Ionizing Radiation Exposure in the Medical Setting
Note: This report represents information on this subject as of June 2006.
Full Text
Resolution 521 (A-05), introduced by the Washington Delegation and adopted at the 2005 American Medical Association Annual Meeting, asked our AMA to work with the public health, radiology, and radiation oncology specialty societies and all other interested parties to study the issue of radiation exposure by the American public and develop a plan, if appropriate, to allow the ongoing monitoring and quantification of radiation exposure sustained by individual patients in medical settings.
In 2006, shortly after the adoption of Resolution 521 (A-05), the National Academy of Sciences’ National Research Council (NRC) published its seventh in a series of reports systematically examining the impact of low-emission radiation on human health.1 Most experts in radiation exposure consider this report the definitive and authoritative resource on radiation risk. This Council report reviews and summarizes the evidence presented in that NRC report. Additionally, this report examines the peer-reviewed literature published in 2005 and early 2006 to ensure that any new data are appropriately considered. Finally, this report provides recommendations for consideration by the House of Delegates.
Data Sources
• The National Academy of Sciences’ National Research Council report, Health Risks from Exposure to Low Levels of Ionizing Radiation: Biological Effects of Ionizing Radiation VII - Phase II, released in 2006 and available online at http://fermat.nap.edu/books/030909156X/html.
• Literature searches conducted in the PUBMED database for English-language articles published between 2005 and 2006 using the search term low linear energy transfer radiation yielded a total of 51 references; 21 articles directly relevant to the human health impact of low-LET radiation were selected for further review but yielded no new data that differed from that discussed in the NRC report.
Background on Ionizing Radiation
Ionizing radiation is defined as radiation that has enough energy to displace electrons from atoms and molecules. These free electrons can then pass through cells and cause damage. There are no qualitative differences between human-made radiation, such as that used in medical settings, and naturally occurring radiation; thus, it is difficult to exactly quantify potentially negative effects of medical use of ionizing radiation from effects that may have occurred through natural exposure.
Ionizing radiation comprises either electromagnetic radiation, such as x-rays or gamma rays, or subatomic particles, such as protons, neutrons, and alpha particles. Electromagnetic radiation is sparsely ionizing because as it passes through a cell, the nature of its fast electrons cause only a few dozen ionizations. This is in contrast to subatomic particle radiation, which transfers more energy when it traverses the cell. Thus, because the rate of energy transfer is termed linear energy transfer (LET), the former radiation is termed low-LET radiation while the latter is termed high-LET radiation.
Radiation exposures are quantified in terms of the absorbed dose, which is the ratio of energy imparted to the mass of the body target. This unit of measure is the joule/kilogram (J/kg) or gray (Gy). Because high-LET radiations cause more damage per Gy, a weighted quantity, equivalent dose (or when averaged over all exposed organs, effective dose) is used. The weighting factor is dimensionless, so the unit remains J/kg for equivalent or effective dose values, but by convention the term sievert (Sv) rather than Gy is used with these dose measures. In terms of the absolute energy imparted, a Sv and a Gy are essentially equivalent and represent exposure of 1 joule (of energy)/kg (body weight/organ weight).
The NRC defines a low dose of low-LET radiation as within the range of near zero up to about 100 mSv (milliSievert). As noted above, high-LET or mixed radiations (comprising both high-LET and low-LET sources) are typically measured in Sv, while low-LET radiation can be measured in either Sv or Gy.
Human Exposure to Natural Ionizing Radiation
The average annual exposure to ionizing radiation worldwide ranges from 1 to 10 mSv, with a median value of about 2.4 mSv. Low-LET radiation accounts for about 1 mSv of that exposure. About 50% of exposures are due to radon gas and its corresponding decay products. Average annual exposures in this country are slightly higher, with a median of about 3 mSv, due to higher average radon levels in the United States. Like the rest of the world, about half of U.S. exposures are due to radon gas. After radon, the next highest percentage of natural radiation comes from exposure to cosmic rays, followed by terrestrial sources, and “internal” emissions.
Cosmic rays are particles that travel through the universe and emerge from sources such as the sun and supernovas. Terrestrial sources of exposure include radiation from rocks and soils, which in the United States varies geographically. “Internal” emissions come from radioactive isotopes (eg, uranium, thorium, and carbon-14) that may be found in food and water and from the human body itself.
Human Exposure to Human-made Radiation
In addition to exposures to natural ionizing radiation, humans are exposed to radiation from various human-made sources such as x-ray equipment and other radioactive materials used in medicine, research, and industry. It is estimated that in the United States, about 82% of human exposure to ionizing radiation comes from natural sources as described above, while the other 18% is due to exposure to human-made sources of radiation.
Of the 18% of radiation exposures from human-made sources, about 58% are due to medical x rays, another 21% to nuclear medicine, and 16% to consumer products such as tobacco, the domestic water supply, building materials, and television and computer screens. The remaining 5% is due to occupational exposures, exposures to fallout, and exposures to the nuclear fuel cycle. Finally, some small percentage of exposure occurs due to human activities such as traveling by jet aircraft, living near a coal-fired power plant, and being near x-ray luggage inspection scanners.
There are also a variety of ways that individual exposure to ionizing radiation will differ. Factors that increase a person’s exposure include: (1) increased use of radiation for medical purposes (eg, x-rays, whole body scans, etc); (2) occupational exposure to radiation; and (3) smoking tobacco products. Factors that may decrease one’s exposure include working on a higher floor (less radon gas) and living at lower elevations (less cosmic radiation exposure).
Adverse Health Effects due to Exposure to Ionizing Radiation
While the mechanisms that lead to adverse health effects after exposure to ionizing radiation are not fully elucidated, it is clear that such exposure does increase the risk of adverse effects such as cancer, as well as growth and development effects (from exposures in utero) and hereditary disease. The most thoroughly studied effects are those in the survivors of the Nagasaki and Hiroshima atomic bombings. It is important to remember that these survivors were distant from the actual fallout and therefore exposed to lower levels of radiation. While the ionizing radiation examined in these studies is not strictly low-LET radiation, these studies allow a better understanding of the risks associated with exposure to ionizing radiation. At doses between 100 and 4000 mSv, significantly excess tumors have been found in survivors. For in utero exposure, doses as low as 10 mSv are associated with excess tumors. Studies have shown a linear correlation in the number of excess tumors to the dose of the exposure; that is, as the amount of exposure was increased, the number of excess tumors also increased. It also appears that no low-dose threshold exists for the induction of cancers, but low radiation exposure doses are unlikely to increase the occurrence of radiation-induced cancers.
Studies of patients irradiated for the treatment or diagnosis of diseases have provided more information on radiation risks. Today, about 50% of cancer patients are treated using radiation. Data from cohorts of patients treated with radiation and followed for extended periods have allowed study of the risks of a second primary cancer after treatment with radiation for a primary cancer. Coupled with the extensive recordkeeping associated with radiotherapy for cancer, it is possible to precisely determine the dose-response relationships between exposure and cancer risk.
Long-term studies of patients receiving radiation therapy for benign conditions, such as enlarged tonsils, are also available. Study of these populations allows evaluation of the risk of radiation absent the confounding factor of malignant disease. While diagnostic radiation generally results in small doses to target organs and therefore is not as useful for ascertaining information about radiation risks, computed tomography (CT) can deliver sizeable doses (typically on the order of tens of mSv per scan; cumulative doses of more than 100 mSv have been reported for children). Analysis of all accumulated data has revealed new risk quantifications regarding the role of low doses of low-LET radiation in human disease (less than 100 mSv). The NRC estimates that in a lifetime, 1 in 100 individuals would be expected to develop cancer (solid or leukemia) as a result of exposure to 100 mSv, while approximately 42 of the same 100 individuals would develop solid cancer or leukemia due to causes unrelated to radiation. Based on the linear correlation between dose of exposure and risk of tumorgenesis, lower doses of exposure would result in a proportionately lower cancer risk.
In addition to increased cancer risk, exposure to high doses of ionizing radiation increases the risk of other diseases, such as cardiovascular disease. However, unlike for cancer, there are no direct data to indicate an increased risk of non-cancer diseases at low doses of exposure. While few data exist on the impact of radiation exposure on the children of exposed parents, animal studies clearly indicate that radiation-induced transmissible mutations occur in mice and other animals, and there is no reason to conclude that humans differ in this respect.
Low-linear Energy Transfer Radiation for Medical Purposes
Generally, radiation is used in medicine in two ways: (1) diagnostic examination; and (2) treatment of malignant and/or benign disease. The use of low-LET radiation in diagnostic procedures is still the foremost application of radiation in medicine. Such use may be reduced as more knowledge is gained on the application of non-ionizing radiation methods, such as ultrasound and magnetic resonance imaging. The NRC estimates that approximately 400 million medical diagnostic examinations and 150 million dental x-ray examinations are performed annually in the United States. It is estimated that an individual’s average effective dose of low-LET radiation exposure due to medical technology is 0.5 mSv.
Diagnostic procedures requiring radiation include x-ray techniques such as radiography, fluoroscopy, CT scans, interventional radiology, and bone densitometry. As these techniques provide diagnostic information, they are conducted with the lowest possible dose of radiation to meet the desired objective. The Table identifies the range of typical doses from various diagnostic procedures. As can be seen, the doses of single exposures are very low. However, the NRC notes there is concern about patients who as a result of their disease, receive repeated examinations over time (e.g., CT for renal stones), bearing in mind that the risk of adverse events due to radiation exposure is believed to be cumulative. The recent proliferation of CT screening centers that accept asymptomatic individuals has also raised concerns about radiation risks. In addition, since the widespread availability of fast, helical CT has reduced the need for pediatric sedation and allows for multiplanar reconstruction,2,3 significant increases in the number of pediatric CT procedures performed have been noted.4
Therapeutic exposures are not as common as diagnostic exposures but the intent in these situations is to deliver a lethal dose of radiation, such as in the treatment of cancer where the doses used are far higher than in diagnostic procedures. Thus, doses from radiotherapy to target organs are usually above 1 Gy and typically range from 50 to 60 Gy for treating malignant disease.
Risks From Medical Radiation Exposure. The NRC reviewed most of the published data in order to derive some quantitative risk estimates on the health effects of medical exposure to radiation. Studies of secondary cancer following radiotherapy have predominantly been in patients who received treatment for malignant conditions with favorable long-term prognosis, such as cervical cancer, breast cancer, and childhood cancers. However, more valuable data have been obtained on the carcinogenicity of low-LET radiation from studies in patients, including children, with benign diseases, such as enlarged tonsils, hemangioma, and tinea capitis. Doses used in such treatment tend to be lower than those used in the treatment of malignant conditions, survival following treatment is good, and there are minimal confounding effects from a malignant disease. In particular, the NRC identified leukemias and breast and thyroid cancers as the malignancies most associated with medical radiation exposures. For leukemia, the excess relative risk (that is, the [rate of disease in an exposed population divided by the rate of disease in an unexposed population] minus 1) estimates from studies with average doses ranging from 0.1 to 2 Gy are significant, in the range of 1.9 to 5 per Gy. Estimates of excess absolute risk (that is, the rate of disease in an exposed population minus the rate of disease in an unexposed population) are also high and are similar across studies, ranging from 1 to 2.6 per 104 person years.
In most of these studies, the majority of patients were adults when they were exposed; only the tinea capitis and hemangioma studies provided information about childhood exposures. In the hemangioma study where all patients received radiation in infancy, the overall excess relative risk per Gy was similar to that seen in the studies in adults, while in the tinea capitis study, where all exposures were below the age of 15 years, the excess absolute risk was also similar to that described earlier for adults.
In incidence studies for breast cancer, the excess relative risk per Gy of exposure varies depending on the study population, and the dose of, and age at, exposure. Thus, women who received very high doses showed an excess relative risk of about 0.15 per Gy, while women who received much lower doses of irradiation for enlarged thymus at infancy had an excess relative risk of 2.5. This range is consistent with the excess relative risk of 0.08 in studies of women receiving radiation for treatment of ankylosing spondylitis versus 2.7 for women receiving repeated x-rays for monitoring of scoliosis. The significant reduction in risk in women receiving high doses of radiation is consistent with the fact that cell killing is occurring, resulting in the reduction of risk per Gy.
Like excess relative risk, the excess absolute risk also varies depending on the population studied and the exposure patterns. However, a large pooled analysis of several cohort studies designed to estimate radiation-induced breast cancer risk yielded a high combined excess absolute risk of about 9.9 per 104 person years for a 1 Gy exposure at age 50 years. The excess rates were higher for those receiving radiation for mastitis and benign breast disease, with an excess absolute rate of about 15 per 104 person years per Gy, indicating that women treated for benign breast conditions appear to be at higher risk for radiation-induced breast cancer. Conversely, the cohorts receiving radiation for treatment of hemangioma showed lower excess absolute risk (5.1 per 104 person years per Gy), implying that protracted low dose-rate exposures may result in a relative reduction of risk.
For thyroid cancer, a large pooled analysis of data from seven studies including five cohort studies indicated a linear dose response in subjects exposed before the age of 15 years, with a leveling in risk at the higher doses typically used for cancer therapy (above 10 Gy). Overall, a pooled excess relative risk of 7.7 per Gy and a pooled excess absolute risk of 4.4 per 104 person years per Gy were derived. Both these estimates were significantly affected by the age at exposure, with a strong decrease in risk with increasing age at exposure. Significantly, the NRC concludes that there is no apparent safe lower limit of exposure, with lower exposure doses correlating to lower, but not negligible, risk.
Unfortunately, much of the data on risk from radiation exposure are confounded by the latency period between exposure and development of malignancy, which ranges from 2-5 years for leukemia and up to 20 years for solid tumors such as breast cancer. However, it is clear that the studies evaluated by the NRC provide good information on the magnitude of risk estimates, which appear to be significant with respect to leukemia and breast and thyroid cancers, and on the factors that may alter that risk.
The NRC report also discusses new risk estimates based on data obtained from in utero exposure studies in animals. These data filled important gaps in the current understanding of heritable genetic effects of in utero exposure to ionizing radiation in humans, which were based on studies of the Japanese atomic bomb survivors and those exposed in the Chernobyl accident. For the first post-radiation generation progeny, the current genetic risks from continuing exposure to low-LET, low dose, or chronic radiation range is of the order of about 750 to 1500 cases for autosomal dominant and X-linked diseases (versus 16,500 naturally occurring cases) and zero for autosomal recessive diseases (versus 7500 naturally occurring cases). For congenital abnormalities, the risk estimate is about 2000 cases (versus 60,000 naturally occurring cases) and for chronic diseases, it is about 250 to 1200 cases (versus 650,000 naturally occurring cases). Overall, the predicted risks per Gy exposure represent about 0.4% to 0.6% of the baseline frequency of 738,000 per million.
With regard to the second post-radiation generation progeny, assuming conditions of continuous radiation exposure in every generation, the risk is slightly higher for autosomal dominant and X-linked diseases and for congenital abnormalities. However, the overall increase in risk for all classes of disease relative to baseline is small, about 0.53% to 0.91% of the baseline frequency.
What Should Physicians Consider for Their Patients?
In light of the clear association between increased risk for certain cancers, and potentially other disease conditions, due to exposure to low-LET ionizing radiation, physicians should not only be vigilant in their use of diagnostic imaging, they should also be cognizant of the proper risk/benefit messages to communicate to their patients. The American College of Radiology (ACR) has created evidence-based guidelines that the College calls Appropriateness Criteria™ to assist referring physicians and other providers in making the most appropriate imaging or treatment decisions. These criteria are available online at http://www.acr.org/s_acr/sec.asp?CID=1845&DID=16050. The guidelines were developed by expert panels in diagnostic imaging, interventional radiology, and radiation oncology. Currently, there are more than 160 topics addressed within 17 expert panels, ranging from an expert panel on cardiovascular imaging to several workgroups within the radiation oncology panel. It is recommended that physicians considering diagnostic x-ray examinations or radiation therapy for their patients (or considering referring their patients for diagnostic x-ray examinations or radiation therapy) consult the ACR’s Appropriateness Criteria™.
In addition, the ACR advocates strong quality assurance (QA) programs overseen by qualified medical physicists to assure optimal performance of equipment producing the ionizing medical radiations. These QA measures, processes, and procedures are available in the form of ACR Practice Guidelines and Technical Standards. Such a guideline exists for diagnostic reference levels in medical x-ray imaging (available at: http://www.acr.org/s_acr/bin.asp?CID=1073&DID=12279&DOC=FILE.PDF [PDF, 95 KB, requires Adobe® Reader®]).
It is also important that physicians continue to provide their patients with specific information about the risks and benefits of the radiation exposure they will undergo as part of their medical diagnosis or therapy. Ideally, this should be available as a patient information brochure on the procedure that can be handed to the patient prior to his or her treatment. Although several general education as well as procedure-specific brochures exist and are available on public Web sites, they have not been widely disseminated among the various medical specialties. Examples are the ACR’s Understanding Radiology brochure (http://www.qualityimaging.org/understandingradiology/Radiology_Brochure_v6.pdf [PDF, 140 KB]), the ACR-Radiological Society of North America brochure on Radiation Exposure in X-Ray Examinations (http://www.radiologyinfo.org/pdf/x-ray_safety.pdf [PDF, 197 KB]) and the ACR-Radiological Society of North America brochure on Computed Tomography (CT) – Head (http://www.radiologyinfo.org/pdf/headct.pdf [PDF, 178 KB]). These materials, however, do not address the issue raised in the original resolution regarding quantification of a patient’s cumulative exposure to ionizing radiation as a result of medical procedures, and this information gap should be studied. For the primary care physician referring a patient, it is important that he or she be aware of the concept that 1 additional patient out of every 1000 who receives an abdominal CT scan (the average dose for an abdominal CT scan is 10mSv) may develop a cancer over his or her lifetime. This knowledge will likely spur increased attention to judicious ordering of medical imaging that is dependent on low-LET radiation.
Conclusions
As noted in the NRC Report, the current scientific evidence “is consistent with the hypothesis that there is a linear dose-response relationship between exposure to ionizing radiation and the development of radiation-induced solid cancers.” Furthermore, the report concluded that “it is unlikely that a threshold exists for the induction of cancer but…that the occurrence of radiation-induced cancers at low doses, will be small.” However, the benefits associated with medical radiation are considerable, and indeed medical imaging alone has become an indispensable diagnostic tool for physicians. Accordingly, it is incumbent upon the physician to use imaging modalities that utilize radiation judiciously and at the lowest doses possible, and to look for alternative imaging technologies when available. The ACR’s Appropriateness Criteria™ provide important guidance on this issue to physicians. Finally, physicians should be prepared to educate their patients on the risks and benefits associated with the use of radiation in medical diagnostics and therapy.
RECOMMENDATIONS
The following statements, recommended by the Council on Science and Public Health, were adopted by the AMA House of Delegates as AMA directives at the 2006 AMA Annual Meeting:
1. The AMA will collaborate with appropriate specialty medical societies and other interested stakeholders to convene a meeting: (a) to examine the feasibility of monitoring and quantifying the cumulative radiation exposure sustained by individual patients in medical settings; and (b) to discuss methods to educate physicians and the public on the appropriate use and risks of low linear energy transfer radiation in order to reduce unnecessary patient exposure in the medical setting. (Directive)
2. The AMA will continue to monitor the National Academy of Sciences’ ongoing efforts to study the impact of low levels of low linear energy transfer radiation on human health . (Directive)
References
1. Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation, National Research Council. Health Risks From Exposure to Low Levels of Ionizing Radiation BEIR VII-Phase 2. Washington, DC: National Academies Press; 2006.
2. Zeman RK, Baron RL, Jeffrey RB Jr, Klein J, Siegel MJ, Silverman PM. Helical body CT: evolution of scanning protocols. Am J Roentgenol. 1998;170:1427-1438.
3. Frush DP, Donnelly LF. Helical CT in children: technical considerations and body applications. Radiology. 1998;209:37-48.
4. Brenner D, Elliston C, Hall E, Berdon W. Estimated risks of radiation-induced fatal cancer from pediatric CT. Am J Roentgenol. 2001;176:289-296.
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Table: Radiation Doses Associated with Typical Imaging Studies (Adapted from the NRC report on health risks from exposure to low levels of ionizing radiation)
| Procedure | |
| Conventional Simple X-Rays (Single Image) | |
| Chest Films | 0.02 |
| Bones and Skull |
|
| Limbs/joints | 0.06 |
| Head | 0.07 |
| Pelvis/hip | 0.83 |
| Cervical spine | 0.3 |
| Thoracic spine | 1.4 |
| Lumbar spine | 1.8 |
| Abdomen | 0.53 |
| Screening Mammogram | 0.13 |
|
|
|
| Conventional Complex X-Rays |
|
| GI Series |
|
| Lower GI | 6.4 |
| Upper GI | 3.6 |
| Barium enema | 10-13 |
| Intravenous Urogram | 1.5 |
|
|
|
| Computer Tomography (CT) |
|
| Head | 2.0 |
| Abdomen | 10 |
| Chest | 20-40 |
| Pulmonary angiography | 20-40 |
|
|
|
| Spiral CT | 10-20 |
| PET-CT | 25 |
|
|
|
| Angiography (carotid, coronary, aortic, peripheral, abdominal) | 10-200 |
|
|
|
| Interventional Procedures (angioplasties with stent placement; percutaneous dilatations, closures, biopsy procedures) | 10-300 |
|
|
|
| Internal Emitters | 3-14 |
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