How to Understand and Communicate Radiation Risk

Donald J. Peck, PhD Henry Ford Health System, Detroit, MI

Ehsan Samei, PhD Duke University Medical Center, Durham, NC

Many medical imaging examinations involve exposure to ionizing radiation. The exposure amount in these exams is very small, to the extent that the health risk associated with such low levels of exposure is frequently debated in scientific meetings. Nonetheless, the prevailing scientific view is that there is a finite (though small) amount of risk involved with such exposures. The risk is increased with the amount of exposure, with repeated exposures, and when the patient is young. This material aims to provide a brief overview of the risk associated with medical imaging examinations that involve ionizing radiation.

A. Radiation Biology Review

Ionizing radiation can cause tissue damage. Tissue damage occurs through the change in chemical properties of molecules in the tissue following exposure to radiation. The major contributor to damage from radiation is through radiation changing a water molecule into a new form called a “free radical.” Free radicals are highly chemically active and as such can have reactions with genetic molecules of the cell (i.e., with the DNA). This can cause damage to the DNA, most of which is readily repaired by the cell. If it is not, it can result in cell death. Alternatively, if the DNA damage is repaired erroneously, it can result in an alteration of the genetic encoding, leading to hereditary changes or cancer induction.

Changes that result in cell death are termed “Deterministic Effects,” while changes to the DNA encoding that lead to other adverse changes are termed “Stochastic Effects” (see Figure 1).

 

Figure 1

A.1. Deterministic Effects (Cell Death)

Cells are dying all of the time in the body from physical, chemical, and other causes (i.e., “natural causes”). In most cases these cells are replaced or the body adapts to function normally when this occurs. But if too many cells die, the damage caused may not be compensated for very easily. Whether the organ can continue to function depends on how much of the organ is damaged and the number of cells within the organ that are damaged. This is due to the structure of tissues and organs into functional subunits (FSU). An FSU is a set of tissues or organs whose ultimate function is dependent on the overall workings of each subunit; e.g., proper digestion of food requires the entire digestive tract to function properly – stomach, intestines, etc. But when the tissue or organ has parallel functional structure, rather than in series like the digestive tract, damage from any source can be compensated for more easily and normal function can be maintained. Therefore, whole organ irradiation or irradiation of an entire FSU reduces or eliminates the ability of a tissue or organ to be repaired. Yet if some part of the FSU is left unexposed, partial function can continue and repair of the damage may be possible. Deterministic effects can be thought of as effects in which the outcome can be determined; i.e., the effects are predictable. Deterministic effects will occur if the radiation deposits enough energy in tissue to disrupt the tissue’s FSU enough. The amount of energy required to cause these changes is different for different tissues and this amount of radiation is called the threshold dose for tissue damage.

Deterministic effects are usually divided into tissue-specific/local changes and whole-body effects. Examples of tissues that are known to demonstrate deterministic effects from radiation exposure are:

  1. Tissue-specific damage from radiation
    1. Lens of the eye
    2. Detectable opacities
    3. Cataract formation
    4. Skin
      1. Skin reddening (erythema)
      2. Hair loss (depilation)
      3. Skin cell death with scarring (necrosis)
      4. Reproductive organs
      5. Infertility
      6. Whole-body radiation damage (only occurs in extremely high radiation exposures, beyond those produced by any diagnostic imaging system)
      7. Bone marrow damage/reduction of blood cell production
      8. Gastrointestinal mucosa lining loss
      9. Central nervous system tissue damage

      The amount of radiation required to produce these deterministic effects has been derived from studies in experimental cell cultures and animal studies as well as human epidemiology studies. From these studies, the dose thresholds have been established where the effect is observed in 1% of a population (see Table 1). This means these values represent the amount of radiation energy absorbed by the tissue where if 100 people were exposed to this level of radiation, only a single individual would experience this effect. The unit used for absorbed radiation dose in Table 1 is the Gray (Gy). This value is the standard international measure for absorbed radiation energy. We will see later that this unit must be converted to another unit to understand the stochastic effects (i.e., genetic and cancer effects) of radiation.

      Table 1: Dose Threshold for Deterministic Effects*
      Tissue Total acute dose threshold (Gy) Time to develop effect
      Lens of eye
      Detectable opacities 0.5–2 > 1 year
      Cataract formation 5.0 > 1 year
      Skin
      Skin reddening 3–6 1–4 weeks
      Temporary hair loss 4 2-3 weeks
      Skin death and scarring 5-10 1-4 weeks
      Testes
      Temporary sterility 0.15 3-9 weeks
      Permanent sterility 3.5–6 3 weeks
      Ovaries
      Permanent sterility 2.5–6 < 1 week
      Gastrointestinal
      Mucosa lining loss 6 6-9 days
      Bone Marrow
      reduction of blood cell production 0.5 1-2 months
      * 1% incidence level based on ICRP publication 103 (2007)

      Reviews of biological and clinical studies have shown that below 0.1 Gy no deterministic effects from radiation exposure have been proven. This is primarily due to the fact that cellular repair mechanisms occur continuously and this prevents deterministic effects at low radiation exposure levels.

      The effects from radiation exposures at X-ray energies do not occur during or immediately after the exposure to radiation. This is shown in the column labeled “Time to develop effect” in Table 1. Unlike the exposure to the sun that causes skin reddening within hours of the exposure, the effects from these high-absorbed radiation doses require weeks to years following the exposure to produce the effects listed. Therefore, the effect will not be seen at the time of exposure and when the effect does occur the correlation to the radiation exposure may not be easily determined.

      A.2. Stochastic Effects (Genetic Changes and Cancer)

      Stochastic effects are random or probabilistic in nature. By being random, the occurrence of individual events cannot be predicted. Stochastic effects can be divided into two groups, genetic and carcinogenic effects. Based on the random nature of these effects, the production of genetic changes or induction of cancer in an individual cannot be determined for certain, regardless of the amount of energy absorbed; only the probability or the likelihood can be ascertained. Furthermore, there isn’t a threshold dose above which these effects will definitely occur.

      Genetic Changes

      The exposure to radiation can cause damage in germ cells that ultimately result in mutations in the exposed person’s fetus if she is pregnant. Such mutations are not radiation-specific; the radiation only produces DNA sequencing errors that might have occurred naturally. Therefore, instead of producing unique mutations, damage from radiation exposure only results in a higher frequency of normal/spontaneous mutations. This means radiation does not cause the production of monsters as seen in the movies. In addition, a large number of animal and human studies have shown that the adverse effects from radiation exposure are negligible in subsequent generations.

      Specifically, there is no direct evidence at any radiation dose that exposure of parents leads to excess genetic disease in their offspring. Therefore, radiation exposure can cause mutations in children if the reproductive cells of the parent are exposed, but the child does not carry any adverse genetic trait produced by the radiation exposure that can be passed on to their offspring. Given these facts, it is very important to inform your physician and the person performing the X-ray study if you are or may be pregnant. As we will discuss later the exposure of a fetus to radiation is a major concern for radiation effects that may occur in a child if the child is exposed before birth.

      Cancer Induction

      Cancer induction is arguably the most important and the most feared radiation effect. From the discovery of ionizing radiation there has been documented evidence of radiation-induced cancer in animal and human studies. The initial human experiences were all at high radiation dose levels from people working with radiation or using radiation without the knowledge of its potential harm. In addition, long-term follow-up studies of the Japanese survivors of the atomic bomb attacks on Hiroshima and Nagasaki and the early medical usage of radiation in treatment and diagnostic studies have shown increased cancer incidence in the exposed populations.

      All radiation effects have a latency period between the time of exposure and the onset of the effect, as seen with deterministic effects in Table 1. For cancer induction, the latency period is on the order of years, with leukemia having the shortest latency period (5 to 15 years) and solid tumors having the longest latency period (10 to 60 years). Therefore, it is very difficult to prove that a cancer is directly related to earlier radiation exposure, because other factors encountered during the latency period may be the actual cause of the cancer. This is particularly true when the exposures are at low radiation levels such as those received in diagnostic radiology and cardiology studies.

      Currently, at low radiation exposure levels no study has been comprehensive enough to demonstrate stochastic effects conclusively. But as stated above, at very high radiation exposure levels there is good data that proves the induction of cancer from the exposure. So the estimation of risk for cancer induction at low radiation exposure must be extrapolated from the high exposure-level data. This is where most of the controversy concerning radiation effects exists. The most conservative estimation of risk from radiation exposure assumes the effects from low radiation exposure are a simple scaled version of the high-exposure results (i.e., a linear or straight-line extrapolation from the high- to the low-exposure results). Most groups that monitor and analyze radiation exposures use this linear extrapolation model to estimate cancer induction from radiation.

      Risk models

      Currently there are two models used to assess risk of stochastic effects from radiation exposure; these are the absolute and relative risk models.

      Absolute risk is defined as the probability that a person who is disease-free at a specific age will develop the disease at a later time following exposure to a risk factor; e.g., the probability of cancer induction following exposure to radiation.

      The age-adjusted cancer incidence rate in the United States from 2001 to 2005 was 467 cancers per year per 100,000 men and women

      (US Surveillance, Epidemiology and End Results (SEER) Program, see http://seer.cancer.gov)

      The relative risk model assumes radiation increases the natural incidence of a cancer and it is expressed as a fraction or multiple of the naturally occurring risk. This value is always greater than 1 (unless the radiation is assumed to produce a beneficial effect -- this hypothesis is known as Hormesis and it will not be considered here). Most advisory publications use the relative risk because it has some mathematical and statistical advantages when derived from epidemiological studies. The current accepted values of relative risk are given in Table 2. Note that a new unit of radiation exposure is used called the Sievert (Sv). This unit is used when defining the effective dose from radiation exposure. It is well known that different tissues react differently to radiation; i.e., some tissues are more sensitive to radiation damage than others. The sensitivity of tissue to radiation is related to the type of radiation (X-rays, alpha particles, etc.) that exposes the tissue and the tissue type that is exposed due to the tissue’s cell age, mitotic cycle and other factors. Therefore, when discussing radiation effects relative to cancer induction, the absorbed radiation energy is normalized to the tissue’s sensitivity. This normalization is designated by the use of the Sievert instead of the Gray (the Gray designates the absorbed energy in the tissue only – see Deterministic Effects Section).

      Also, when converting the absorbed dose in Gray to the effective dose in Sievert, the geometry of the exposure needs to be known to account for each tissue type that is in the radiation field of view and all tissues that are in or near the radiation field of view that may be exposed to scatter radiation. With the knowledge of what tissue is exposed and the tissue’s sensitivity to radiation, models based on an average human determine the conversion factor from Gray to Sievert. Most imaging studies only involve exposure to a portion of the body, and even so, a fraction of the radiation does not interact with the tissue in the patient at all, but travels through the patient to create the image. Therefore, the conversion from the Gray to Sievert results in each tissue receiving only a fraction of the total energy that entered the patient. The range of values for this conversion is from 1% for radiation-insensitive tissue up to 12% for the most sensitive tissue. Therefore, the effective dose (i.e., Sv) is always less than the absorbed dose (i.e., Gy).

      Table 2: Nominal Risk for Cancer Effects *
      Exposed population Excess relative risk of cancer
      (per Sv)
      entire population 5.5% – 6.0%
      adult only 4.1% – 4.8%
      *relative risk values based on ICRP publications 103 (2007) and 60 (1990)

      Since the results of radiation effects on cell cultures, animal studies, and human epidemiology studies may be interpreted differently, you may see variations in the published relative risk values, but they are all within a few percentage points of each other. Many authors use an average value of 5% per Sievert when discussing the risk of cancer from radiation exposure.

      Note that the values in Table 2 are for adults, or all people (i.e., entire population). It is known that the sensitivity to radiation varies based on the cell age and mitotic cycle. This suggests that children should have a higher relative risk when compared with adults due to their increased growth rate and ongoing cellular differentiation. This is why there is an increase in the relative risk values for the “entire population” in Table 2. In Figure 2, the estimated lifetime risk that radiation will produce cancer (carcinogenesis) is presented relative to the person’s age. This shows that children have a 10% - 15% lifetime risk from radiation exposure while individuals over the age of 60 have minimal to no risk (due to the latency period for cancer and the person’s life expectancy).

       

      Figure 2 Adapted from ICRP Publication 60 (1990)

      These data also demonstrate that you cannot simply use the average relative risk shown in Table 2 to estimate the increased incidence of cancer due to radiation exposure. In order to do this analysis correctly you need take into consideration the age of all individuals in the irradiated group.

      A.3. Special Considerations for Embryo/Fetus

      The early development of life is a time when rapid cell division and differentiation are occurring. Therefore, radiation sensitivity is high for the developing embryo/fetus, and radiation protection needs to be considered differently than for the general public. Table 3 provides a review of the stage and deterministic effects that may occur in the embryo/fetus following exposure to different levels of radiation. Similar to what was shown in Table 1, deterministic effects below an absorbed dose of 0.1 Gy are not found, even in the embryo/fetus.

      Table 3: Summary of Suspected In-Utero Induced Deterministic Radiation Effects*
      Menstrual or gestational age Conception age <0.05 Gy 0.05-0.1 Gy >0.1 Gy
      0 - 2 weeks Prior to conception None None None
      3rd and 4th weeks 1st - 2nd weeks None Probably none Possible spontaneous abortion
      5th - 10th weeks 3rd - 8th weeks None Potential effects are scientifically uncertain and probably too subtle to be clinically detectable Possible malformations increasing in likelihood as dose increases
      11th - 17th weeks 9th - 15th weeks None Potential effects are scientifically uncertain and probably too subtle to be clinically detectable Increased risk mental retardation or deficits in IQ that increase in frequency and severity with increasing dose
      18th - 27th weeks 16th - 25th weeks None None IQ deficits not detectable at diagnostic doses
      >27 weeks >25 weeks None None None applicable to diagnostic medicine
      *Taken from “ACR Practice Guideline for Imaging Pregnant or Potentially Pregnant Adolescents and Women with Ionizing Radiation”, derived from ICRP Publications 84 (2001) and 90 (2004).

      Although deterministic effects are not seen at low dose levels in the embryo/fetus, there have been many studies that have shown an increased incidence of cancer (i.e., stochastic effects) in children following in-utero exposure to radiation. ,Pre-natal radiation exposures resulted in an increased cancer rate in the offspring of survivors of the atomic bombings in Hiroshima and Nagasaki. In other epidemiological studies there have also been good statistical results that demonstrate an increased cancer rate in children following pre-natal radiation exposure from diagnostic radiology studies. Unfortunately, these epidemiological studies do not provide very good data on the specific absorbed dose received by the fetus or embryo. This limits the ability to accurately characterize the dose vs. response, as has been done for deterministic effects. But since the doses received in these epidemiology studies were in the diagnostic radiology range, they suggest that low levels of radiation exposure to the embryo/fetus definitely increase the risk of childhood cancer.

      B. Dose Metrics

      In the context of dose quantities relevant to the topic of radiation risk, two types of quantities are of importance: dose limits and reference levels. Dose limits refer to the maximum level of dose that the general public can receive from a source other than natural background radiation levels and those received by occupational workers in their job. The reference levels reflect the typical dose values expected in the majority of imaging studies. Both quantities can be described in terms of a number of dose-related metrics, including absorbed dose, effective dose, exposure, or any modality-specific dose index (e.g., CTDI for CT imaging).

      B.1. Dose Limits

      Limits for exposure to radiation should be at a level below the threshold where deterministic effects occur; i.e., below 0.1 Gy. Furthermore, the limit should exclude exposures from background radiation. Because radiation is around us all of the time from the sun and naturally occurring sources it would not make sense to try to limit radiation exposure below natural background levels.

      As the main concern are stochastic effects, the effective dose with units of Sieverts or 1/1000 of a Sievert (i.e., mSv) is most commonly used. Using the threshold dose as a starting point, dose limits are determined using the Principles of Justification and Optimization.

      • Principle of Justification: Any decision that alters the radiation exposure to an individual or population should have an outcome that does more good than harm. This means that any radiation source should provide a benefit with its use, either to the individual or to society at large, and the risk of any detrimental effects must be small relative to any benefit.
      • Principle of Optimization: The application of radiation in any situation should be developed to minimize the risk of exposure while maximizing the benefit. Overly conservative reduction of radiation exposure from medical procedures may limit the diagnostic quality of the procedure, resulting in a reduction in the patient’s overall medical outcome. When the medical benefit is retained or maximized, the risk should be as low as possible. The Principle of Optimization is analogous to the As Low As Reasonably Achievable (ALARA) concept.

      It is important to understand that dose limits are not levels of exposure that should be considered acceptable in an occupation or that can be safely received by the general public. They are rather maximum limits consistent with the current state of medical practice. In general, the concept of ALARA should still be used when developing radiation protection procedures/policies, and dose limits should be thought of as the maximum exposure that should be allowed in any situation. In most states the ALARA concept requires investigating dose-reduction methods, even if only a fraction of the limit is received by any individual.

      B.2. Reference Levels

      The risk and benefit from medical exposures are received by the same individual. Since the individual’s situation, body habitus, and medical needs are unique, dose limits do not make sense for medical exposures. Yet an average radiation exposure level received from a diagnostic or interventional procedure can be used to evaluate whether the dose being used for a procedure is within an acceptable range. These exposure levels are called reference levels and they are exam-specific.

      Reference levels for medical exposures are usually set at the uppermost value seen in normal practice. These values should be considered the typical operational dose for an average-size patient; dose for the majority of exams will be below these values. In normal circumstances, any exam that exceeds the reference level should be investigated to determine methods to reduce the exposure for that exam type at the institution. However, an individual’s exposure may be above the reference level due to variations in the patient’s size and complications that may occur during the exam. If the variation in the patient and/or procedure can be considered non-routine and justified, an exposure above the reference level may not need to be considered for corrective action.

      Table 4: Diagnostic Exam Reference Levels
      Exam Reference level
      PA chest 25 mR
      AP abdomen 600 mR
      Fluoroscopy of the abdomen 6.5 R/min
      CT head 75 mGy CTDI
      CT abdomen 25 mGy CTDI
      CT Pediatric abdomen 20 mGy CTDI
      * Taken from ACR Practice Guideline for Diagnostic Reference Levels in Medical X-Ray Imaging - Revised 2008 (res. 3)

      There are a number of sources for reference level values. Some of these can be found in the Section on Information Sources and Recommended References and Citations. The American College of Radiology (ACR) has developed a comparative scale for the Relative Radiation Level (RRL) values based on effective dose (Table 5), which may be used toward reference levels or simple comparison of exams.

      Table 5: Relative Radiation Level Scale
      Relative Radiation Level Effective dose range
      None 0
      Minimal Less than 0.1 mSv
      Low 0.1 – 1.0 mSv
      Medium 1.0 – 10 mSv
      High 10 – 100 mSv
      * Adapted from ACR Appropriateness Criteria, Radiation Dose Assessment Introduction 2008

      Tables 6 – 9 give a sample of the Relative Radiation Level and the range of effective dose values reported for specific diagnostic and interventional radiology examinations. These values are for an average adult patient using typical equipment and techniques. The average effective dose in all of these exams is below 100 mSv (i.e., 0.1 Sv). Therefore, deterministic effects should not be seen for an average patient/exam receiving diagnostic radiology exams. But the potential for stochastic effects must always be considered when an examination is planned.

      Table 6: Average Effective Dose in Diagnostic Radiology*
      Exam Relative Radiation Level Range of values (mSv)
      Extremity 0.0002 - 0.1
      Chest X-ray PA / LAT 0.007 - 0.24
      Mammography 0.1 – 0.6
      Abdomen / Pelvis 0.04 - 1.2
      Thoracic / Lumbar Spine 0.5 – 1.8
      IVU 0.7 – 3.7
      Upper GI w/fluoroscopy 1.5 - 12
      Barium enema w/fluoroscopy 2 - 18

      Table 7: Average Effective Dose in CT*
      Exam Relative Radiation Level Range of values (mSv)
      Head 0.9 – 4
      Chest (standard) 4 – 18
      Chest (high resolution,
      e.g., pulmonary embolism)
      13 – 40
      Abdomen 3.5 – 25
      Pelvis 3.3 – 10
      Coronary Angiogram 5 – 32
      Virtual Colonoscopy 4 – 13
      Calcium Scoring 1 - 12

      Table 8: Average Effective Dose in Interventional Radiology*
      Exam Relative Radiation Level Range of values (mSv)
      Head/Neck angiography 0.8 – 19.6
      Coronary angiography (diagnostic) 2 – 15.8
      Coronary angioplasty, stent placement, RF ablation 6.9 – 57
      TIPPS 20 – 180

      Table 9: Average Effective Dose in Nuclear Medicine*
      Exam Relative Radiation Level Effective dose/ administered activity (mSv/MBq)
      Brain (Tc99m) 0.0093 – 0.0077
      Brain PET (18F-FDG) 0.019
      Thyroid scan (123I) 0.075 (w/15% uptake)
      Thyroid scan (Tc99m) 0.013
      Cardiac Stress Test
      (depending on isotope/protocol)
      0.0085 – 0.22
      Cardiac PET (18F-FDG) 0.019
      Lung Perfusion (Tc99m) 0.011
      GI Bleed 0.007
      Renal
      (depending on isotope/protocol)
      0.0049 – 0.0088
      Bone 0.0057

      C. Balancing Benefit and Risk

      With an understanding of the effects of radiation and the doses for standard examinations, a physician (possibly with the help of a radiologist) can make a determination of which examination provides the most benefit to the patient at the lowest possible dose. To do this, the physician needs to consider the following criteria:

      1. Patient’s clinical conditions -- what are the benefits of using radiation?
      2. Availability of equipment. New and changing radiation equipment technology makes the availability of some procedures limited.
      3. Availability of personnel. Personnel must have the appropriate training on the equipment to perform the procedure desired.
      4. Alternative exams: All other non-ionizing radiation options (e.g., ultrasound, MRI, EEG, EKG, etc.) for the specific situation cannot provide the desired outcome

      The radiologist and the referring physician ultimately make the choice on which examination to perform, taking into consideration all information known for the specific patient’s situation and the imaging options. In general, the use of non-radiation tests should be considered before using radiation, and less invasive procedures should be considered before more invasive techniques are chosen.

      The ACR has organized several expert panels to develop criteria for determining appropriate imaging examinations for specific medical conditions. The ACR Appropriateness Criteria® are based on the complexity and severity of a patient’s clinical condition and include those exams that are generally used for evaluation of these conditions. All exams that have potential to be used in the specific situation are then ranked based on the appropriateness criteria, taking into account the procedures’ risks vs. benefit as defined by the panel of experts. For example, Table 10 shows the Appropriateness Criteria for diagnosing abdominal pain and fever in an adult patient. Table 11 shows an alternate set of criteria if the patient is pregnant.

      Table 10: Acute Abdominal Pain and Fever
      Patient presenting with fever, non-localized abdominal pain and no recent operation*
      Exam Rating
      1 = least appropriate
      9 = most appropriate
      RRL scale
      CT abdomen and pelvis w/contrast 8
      CT abdomen and pelvis w/o contrast 6
      US abdomen 6 None
      X-ray abdomen 6
      X-ray upper GI series with small bowel 5
      X-ray colon contrast enema 5
      Nuclear ImagingGa-67 of abdomen 5
      Nuclear Imaging Tc99m WBC abdomen and pelvis 5
      MRI abdomen and pelvis w/o contrast 5 None
      MRI abdomen and pelvis w/contrast 5 None
      Interventional arteriography visceral 2
      *adapted from ACR Appropriateness Criteria October 2008.

      Table 11: Acute Abdominal Pain and Fever in a Pregnant Patient
      Patient presenting with fever, non-localized abdominal pain and no recent operation*
      Exam Rating
      1 = least appropriate
      9 = most appropriate
      RRL scale
      US abdomen 8 None
      MRI abdomen and pelvis w/o contrast 7 None
      MRI abdomen and pelvis w/contrast 7 None
      CT abdomen and pelvis w/contrast** 5
      CT abdomen and pelvis w/o contrast 5
      X-ray abdomen 4
      X-ray upper GI series with small bowel 2
      X-ray colon contrast enema 2
      Nuclear Imaging Ga-67 of abdomen 2
      Nuclear Imaging Tc99m WBC abdomen and pelvis 2
      Interventional arteriography visceral 2
      *adapted from ACR Appropriateness Criteria® October 2008
      ** only after all exams that do not use ionizing radiation have been used or ruled out as possible.

      The ACR Appropriateness Criteria are for generic patient situations and do not take into account secondary conditions the patient may have. For example, it is known that some inherited syndromes (e.g., Down syndrome, Fanconi’s anemia, Ataxia-telangiectasia) result in increased sensitivity to radiation. In addition, these criteria assume all equipment options are available at a given site.

      To get a better understanding of the examination being ordered, the ACR and the Radiological Society of North America (RSNA) have established the RadiologyInfo.org website (www.radiologyinfo.org) to inform and educate the public about radiologic procedures. This site explains how various X-ray, CT, MRI, ultrasound, radiation therapy, and other procedures are performed. It also addresses what the patient may experience and how to prepare for the exams. The website contains over 100 radiologic procedures and is updated frequently with new information.

      Improved awareness and recommendations for imaging pediatric patients have also been initiated by the Alliance for Radiation Safety in Pediatric Imaging. This initiative is called Image Gently. The goal of this campaign is to raise awareness of radiation dose when imaging children and to suggest methods/processes to provide acceptable images at the lowest doses.

      D. Perception of Risk

      There is risk in all aspects of life. The best that can be hoped for is to minimize the risks that have the greatest potential for disrupting one’s life. When a risk has a benefit to an individual or to society the risk may be justified. But how can both the risks and the benefits be explained? This requires knowledge of how people perceive risk and how to communicate the risks and the benefits to different populations.

      D.1. How to Convey Technical Information to the Public

      Medical environments are full of technical jargon that is not understood by the public or even all personnel within different medical professions. Technical information must be conveyed in simple, clear terms. In addition, care must be taken to make sure important ideas are emphasized and not lost in the discussion. In general the following principles should be used when trying to convey technical information to the public:

      • Avoid using technical/medical jargon
      • Translate technical/medical terms (e.g., dose) into everyday language
      • Write short sentences that convey a single point
      • Use headings and other formatting techniques to provide a clear and organized structure to the presentation of information

      D.2. Risk Communication vs. Risk Education

      Risk communication differs from risk education. When you are attempting to discuss risk you need to understand the value systems of the people you are talking to. This requires an understanding of how different groups may interpret risk.

      Risk Ranking

      Differences between how scientists and non-scientists rank risk is one of the major problems of risk communication. In general, if scientists and non-scientists are asked to rank a series of health risks, the rank orders of the lists are considerably different. This is demonstrated in Table 12, where three different groups were asked to rank 30 activities/sources of risk from the most risky (ranked as 1) to the least risky (ranked at 30). The top 10 risky activities are highlighted. At best, there is a correlation coefficient of 0.6 between the scientific community (i.e., professional society members) and the other groups. At the time this study was conducted, X-ray exposure was ranked 24th in risk by the experts and 17th or 22nd bin risk by the other groups. Because it is clear that different groups will assess risk differently we need to understand where these differences come from in order to communicate risks and benefits better.

      Table 12: Perception of Risk*
      Activity (ranked by experts) League of Women Voters College students Professional society members
      Motor Vehicles 2 5 3
      Smoking 4 3 4
      Alcohol 6 7 5
      Handguns 3 2 1
      Surgery 10 11 9
      Motorcycles 5 6 2
      X-rays 22 17 24
      Pesticides 9 4 15
      Electric Power 18 19 19
      Swimming 19 30 17
      Contraceptives 20 9 22
      Private Aviation 7 15 11
      Large Construction 12 14 13
      Food Preservatives 25 12 28
      Bicycles 16 24 14
      Commercial Aviation 17 16 18
      Police Work 8 8 7
      Fire Fighting 11 10 6
      Railroads 24 23 29
      Nuclear Power 1 1 8
      Food Coloring 26 20 30
      Home Appliances 29 27 27
      Hunting 13 18 10
      Antibiotics 28 21 26
      Vaccinations 30 29 29
      Spray Cans 14 13 23
      Football 23 26 21
      Power mowers 27 28 25
      Mountain Climbing 15 22 12
      Skiing 21 25 16
      *adapted from Slovis P, Science Vol. 236 No. 4799 (1987)


      Objective Risks Vs. Subjective Risks

      There are two basic translations of how risk is interpreted. These are objective in structure (how the scientific community normally interprets risk) and subjective in nature, which is often used by the general public. Objective assessment of risk is what we have used in this document and is based upon peer-reviewed scientific analysis of risks. Yet the general public may be getting their risk assessment from less technical information sources such as non-peer reviewed publications and journals (newspapers, magazines, etc.), non-peer-reviewed internet sites (e.g., Wikipedia), non-documentary based television shows (e.g., Gray’s Anatomy, ER), and personal communication in social settings (e.g., discussion with friends). In addition, unlike scientific analysis, the public is unlikely to recall where a fact was presented to them and may not be able to recall whether the National Enquirer or the proceedings of the National Academy of Sciences presented the fact. As a result, equal weight may be given to data presented by any source.

      The most important point the physician needs to understand is that although they may know the objective risk of an examination or procedure, they need to be talk with the patient based on the patient’s subjective risk assessment. The methods to do this are educational and motivational rather than scientific.

      Furthermore, in medical situations the patient and their family or friends are often confused and under high stress. In these situations several things need to be considered:

      • People often have difficulty processing information and do not “hear” what is being said to them
      • People often become distrustful of anything a person is saying, and therefore do not listen to what is being said
      • People often give greater weight to negative information than to positive information

      In risk perception theory, perception equals reality. This means there may be no correlation between public perceptions of risk and scientific or technical information. Therefore, you must discuss the risk based on the perception. In order to accomplish this, Table 13 reviwes several fundamental dos and don’ts of communicating risk.

      Table 13: Checklist on Dos and Don’ts When Communicating Risks*
      Category Dos Don’ts
      Truthfulness Tell the truth Do not lie or avoid the truth
      Absolutes Avoid absolutes --nothing is absolute Do not use the terms “never” or “always”
      Jargon Define all terms and acronyms Do not use standard medical terminology
      Negative Use positive or neutral terms Do not use negative terms or negative associations
      Temper Remain calm Do not let your feelings interfere with your ability to communicate
      Clarity Ask whether you are being understood Do not assume understanding
      Abstraction Use examples, metaphors, and analogies to aid understanding Do not talk of theoretical concepts without using clear non-technical justification
      Attack Only attack the issue Do not attack the person or organization that may have made incorrect statements
      Promise Promise only what you are certain will occur Do not make promises that you cannot back up and follow through on to ensure they occur
      Speculation Provide information only on what is being done and what you know Do not discuss worst-case scenarios and unintended possible outcomes, unless required by protocol
      Risk/benefit comparison Make risk and benefit statements separately Do not discuss the risk relative to the benefit
      Risk comparisons Use tested comparison messages, cite trustworthy data/groups Do not compare unrelated risks
      *adapted from EPA Workbook on Risk Communication in Action (2007)

      D.4. Risk Comparison

      When making risk comparisons, the best practice is to use the following criteria:

      • Make comparison of the same risk at two different times or circumstances
      • Make comparison with a standard that is understood by the listener
      • Make comparison with different estimates of the same risk

      Often you will read risks compared for unrelated situations. For example, in Table 14 the odds of dying from accidental death are shown. Note the lifetime odds of dying from an injury for a person born in 2005 were 1 in 22 (i.e., 4.5%). This suggests the odds of dying from accidental causes are similar to getting cancer from an exposure of 1 Sv at a later time (see Table 2). The latency is a factor that weights heavily on the risk perception and as such, equal risk factors of different timescale cannot be directly compared. Furthermore, an accidental injury cannot be related to a decision about a medical procedure where the risk of not performing it would have its own associated risk. Herein lies one of the main challenges in communicating risk associated with medical exposures. The exposure involves a finite stochastic risk with a very long latency period whereas not performing the procedure would have another risk with possibly a shorter time horizon.

      Table 14: Odds of Death From Injury* (Poor comparison for radiation risk)
      Type of incident / Manner of injury Number of deaths in 2005 Probability of occurrence
      All causes of mortality from injuries 176,406 4.5%
      Transport accidents 48,441 1.3%
      Automobile 14,584 0.4%
      Pedestrian 6,074 0.2%
      Air travel 590 0.02%
      Non-transportation accidents 69,368 1.8%
      Falls 19,656 0.5%
      Being struck by objects 2,845 0.07%
      Intentional self-harm 32,637 0.9%
      Assault 18,124 0.5%
      Complications from medical care 2,653 0.07%
      *adapted from National Safety Council, http://www.nsc.org/research/odds.aspx

      If the issue is primarily communicating the risk of medical radiation alone, a good approach would be to give the risk of exposure to radiation in a radiology exam as an equivalent amount of exposure to the natural background radiation. This is shown in Table 15

      Table 15: Comparison of Adult Exam Dose to Background Radiation Level
      Exam Reference level
      (time to receive equivalent background radiation)
      Chest X-ray PA / LAT 2.4 days / 12 days
      Mammography 1 ½ months
      Abdomen / Pelvis X-ray 3 months
      Head CT 8 months
      Lung Perfusion (Tc99m) 8 months
      Thyroid scan (Tc99m) 1 ½ years
      Brain (Tc99m) 2 years
      Abdominal CT 2 ½ years
      Cardiac Stress Test
      (depending on isotope/protocol)
      3 years – 13 ½ years
      Cardiac PET (18F-FDG) 5 years
      High resolution Chest CT
      (e.g. pulmonary embolism, angiogram)
      5 years
      * Using an average background radiation level of 3 mSv/yr and Tables 8-11

      Other comparisons that might be acceptable would be the estimated risk from cancer induction from naturally occurring or human-induced carcinogens; i.e., radon, arsenic, smoking, etc.

      E. Benefits Vs. Risk of Not Using Radiation

      It would be difficult to address all the benefits that are associated with the utilization of radiation in our everyday lives, including its use in medicine. Suffice it to say that radiation is extremely beneficial in many aspects of life when used appropriately. With respect to the use of radiation for diagnosis, assistance in medical interventional procedures, and therapy, the benefits need to be weighed relative to the potential risks. As we have discussed, the risk of radiation-induced effects are not well understood at the levels of radiation used for diagnostic and interventional procedures. But there are clearly risks associated with not performing an exam that should also be considered.

      The risks to consider of NOT performing an exam include missing a diagnosis and/or initiating treatment too late to improve the medical outcome. The potential to reduce a patient’s overall life expectancy due to a disease must also be considered in conjunction with the latency period for radiation-induced cancer and the age of the patient.

      F. Information Sources

      There are many organizations and advisory groups that monitor and assess radiation use and the risks associated with its use. These sources should be considered when developing teaching material or when determining whether radiation information being presented is valid. A brief description of each of these organizations based on their mission statement is given below.

      • American Association of Physicists in Medicine (AAPM - www.aapm.org )

        Association involved in the advancement of the practice of physics in medicine and biology by encouraging innovative research and development, disseminating scientific and technical information, fostering the education and professional development of medical physicists, and promoting the highest quality medical services for patients.

      • American College of Radiology (ACR - www.acr.org)

        A professional society involved in maximizing the value of radiology, radiation oncology, interventional radiology, nuclear medicine, and medical physics by advancing the science of radiology, improving the quality of patient care, positively influencing the socio-economics of the practice of radiology, providing continuing education for radiology and allied health professions, and conducting research for the future of radiology.

      • Conference of Radiation Control Program Directors (CRCPD - www.crcpd.org )

        A non-profit professional organization dedicated to radiation protection and the consistent promotion of methods to resolve radiation protection issues, to encourage high standards of quality in radiation protection programs, and to provide leadership in radiation safety and education.

      • International Commission on Radiation Protection (ICRP - www.icrp.org)

        An independent registered charity established to advance for the public benefit the science of radiological protection, in particular by providing recommendations and guidance on all aspects of protection against ionizing radiation.

      • National Council on Radiation Protection and Measurements (NCRP - www.ncrponline.org)

        Chartered by the U.S. Congress to collect, analyze, develop, and disseminate in the public interest information and recommendations about protection against radiation and radiation measurements, quantities and units, particularly those concerned with radiation protection.

      • National Research Council (NRC) of The National Academies of Sciences (NAS - http://sites.nationalacademies.org/nrc/index.htm)

        A private, nonprofit institution that provides science, technology, and health policy advice to governments.

        • See Biological Effects of Ionizing Radiation Reports (e.g. BEIR VII Phase II).

        • Nationwide Evaluation of X-ray Trends (NEXT - www.fda.gov/cdrh/radhealth/next.html)

          Joint effort of the FDA Center for Devices and Radiological Health (CDRH) and the Conference of Radiation Control Program Directors (CRCPD) to characterize the radiation doses patients receive and to document the state of the practice of diagnostic radiology.

        • Radiological Society of North American (RSNA - www.rsna.org)

          A professional society involved in promoting and developing the highest standards of radiology and related sciences through education and research. The Society seeks to provide radiologists and allied health scientists with educational programs and materials of the highest quality, and to constantly improve the content and value of these educational activities.

          • See www.radiologyinfo.org – website designed for the general public to answer questions related to the many radiologic procedures and therapies.

          • Society of Pediatric Radiology (www.pedrad.org)

            Alliance for Radiation Safety in Pediatric Imaging, i.e., Image Gently campaign

            (www.pedrad.org/associations/5364/ig/)

            Alliance of organizations and people with a goal to change the practice of imaging children through increasing awareness of the opportunities to lower radiation dose in imaging. The Alliance began as a committee within the Society for Pediatric Radiology.

          • United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR - www.unscear.org)

            Established by the General Assembly of the United Nations to assess and report levels and effects of exposure to ionizing radiation.

          • Center for Devices and Radiological Health (CDRH - www.fda.gov/cdrh)

            Component of the U.S. Food and Drug Administration (FDA) that provides independent, professional expertise and technical assistance on the development, safety and effectiveness, and regulation of medical devices and electronic products that produce radiation.

          • U.S. Surveillance, Epidemiology and End Results Program (SEER - http://seer.cancer.gov)

            An adjunct to the National Cancer Institute that is a source for cancer statistics in the United States.

          G. Recommended References and Citations

          • Broadbent M.V., Hubbard L.B., “Science and Perception of Radiation Risk,” Radiographics, March 1992.
          • Gail M.H., “Models of Absolute Risk, Use, Estimation and Validation,” Cancer Chemoprevention, Volume 2: Strategies for Cancer Chemoprevention, edited by Kelloff G.J., Hawk E.T., Sigman C.C., Humana Press, 2005.
          • Hall E.J., Garcia A.J., Radiobiology for the Radiologist, sixth edition. Lippincott Publishing, 2006.
          • Hendee W.R., “Personal and Public Perceptions of Radiation Risks,” Radiographics, November 1991.
          • ICRP 103 Recommendations of the ICRP, Volume 37 (2-4), 2007.
          • ICRP 60 Recommendations of the ICRP, Volume 21 (1-3), 1991.
          • ICRP 84 Pregnancy and Medical Radiation, Volume 30 (1), 2001.
          • ICRP 90 Biological Effects After Prenatal Irradiation, Volume 33 (1-2), 2004.
          • ICRP 99 Low-Dose Extrapolation of Radiation-Related Cancer Risk, Volume 35 (4), 2005.
          • ICRP 105 Radiological Protection in Medicine, Volume 37 (6), 2007.
          • ISO 14971, Medical Devices—Risk Management—Application of Risk Management to Medical Devices, 2007.
          • Kaste S.C.; “Imaging Challenges: A U.S. Perspective on Controlling Exposure to Ionizing Radiation in Children With Cancer,” Pediatric Radiology, volume 39, supplement 1, 2009.
          • Kleinerman R.A., “Radiation-Sensitive Genetically Susceptible Pediatric Sub-Populations,” Pediatric Radiology, volume 39, supplement 1, 2009.
          • Kuhn J.P., Slovis T.L., Haller J.O., Caffey's Pediatric Diagnostic Imaging, tenth edition, Mosby Publishing, 2003.
          • Kuperman V.Y., “General Properties of Different Models Used to Predict Normal Tissue Complications Due to Radiation,” Medical Physics, October 2008.
          • Linet M.S., Kim K.P., Rajaraman P., “Children's Exposure to Diagnostic Medical Radiation and Cancer Risk: Epidemiologic and Dosimetric Considerations,” Pediatric Radiology, volume 39, supplement 1, 2009.
          • Little M.P., Muirhead C.R., “Absence of Evidence for Threshold Departures From Linear-Quadratic Curvature in the Japanese A-Bomb Cancer Incidence and Mortality Data,” International Journal of Low Radiation, volume 1, 2004.
          • Little M.P., Muirhead C.R., “Curvature in the Cancer Mortality Dose Response in Japanese Atomic Bomb Survivors: Absence of Evidence of Threshold,” International Journal of Radiation Biology, January 1998.
          • Little M.P., Muirhead C.R., “Evidence for Curvilinearity in the Cancer Incidence Dose-Response in the Japanese Atomic Bomb Survivors,” International Journal of Radiation Biology, January 1996.
          • Little M.P., Muirhead C.R., Charles M.W., “Describing Time and Age Variations in the Risk of Radiation–Induced Solid Tumour Incidence in the Japanese Atomic Bomb Survivors Using Generalized Relative and Absolute Models,” Statistics in Medicine, January 1999.
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          • Limitation of Exposure to Ionizing Radiation, NCRP Report 116., National Council on Radiation Protection and Measurements, Bethesda, MD, 1993.
          • Health Risks From Exposure to Low Levels of Ionizing Radiation, National Research Council, BEIR VII Phase 2, National Academies Press, 2006.
          • Patel S.J., Reede D.L., Katz D.S., et al., “Imaging the Pregnant Patient for Non-Obstetric Conditions: Algorithms for Radiation Dose Considerations,” Radiographics, November/December 2007.
          • Paterson A., Frush D.P., “Dose Reduction in Paediatric MDCT: General Principles,” Clinical Radiology, June 2007.
          • Pierce D.A., Preston D.L., “Radiation-Related Cancer Risks at Low Doses Among Atomic Bomb Survivors,” Radiation Research, August 2000.
          • Reckelhoff-Dangel C., Peterson D., “EPA Risk Communication in Action,” The Risk Communication Workbook, U.S. Environmental Protection Agency Office of Research and Development, 2007.
          • Slovis P., “Perception of Risk,” Science, April 17, 1987.
          • Vaeth M., Pierce D.A., “Calculating Excess Lifetime Risk in Relative Risk Models,” Environmental Health Perspectives, July 1990.
          • Verdun F.R., Bochud F., Gudinchet F., et al., “Radiation Risk: What You Should Know to Tell Your Patient,” Radiographics, November/December 2008.