardiology/radiology accounted for 24 and 7 % of overall (6.2 mSv per year) exposure (summing up natural, medical and industrial or military exposure).3 In absolute terms, the contribution per capita per year of interventional cardiology alone was 0.44 mSv (24 chest X-rays) in 2006. Radiation is a proven carcinogen4 and medical exposure to ionising radiation may elevate a person’s lifetime risk for developing cancer.5 Of all nuclear medicine diagnostic exposures (yellow part of the bar in Figure 1), 83 % come from nuclear cardiology. The attributable cancer risk of contracting cancer from medical radiation is estimated to be around 5–10 %,6,7 with approximately 29,000 future cancers (2 % of all cancers) in the US related to CT scans.8 A balanced public health approach seeks to upport the benefits of these often life-saving medical imaging exams while minimising risk.9
The Contribution of Cardiology to Global Radiological Exposure
In adult cardiology, patients, interventional cardiology procedures account for 12 % of examinations and 48 % of the total collective dose (see Figure 2).10 In patients with acute myocardial infarction, diagnostic cardiac catheterisation and percutaneous coronary interventions contribute for 67 % of average cumulative radiation exposure for acute myocardial infarction.11
In children with congenital heart disease, invasive radiology (with diagnostic and interventional catheterisation) accounts for 6 % of all radiological examinations and 84 % of the collective dose, in a study carried out in an Italian paediatric hospital.12 In both adult and paediatric heart patients, the cumulative cardiological exposure is significant, in the range of 60 mSv (3,000 chest X-rays) per hospital admission in adults.10
The number of professionally exposed subjects in the catheter laboratory also continues to rise. According to 2008 United Nations Scientific Committee of the Effects of Anatomic Radiation (UNSCEAR) estimates, the number of occupationally exposed workers totals 22.8 million (plus military personnel) and 7.35 million of these are medical workers.13 Cardiac catheter laboratory staff (physicians, nurses and technicians) are progressively more represented, with higher values of exposure: 10 % of monitored population, 31 % of those with exposure >1 mSv and 67 % of workers with yearly exposure >6 mSv in the Tuscany region of Italy.14
Recommendations of Regulatory Bodies and Scientific Societies on Unnecessary Radiation Exposure
Advances in imaging technology have led to an explosive growth of the performance of cardiovascular imaging and interventional fluoroscopy. This growth is challenging since it may lead to overuse or inappropriate use of new technologies. This implies potential harm for patients undergoing imaging (who take the risks of an imaging study without a commensurate benefit), excessive delay in the waiting lists for other patients needing the examination and an exorbitant cost for society, with no improvement and possibly with poorer care quality.15 As recommended in April 2010 by the US President’s Cancer Panel, all possible action should be taken by healthcare providers to minimise radiation exposure from medical sources, recognised as one of the six major causes of environmental cancer. These principles should be applied universally in clinical practice and more strictly in the medical environment, where the dose and the risk are highest – such as in cardiac imaging and interventional cardiology. Cardiologists have considerable power to abate the concentration of X-ray carcinogens that pollute the catheterisation laboratory and “needlessly increase healthcare costs, cripple our nation’s productivity and devastate our lives.”16
The risk of cancer, more so than the far smaller risk of severe hereditary disorders, is at the core of the radiation protection system for staff and patients. At low levels of radiation exposure, radiation damage is stochastic or statistical in nature; it is possible to predict the proportion of a given population of exposed patients who will be affected, but impossible to predict beforehand which particular individuals will succumb. The likelihood of inducing the effect, but not the severity, increases in relation to dose and may differ among individuals.17,18
The current risk estimates are based on several authoritative reports: International Commission on Radiographical Protection-103 (ICRP-103), UNSCEAR 2008 and the Seventh Report of the committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation (Biological Effects of Ionizing Radiation [BEIR] VII report).1,4,19 These are estimates – not fears – and are based on more than 100,000 studies, including studies of 87,000 Hiroshima and Nagasaki atomic bomb survivors and 407,000 nuclear power industry workers. The epidemiological evidence linking increased cancer risk to radiation exposure is now considered conclusive for a dose level above 100 mSv (which is sometimes reached by a patient in a single hospital admission and possibly even with a single test, such as multiphase abdominal computed tomography). ICRP and BEIR VII endorsed the linear no-threshold model, accepting therefore that there is no safe dose and the greater the dose, the higher the risk. For any given radiation exposure, the cancer risk is higher in females (+38 %) than males at all ages, three- to four-fold higher in children than adults and 50 % lower in an 80-year-old compared with a 50-year-old person (see Figure 3).20 The radiation-induced cancer is clinically undistinguishable from a spontaneously occurring cancer. The risk is low, and certainly outweighed by the benefit if the procedure is appropriate, but not negligible. If 100 subjects are exposed to 100 mSv (corresponding to 5,000 chest X-rays), 42 will have a spontaneous cancer (independently of radiation exposure) and one will have a radiation-induced cancer (Figure 4). These estimates still have a considerable margin of uncertainty, with a 2–3 confidence interval (CI) of attributable risk estimates. The mostly accepted range of uncertainty is one in 30 to one in 300.
Several genetic, environmental and dietary variables can affect the variability of damage observed to any given level of radiation and current research is targeting shifting epidemiology estimates to personalised measures of DNA and chromosomal damage21,22 focused on identifying inter-individual differences that could modulate radiation risks in order to obtain better estimates of the extent of the differences.23,24 Radiation-associated chromosomal damage in interventional cardiologists is amplified by smoking and by genetic polymorphisms of genes involved in DNA repair.18 If the risk is personalised, it will be possible to implement tailored preventive and chemopreventive strategies.
Doses of Common Procedures
The effective dose for a radiological investigation provides a rough estimate related to the total radiation risk, no matter how the radiation dose is distributed around the body, provided that the representative patients or patient populations for which the effective doses are derived are similar with regard to age and sex. Typical effective doses for common cardiological testing are reported in Table 1.15,25–27
Typical effective doses for some common diagnostic radiology procedures range from a factor of about 1,000 from the equivalent of one to two days of natural background radiation (e.g. 0.02 mSv for a chest radiography) to the equivalent of several years (e.g. for abdominal CT). Within the same procedure, for instance, percutaneous coronary intervention or cardiac CT, the dose ranges over a factor of 10 for patients28,29 and – for interventional cardiology – for staff.30 Physicians should be made aware of the exposures they deliver to their patients and how they compare to established norms.17
How to Improve the Radiological Climate in Cardiology
The International Atomic Energy Agency recently launched a campaign to increase justification of radiological examination using the ‘3A’s strategy’: awareness, appropriateness and audit.31 This strategy deployed in the cardiac imaging or catheterisation laboratory has the potential to yield surprising safety dividends. In the US, the issue of radiological responsibility was addressed with the ‘Image Gently, Step Lightly’ campaign, which focused on the risks of unnecessary and excessive medical radiation from CT imaging and interventional radiology administered to our paediatric patients32 – in whom radiation damage is most severe.
Extensive data show substantial unawareness of radiological doses and risks, not only of patients but also of prescribing and practicing doctors. Results of recent surveys show that most doctors grossly underestimate the radiation doses (usually by up to 500 times) and the corresponding cancer risks for most commonly requested tests.33,34 About 80 % of interventional cardiology fellows did not know their own radiation exposure.35 However, radioprotection knowledge is the best shield against unnecessary radiation exposure for both doctors and patients.
Some of the potential damage in patients and professionally exposed staff is obviously unavoidable due to the high radiation burden of the procedures, with the operator close to the radiation source and, not infrequently, with the patient in clinically critical condition. However, another part of the exposure is avoidable and is due to exposure deriving from inappropriate instructions and lack of implementation of safety principles in the catheterisation or cardiac imaging laboratory.
Audit Doses (of Laboratory and Operators)
As dose is determined both by physician-operator behaviour and by variables outside the operator’s control, it is impossible to identify a value that denotes a boundary between appropriate and inappropriate practices. Few data are available that permit benchmarking for these parameters. However, we know that for the same procedure in the same laboratory dose variations can occur that are up to 10 times the median value. This means that the use of ‘reference value’ for dose reconstruction doses is not appropriate for individual patients or procedures. Each laboratory and each operator should very carefully assess his/her standing for a given examination compared with benchmark values. Only if you know what you are doing, and what your values are compared with your peers, can you improve your radioprotection performance.
Appropriateness of Procedures – The Justification Principle
Various professional organisations, including the American College of Cardiology (ACC) and the European Society of Cardiology (ECS), have developed and are working to disseminate imaging referral criteria called ‘appropriateness criteria’.38 Existing criteria for appropriate ordering of medical imaging exams have not yet been broadly adopted by the practicing medical community and according to recent estimates, 30–50 % of all examinations are partially or totally inappropriate, i.e. risks and costs outweigh benefits. The acute and long-term risks of a cardiac imaging or of an interventional procedure are the same, whether it is appropriately or inappropriately performed. In 2010, the US Food and Drug Administration (FDA) recommended supporting various professional organisations, including the American College of Radiology (ACR) and the American College of Cardiology (ACC), in their effort to develop and adopt appropriate use criteria for interventional fluoroscopy. They suggested that an electronic FDA decision support tool for ordering imaging procedures could incorporate these criteria to improve quality and consistency in clinical decision-making.
This crucial aspect remains an important field for a proactive role in scientific societies. At least in Europe, this striving for appropriateness is also propelled by the law, which clearly states that “if an exposure cannot be justified, it should be prohibited” (articles 3 of Euratom 97/43 directive) and (article 5) that “both the prescriber and the practitioner are responsible for the justification of the test exposing the patient to ionizing radiation”. Any responsible prescription of ionising tests should strictly adhere to the principle of justification.
Following the principle of keeping radiation dose as low as reasonably achievable (ALARA), recently the imaging industry focused on the ‘dose war’, which involves attempts to maintain the same diagnostic power with the lowest possible dose (see Figure 5). For instance, in nuclear medicine the high (20–40 mSv) dose of Thallium perfusion imaging was substantially reduced with technetium tracers such as Sestamibi (9–11 mSv), more recently coupled with new reconstruction algorithms, by introducing stress-only protocols and by implementing semiconductor detectors into latest generation gamma cameras allowing massive scan shortening or dose reduction. Even more substantial reduction (up to 2–3 mSv) can be achieved with N-13 ammonia stress/rest with PET imaging.39 Similar striking dose reduction has been achieved in CT coronary angiography, with several strategies such as automated exposure control, electrocardiographically controlled tube modulation and reduced tube voltage (from 120 to 100 KV) in non-obese patients. Very recent introduction of high-pitch spiral scanning has enabled to lower radiation dose to <1 mSv.39 In interventional cardiology, the cultural aspect of radioprotection is as equally important as technological advances. A reduction of 90 % of occupational doses has been achieved simply by a training programme in radioprotection.36–37 In cardiac radiofrequency ablation, the dose exposure fell from 15 mSv to near zero radiation exposure with non-fluoroscopic navigation techniques.
One of the most important ethics of practicing medicine is primum non nocere, and so it is incumbent upon us to strive for better ways to minimise radiation and its consequent risks. The cultural approach of ignoring or minimising long-term cancer risks linked to the use of ionising radiation has led to serious, avoidable health damage for patients, invasive cardiologists and staff. Now it is becoming increasingly clear that the long-term radiation-induced cancer risk should be included in the ‘risk’ side of the risk–benefit balance of our medical acts and therefore a clear perception of the radiation issue is vital for both patients and doctors. Effective radiation exposure per episode of care represents a potential safety metric for patients with common clinical conditions. A good cardiologist – and even more so, a good imaging or interventional cardiologist – cannot be scared of radiation, but must be very wary of radiation’s risks.