M e d i c a l P hy s i c s a n d I n f o r m a t i c s • O p i n i o n Huda Radiation Risks
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Medical Physics and Informatics Opinion
Radiation Risks: What Is to Be Done? Walter Huda1 Huda W
OBJECTIVE. What is currently known about radiologic risks is reviewed, policies that should be adopted based on our current knowledge are proposed, and how these policies can be applied to adequately protect patients in everyday clinical practice is described. CONCLUSION. All activities in life (e.g., driving automobiles) are associated with risks, and medical imaging is no different, so the most important message to convey to patients is whether a proposed examination is worthwhile. Our collective goal should be ensuring that all radiologic examinations are justified and are as low as reasonably achievable (ALARA), which maximizes the benefits of medical imaging for our patients.
T
Keywords: ALARA, benefits, justification, policy, radiation risks DOI:10.2214/AJR.14.12834 Received March 5, 2014; accepted without revision March 14, 2014. 1 Department of Radiology and Radiological Science, Medical University of South Carolina, 96 Johnathan Lucas St, MSC 323, Charleston, SC 29425-3230. Address correspondence to W. Huda ([email protected]).
AJR 2015; 204:124–127 0361–803X/15/2041–124 © American Roentgen Ray Society
124
he issue of radiation risks in radiology continues to be debated in the medical imaging community [1, 2]. Because patients and operators are exposed to ionizing radiation, which have increased population doses from medical imaging in the United States by 600% in one generation, this debate is of obvious importance [3, 4]. The current arguments focus on the issue of whether radiation risks to patients and operators in medical imaging are real. In the absence of a consensus on the science of radiation risks, the imaging community needs to develop radiation protection policies and agree about how these policies are to be applied in clinical practice. In this article, what is currently known about radiologic risks is reviewed, policies that should be adopted based on our current knowledge are proposed, and how these policies can be applied to adequately protect patients in everyday clinical practice is described. Radiation Risks Whether a patient undergoing a radiologic examination with x-rays (e.g., CT) has a risk of a delayed cancer is problematic because of a lack of scientific consensus [1, 2]. It is therefore important to recognize that the first issue to be addressed is how to act in the presence of scientific uncertainty when it is possible to make two kinds of error. The first is to assume that the risks are real and to then discover that they do not exist. The second is to assume that the risks are nonexistent and to subsequently
discover that these risks are real. The precautionary principle guides us to act on the assumption that the risks are real irrespective of the scientific merits of any scientific arguments about radiation risks [5]. Assuming that the risks are nonexistent and subsequently being proved wrong would likely be unacceptable to most medical imaging practitioners. The most important scientific bodies that review the basic science of radiation risks and recommend appropriate policies include the International Commission on Radiological Protection (ICRP), the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), and the Committee on the Biological Effects of Ionizing Radiation (BEIR) of the U.S. National Academy of Sciences. The ICRP [6] states the following in publication 103: …the practical system of radiation protection recommended by the Commission will continue to be based on the assumption that, at doses below about 100 mSv, a given increment in dose will produce a directly proportionate increment in the probability of incurring cancer. UNSCEAR [7] stated the following in a 2006 report: Most recent data for the survivors of the atomic bombings are largely consistent with linear or linear-quadratic dose trends over a wide range of doses.
AJR:204, January 2015
Radiation Risks
… 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. It is thus clear that these bodies recommend that, for radiation protection purposes, we need to act on the assumption that radiation risks in medical imaging actually exist. Over the past decade, several studies have provided insights about the issue of radiation risks at low doses, including the issue of protracted occupational exposures. Brenner et al. [9] showed that statistically significant risks, albeit small, were observed in a group of atomic bomb survivors who received an average dose as low as 34 mSv. The results of a study of radiation workers in the United Kingdom (National Registry for Radiation Workers) who were exposed to radiation over their working lives showed that their radiation risks for leukemia and solid tumors [10] were consistent with the current radiation risk estimates obtained from studying atomic bomb survivors. A recent study of children who underwent CT in the United Kingdom showed clear evidence of increased leukemia and brain tumors [11]. A recent Australian study also showed an increased cancer incidence with the increasing number of pediatric CT studies performed [12]; the increased incidence was attributed mainly to radiation, with an estimated average patient effective dose per examination of 4.5 mSv [12]. Imaging Policy One important consequence of assuming that radiation risks are real is that any patient exposure in a radiologic examination must be justified. This principle—which requires patient benefits to exceed all risks including radiation—is well established in radiation protection, including medical practice [13]. Identification of justified medical imaging examinations is the responsibility of individuals who are knowledgeable about the most appropriate examinations for a clinical indication, the diagnostic information that is likely to be generated, and the corresponding patient radiation doses and associated risks [14]. American Board of Radiology–certified radiologists, for example, are trained to identify which examinations are appropriate [15],
Fig. 1—Scatterplot shows radiation risk estimates per 100,000 individuals exposed to uniform wholebody–equivalent dose of 10 mSv based on risk estimates for North American population provided by Committee on Biological Effects of Ionizing Radiation (BEIR) in BEIR VII [8] and assuming linear no-threshold dose response.
600
500 Cancer Incidence (No. of Cases per 100,000 Individuals Exposed to Uniform 10-mSv Whole-Body Dose)
Downloaded from www.ajronline.org by UT Southwestern Medical Center on 01/07/15 from IP address 129.112.109.42. Copyright ARRS. For personal use only; all rights reserved
The BEIR Committee, in BEIR VII [8], concluded the following:
400
300
Male Female
200
100
0
0
20
40
60
80
100
Age (y)
what kind of diagnostic information might be obtained, and the corresponding patient doses and risks [16, 17]. Knowledgeable individuals know whether they would proceed with a given examination for any clinical indication for a close family member, a very good sign that an examination is truly worthwhile. An important corollary of the need to justify imaging examinations is that this justification can be achieved only when practitioners have a quantitative understanding about the magnitude of the radiation risks [18]. Radiologists must be educated about the magnitude of the current radiation risk estimates, which depend on the amount of radiation used and on the age and sex of exposed individuals. Figure 1 shows cancer incidence risks for a uniform whole-body–equivalent dose of 10 mSv based on the current BEIR VII [8] risk estimates assuming a linear nothreshold dose response. The purpose of Figure 1 is to help imaging practitioners understand the magnitude of current radiation risks and how these risks are affected by patient demographics, which will help them identify indicated examinations [19]. When one is ignorant of the magnitude of the ra-
diation risk, how can one determine that the estimated benefit exceeds this unknown risk [20]? In addition, it is also important that imaging practitioners also understand that there are large uncertainties in these risk estimates. Markedly different risk numbers will be obtained for different exposed populations [8, 21, 22], and it is believed that the risks shown in Figure 1 could easily be higher or lower by a factor of 2–3 [23–25]. Given the assumed existence of radiation risks, it is necessary to ensure that no more radiation is used than required to obtain the needed diagnostic information. Optimizing radiologic examinations in this manner is generally referred to as keeping exposures as low as reasonably achievable (ALARA) [13]. However, any optimization of radiologic imaging must focus on diagnostic imaging performance rather than on patient dose and risks per se [26]. The reason for this focus is that dose reduction is appropriate only when the diagnostic information from the radiologic examination is not compromised [27]. Although the downside of using too much radiation is obvious to all, using too little radiation can adversely impact the diagnostic informa-
AJR:204, January 2015 125
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Huda tion, which can result in harm to the patient. Assuming that radiation risks exist also implies that it is worthwhile to allocate valuable resources to optimize radiologic examinations [28]. Specifically, having quantitative knowledge of the estimated radiation risks permits the potential benefit of optimization efforts to be compared with alternative uses of our limited resources [29]. Clinical Practice Radiologists must be knowledgeable about radiation risks to be able to help patients who have questions regarding planned CT examinations for themselves or for their children [30]. For indicated and questionable examinations, the emphasis should not be on the existence of any radiation risks, but rather on whether a planned examination is worthwhile. Understanding whether a radiologic examination is worthwhile requires a mastery of what is known about risks by the radiologist (i.e., Fig. 1) and of the corresponding uncertainties. Thus, a mastery of radiation risks helps imaging practitioners to be respectful of ionizing radiation and to avoid the extremes of being either too fearful or too blasé about these risks. For clearly indicated CT examinations in pediatric patients (e.g., closed head injury with loss of consciousness and blown pupil or suspected ureteral stone), it would be appropriate to explain to the parent that a CT examination is absolutely worthwhile because the patient benefit is overwhelming and any risks are of negligible concern. On the other hand, consider clinical problems in which a CT examination would be more difficult to justify, such as head CT for a child with a headache without neurologic findings or chest CT before radiography for a child who needs to be evaluated for pneumonia. For the latter two examples of planned CT examinations, discussions with parents should convey that there would likely be little benefit from the examination and that exposing their child to radiation would not be worthwhile. In short, why expose a child to a possible radiation risk if the diagnostic payoff is most likely to be negligible in comparison? Protecting patients by adopting policies of justification and of ALARA implies that it is inappropriate to only compute the total number of cancers in a patient population that undergoes radiologic examinations [31] because these computations ignore the likely enormous collective benefits associated with indicated examinations [27]. Medical
126
radiation protection practice, when focused on the individual patient, always recognizes that the patient is harmed when an indicated examination is not performed [32]. Estimating individual risks is helpful for educating imaging practitioners, whereas computing population risks that neglect the collective patient benefits has little merit. For examinations that are not indicated, however, patient population risk estimates are appropriate [33] because these risk estimates quantify the estimated collective harm associated with nonindicated examinations that result in very little positive patient benefit. Radiation risk models other than the linear no-threshold model—which has been adopted for use in medical radiation protection practice by groups such as ICRP, BEIR, and UNSCEAR and is the basis for the policies and practices advocated here—exist [34, 35]. Proponents of models of radiation risk that differ from the one articulated in this article need to explain how they would propose modifying the policies of justification and ALARA described in this article. Debating how to act in our clinical practice for the benefit of our patients is more relevant than heatedly debating whether radiation risks in radiology really exist. Conclusion Radiation protection in medical imaging can be reduced to two simple principles that are easy to understand, straightforward to implement, and likely to be acceptable to most medical imaging practitioners: Patient examinations need to be justified by a net patient benefit, and unnecessary radiation should be eliminated (i.e., ALARA). Current radiation risks estimates, however, are relatively small. All activities in life (e.g., driving automobiles) are associated with risks, and medical imaging is no different, so the most important message to convey to patients is whether a proposed examination is worthwhile. Our collective goal should be ensuring all radiologic examinations are justified and are ALARA, which maximizes the benefits of medical imaging for our patients. Acknowledgments I thank G. D. Frey, D. P. Frush, J. Hill, E. Samei, and S. V. Tipnis for useful discussions that contributed to this article. References 1. Brenner DJ, Hall EJ. Cancer risks from CT scans: now we have data, what next? Radiology 2012;
265:330–331 2. Hendee WR, O’Connor MK. Radiation risks of medical imaging: separating fact from fantasy. Radiology 2012; 264:312–321 3. Boone JM, Hendee WR, McNitt-Gray MF, Seltzer SE. Radiation exposure from CT scans: how to close our knowledge gaps, monitor and safeguard exposure—proceedings and recommendations of the Radiation Dose Summit, sponsored by NIBIB, February 24–25, 2011. Radiology 2012; 265:544–554 4. Mettler FA Jr, Bhargavan M, Faulkner K, et al. Radiologic and nuclear medicine studies in the United States and worldwide: frequency, radiation dose, and comparison with other radiation sources—1950–2007. Radiology 2009; 253:520–531 5. Semelka RC, Armao DM, Elias J Jr, Huda W. Imaging strategies to reduce the risk of radiation in CT studies, including selective substitution with MRI. J Magn Reson Imaging 2007; 25:900–909 6. International Commission on Radiological Protection. The 2007 recommendations of the International Commission on Radiological Protection: ICRP publication 103. Ann ICRP 2007; 37:1–332 7. United Nations Scientific Committee on the Effects of Atomic Radiation. Effects of ionizing radiation: UNSCEAR 2006 report, volume 1. Vienna, Austria: United Nations, 2008:137 8. Committee to Assess Health Risks From Exposure to Low Levels of Ionizing Radiation, Board on Radiation Effects Research, Division of Earth and Life Studies, National Research Council of the National Academies. Health risks from exposure to low levels of ionizing radiation: BEIR VII Phase 2. Washington, DC: National Academies Press, 2006:10 9. Brenner DJ, Doll R, Goodhead DT, et al. Cancer risks attributable to low doses of ionizing radiation: assessing what we really know. Proc Natl Acad Sci U S A 2003; 100:13,761–13,766 10. Muirhead CR, O’Hagan JA, Haylock RG, et al. Mortality and cancer incidence following occupational radiation exposure: third analysis of the National Registry for Radiation Workers. Br J Cancer 2009; 100:206–212 11. Pearce MS, Salotti JA, Little MP, et al. Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study. Lancet 2012; 380:499–505 12. Mathews JD, Forsythe AV, Brady Z, et al. Cancer risk in 680,000 people exposed to computed tomography scans in childhood or adolescence: data linkage study of 11 million Australians. BMJ 2013; 346:f2360 13. International Commission on Radiological Protection. ICRP publication 105: radiation protection in medicine. Ann ICRP 2007; 37:1–63 14. Sierzenski PR, Linton OW, Amis ES Jr, et al. Applications of justification and optimization in
AJR:204, January 2015
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Radiation Risks medical imaging: examples of clinical guidance for computed tomography use in emergency medicine. JACR 2014; 11:36–44 15. American College of Radiology website. ACR appropriateness criteria. www.acr.org/quality-safety/ appropriateness-criteria. Accessed September 11, 2014 16. Brenner D, Elliston C, Hall E, Berdon W. Estimated risks of radiation-induced fatal cancer from pediatric CT. AJR 2001; 176:289–296 17. Paterson A, Frush DP, Donnelly LF. Helical CT of the body: are settings adjusted for pediatric patients? AJR 2001; 176:297–301 18. Lee CI, Haims AH, Monico EP, Brink JA, Forman HP. Diagnostic CT scans: assessment of patient, physician, and radiologist awareness of radiation dose and possible risks. Radiology 2004; 231:393–398 19. Frush DP, Donnelly LF, Rosen NS. Computed tomography and radiation risks: what pediatric health care providers should know. Pediatrics 2003; 112:951–957 20. Bosanquet DC, Green G, Bosanquet AJ, Galland RB, Gower-Thomas K, Lewis MH. Doctors’ knowledge of radiation: a two-centre study and historical comparison. Clin Radiol 2011; 66:748–751
21. Ivanov VK, Tsyb AF, Mettler FA, Menyaylo AN, Kashcheev VV. Methodology for estimating cancer risks of diagnostic medical exposure: with an example of the risks associated with computed tomography. Health Phys 2012; 103:732–739 22. Mobbs SF, Muirhead CR, Harrison JD. Risks from ionising radiation. Oxfordshire, UK: Health Protection Agency, 2010: report HPA-RPD-066 23. National Council on Radiation Protection & Measurements. Risk estimates for radiation protection. Bethesda, MD: NCRP, 1993: report 115 24. National Council on Radiation Protection & Measurements. Uncertainties in fatal cancer risk estimates used in radiation protection. Bethesda, MD: NCRP, 1997: report 126 25. National Council on Radiation Protection & Measurements. Evaluation of the linear-nonthreshold dose-response model for ionizing radiation. Bethesda, MD: NCRP, 2001: report 136 26. Hricak H, Brenner DJ, Adelstein SJ, et al. Managing radiation use in medical imaging: a multifaceted challenge. Radiology 2011; 258:889–905 27. McCollough CH, Guimaraes L, Fletcher JG. In defense of body CT. AJR 2009; 193:28–39 28. Pettersson HB, Falth-Magnusson K, Persliden J,
Scott M. Radiation risk and cost-benefit analysis of a paediatric radiology procedure: results from a national study. Br J Radiol 2005; 78:34–38 29. Patton DD. Cost-effectiveness in nuclear medicine. Semin Nucl Med 1993; 23:9–30 30. Larson DB, Rader SB, Forman HP, Fenton LZ. Informing parents about CT radiation exposure in children: it’s OK to tell them. AJR 2007; 189:271–275 31. Berrington de González A, Mahesh M, Kim KP, et al. Projected cancer risks from computed tomographic scans performed in the United States in 2007. Arch Intern Med 2009; 169:2071–2077 32. Restrepo CS, Gutierrez FR, Marmol-Velez JA, Ocazionez D, Martinez-Jimenez S. Imaging patients with cardiac trauma. RadioGraphics 2012; 32:633– 649 [Erratum in RadioGraphics 2012; 32:1258] 33. Lin EC. Radiation risk from medical imaging. Mayo Clin Proc 2010; 85:1142–1146; quiz, 1146 34. Luckey TD. Radiation hormesis: the good, the bad, and the ugly. Dose Response 2006; 4:169–190 35. Tubiana M, Feinendegen LE, Yang C, Kaminski JM. The linear no-threshold relationship is inconsistent with radiation biologic and experimental data. Radiology 2009; 251:13–22
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Medical Physics and Informatics Opinion
Radiation Risks: What Is to Be Done? Walter Huda1 Huda W
OBJECTIVE. What is currently known about radiologic risks is reviewed, policies that should be adopted based on our current knowledge are proposed, and how these policies can be applied to adequately protect patients in everyday clinical practice is described. CONCLUSION. All activities in life (e.g., driving automobiles) are associated with risks, and medical imaging is no different, so the most important message to convey to patients is whether a proposed examination is worthwhile. Our collective goal should be ensuring that all radiologic examinations are justified and are as low as reasonably achievable (ALARA), which maximizes the benefits of medical imaging for our patients.
T
Keywords: ALARA, benefits, justification, policy, radiation risks DOI:10.2214/AJR.14.12834 Received March 5, 2014; accepted without revision March 14, 2014. 1 Department of Radiology and Radiological Science, Medical University of South Carolina, 96 Johnathan Lucas St, MSC 323, Charleston, SC 29425-3230. Address correspondence to W. Huda ([email protected]).
AJR 2015; 204:124–127 0361–803X/15/2041–124 © American Roentgen Ray Society
124
he issue of radiation risks in radiology continues to be debated in the medical imaging community [1, 2]. Because patients and operators are exposed to ionizing radiation, which have increased population doses from medical imaging in the United States by 600% in one generation, this debate is of obvious importance [3, 4]. The current arguments focus on the issue of whether radiation risks to patients and operators in medical imaging are real. In the absence of a consensus on the science of radiation risks, the imaging community needs to develop radiation protection policies and agree about how these policies are to be applied in clinical practice. In this article, what is currently known about radiologic risks is reviewed, policies that should be adopted based on our current knowledge are proposed, and how these policies can be applied to adequately protect patients in everyday clinical practice is described. Radiation Risks Whether a patient undergoing a radiologic examination with x-rays (e.g., CT) has a risk of a delayed cancer is problematic because of a lack of scientific consensus [1, 2]. It is therefore important to recognize that the first issue to be addressed is how to act in the presence of scientific uncertainty when it is possible to make two kinds of error. The first is to assume that the risks are real and to then discover that they do not exist. The second is to assume that the risks are nonexistent and to subsequently
discover that these risks are real. The precautionary principle guides us to act on the assumption that the risks are real irrespective of the scientific merits of any scientific arguments about radiation risks [5]. Assuming that the risks are nonexistent and subsequently being proved wrong would likely be unacceptable to most medical imaging practitioners. The most important scientific bodies that review the basic science of radiation risks and recommend appropriate policies include the International Commission on Radiological Protection (ICRP), the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), and the Committee on the Biological Effects of Ionizing Radiation (BEIR) of the U.S. National Academy of Sciences. The ICRP [6] states the following in publication 103: …the practical system of radiation protection recommended by the Commission will continue to be based on the assumption that, at doses below about 100 mSv, a given increment in dose will produce a directly proportionate increment in the probability of incurring cancer. UNSCEAR [7] stated the following in a 2006 report: Most recent data for the survivors of the atomic bombings are largely consistent with linear or linear-quadratic dose trends over a wide range of doses.
AJR:204, January 2015
Radiation Risks
… 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. It is thus clear that these bodies recommend that, for radiation protection purposes, we need to act on the assumption that radiation risks in medical imaging actually exist. Over the past decade, several studies have provided insights about the issue of radiation risks at low doses, including the issue of protracted occupational exposures. Brenner et al. [9] showed that statistically significant risks, albeit small, were observed in a group of atomic bomb survivors who received an average dose as low as 34 mSv. The results of a study of radiation workers in the United Kingdom (National Registry for Radiation Workers) who were exposed to radiation over their working lives showed that their radiation risks for leukemia and solid tumors [10] were consistent with the current radiation risk estimates obtained from studying atomic bomb survivors. A recent study of children who underwent CT in the United Kingdom showed clear evidence of increased leukemia and brain tumors [11]. A recent Australian study also showed an increased cancer incidence with the increasing number of pediatric CT studies performed [12]; the increased incidence was attributed mainly to radiation, with an estimated average patient effective dose per examination of 4.5 mSv [12]. Imaging Policy One important consequence of assuming that radiation risks are real is that any patient exposure in a radiologic examination must be justified. This principle—which requires patient benefits to exceed all risks including radiation—is well established in radiation protection, including medical practice [13]. Identification of justified medical imaging examinations is the responsibility of individuals who are knowledgeable about the most appropriate examinations for a clinical indication, the diagnostic information that is likely to be generated, and the corresponding patient radiation doses and associated risks [14]. American Board of Radiology–certified radiologists, for example, are trained to identify which examinations are appropriate [15],
Fig. 1—Scatterplot shows radiation risk estimates per 100,000 individuals exposed to uniform wholebody–equivalent dose of 10 mSv based on risk estimates for North American population provided by Committee on Biological Effects of Ionizing Radiation (BEIR) in BEIR VII [8] and assuming linear no-threshold dose response.
600
500 Cancer Incidence (No. of Cases per 100,000 Individuals Exposed to Uniform 10-mSv Whole-Body Dose)
Downloaded from www.ajronline.org by UT Southwestern Medical Center on 01/07/15 from IP address 129.112.109.42. Copyright ARRS. For personal use only; all rights reserved
The BEIR Committee, in BEIR VII [8], concluded the following:
400
300
Male Female
200
100
0
0
20
40
60
80
100
Age (y)
what kind of diagnostic information might be obtained, and the corresponding patient doses and risks [16, 17]. Knowledgeable individuals know whether they would proceed with a given examination for any clinical indication for a close family member, a very good sign that an examination is truly worthwhile. An important corollary of the need to justify imaging examinations is that this justification can be achieved only when practitioners have a quantitative understanding about the magnitude of the radiation risks [18]. Radiologists must be educated about the magnitude of the current radiation risk estimates, which depend on the amount of radiation used and on the age and sex of exposed individuals. Figure 1 shows cancer incidence risks for a uniform whole-body–equivalent dose of 10 mSv based on the current BEIR VII [8] risk estimates assuming a linear nothreshold dose response. The purpose of Figure 1 is to help imaging practitioners understand the magnitude of current radiation risks and how these risks are affected by patient demographics, which will help them identify indicated examinations [19]. When one is ignorant of the magnitude of the ra-
diation risk, how can one determine that the estimated benefit exceeds this unknown risk [20]? In addition, it is also important that imaging practitioners also understand that there are large uncertainties in these risk estimates. Markedly different risk numbers will be obtained for different exposed populations [8, 21, 22], and it is believed that the risks shown in Figure 1 could easily be higher or lower by a factor of 2–3 [23–25]. Given the assumed existence of radiation risks, it is necessary to ensure that no more radiation is used than required to obtain the needed diagnostic information. Optimizing radiologic examinations in this manner is generally referred to as keeping exposures as low as reasonably achievable (ALARA) [13]. However, any optimization of radiologic imaging must focus on diagnostic imaging performance rather than on patient dose and risks per se [26]. The reason for this focus is that dose reduction is appropriate only when the diagnostic information from the radiologic examination is not compromised [27]. Although the downside of using too much radiation is obvious to all, using too little radiation can adversely impact the diagnostic informa-
AJR:204, January 2015 125
Downloaded from www.ajronline.org by UT Southwestern Medical Center on 01/07/15 from IP address 129.112.109.42. Copyright ARRS. For personal use only; all rights reserved
Huda tion, which can result in harm to the patient. Assuming that radiation risks exist also implies that it is worthwhile to allocate valuable resources to optimize radiologic examinations [28]. Specifically, having quantitative knowledge of the estimated radiation risks permits the potential benefit of optimization efforts to be compared with alternative uses of our limited resources [29]. Clinical Practice Radiologists must be knowledgeable about radiation risks to be able to help patients who have questions regarding planned CT examinations for themselves or for their children [30]. For indicated and questionable examinations, the emphasis should not be on the existence of any radiation risks, but rather on whether a planned examination is worthwhile. Understanding whether a radiologic examination is worthwhile requires a mastery of what is known about risks by the radiologist (i.e., Fig. 1) and of the corresponding uncertainties. Thus, a mastery of radiation risks helps imaging practitioners to be respectful of ionizing radiation and to avoid the extremes of being either too fearful or too blasé about these risks. For clearly indicated CT examinations in pediatric patients (e.g., closed head injury with loss of consciousness and blown pupil or suspected ureteral stone), it would be appropriate to explain to the parent that a CT examination is absolutely worthwhile because the patient benefit is overwhelming and any risks are of negligible concern. On the other hand, consider clinical problems in which a CT examination would be more difficult to justify, such as head CT for a child with a headache without neurologic findings or chest CT before radiography for a child who needs to be evaluated for pneumonia. For the latter two examples of planned CT examinations, discussions with parents should convey that there would likely be little benefit from the examination and that exposing their child to radiation would not be worthwhile. In short, why expose a child to a possible radiation risk if the diagnostic payoff is most likely to be negligible in comparison? Protecting patients by adopting policies of justification and of ALARA implies that it is inappropriate to only compute the total number of cancers in a patient population that undergoes radiologic examinations [31] because these computations ignore the likely enormous collective benefits associated with indicated examinations [27]. Medical
126
radiation protection practice, when focused on the individual patient, always recognizes that the patient is harmed when an indicated examination is not performed [32]. Estimating individual risks is helpful for educating imaging practitioners, whereas computing population risks that neglect the collective patient benefits has little merit. For examinations that are not indicated, however, patient population risk estimates are appropriate [33] because these risk estimates quantify the estimated collective harm associated with nonindicated examinations that result in very little positive patient benefit. Radiation risk models other than the linear no-threshold model—which has been adopted for use in medical radiation protection practice by groups such as ICRP, BEIR, and UNSCEAR and is the basis for the policies and practices advocated here—exist [34, 35]. Proponents of models of radiation risk that differ from the one articulated in this article need to explain how they would propose modifying the policies of justification and ALARA described in this article. Debating how to act in our clinical practice for the benefit of our patients is more relevant than heatedly debating whether radiation risks in radiology really exist. Conclusion Radiation protection in medical imaging can be reduced to two simple principles that are easy to understand, straightforward to implement, and likely to be acceptable to most medical imaging practitioners: Patient examinations need to be justified by a net patient benefit, and unnecessary radiation should be eliminated (i.e., ALARA). Current radiation risks estimates, however, are relatively small. All activities in life (e.g., driving automobiles) are associated with risks, and medical imaging is no different, so the most important message to convey to patients is whether a proposed examination is worthwhile. Our collective goal should be ensuring all radiologic examinations are justified and are ALARA, which maximizes the benefits of medical imaging for our patients. Acknowledgments I thank G. D. Frey, D. P. Frush, J. Hill, E. Samei, and S. V. Tipnis for useful discussions that contributed to this article. References 1. Brenner DJ, Hall EJ. Cancer risks from CT scans: now we have data, what next? Radiology 2012;
265:330–331 2. Hendee WR, O’Connor MK. Radiation risks of medical imaging: separating fact from fantasy. Radiology 2012; 264:312–321 3. Boone JM, Hendee WR, McNitt-Gray MF, Seltzer SE. Radiation exposure from CT scans: how to close our knowledge gaps, monitor and safeguard exposure—proceedings and recommendations of the Radiation Dose Summit, sponsored by NIBIB, February 24–25, 2011. Radiology 2012; 265:544–554 4. Mettler FA Jr, Bhargavan M, Faulkner K, et al. Radiologic and nuclear medicine studies in the United States and worldwide: frequency, radiation dose, and comparison with other radiation sources—1950–2007. Radiology 2009; 253:520–531 5. Semelka RC, Armao DM, Elias J Jr, Huda W. Imaging strategies to reduce the risk of radiation in CT studies, including selective substitution with MRI. J Magn Reson Imaging 2007; 25:900–909 6. International Commission on Radiological Protection. The 2007 recommendations of the International Commission on Radiological Protection: ICRP publication 103. Ann ICRP 2007; 37:1–332 7. United Nations Scientific Committee on the Effects of Atomic Radiation. Effects of ionizing radiation: UNSCEAR 2006 report, volume 1. Vienna, Austria: United Nations, 2008:137 8. Committee to Assess Health Risks From Exposure to Low Levels of Ionizing Radiation, Board on Radiation Effects Research, Division of Earth and Life Studies, National Research Council of the National Academies. Health risks from exposure to low levels of ionizing radiation: BEIR VII Phase 2. Washington, DC: National Academies Press, 2006:10 9. Brenner DJ, Doll R, Goodhead DT, et al. Cancer risks attributable to low doses of ionizing radiation: assessing what we really know. Proc Natl Acad Sci U S A 2003; 100:13,761–13,766 10. Muirhead CR, O’Hagan JA, Haylock RG, et al. Mortality and cancer incidence following occupational radiation exposure: third analysis of the National Registry for Radiation Workers. Br J Cancer 2009; 100:206–212 11. Pearce MS, Salotti JA, Little MP, et al. Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study. Lancet 2012; 380:499–505 12. Mathews JD, Forsythe AV, Brady Z, et al. Cancer risk in 680,000 people exposed to computed tomography scans in childhood or adolescence: data linkage study of 11 million Australians. BMJ 2013; 346:f2360 13. International Commission on Radiological Protection. ICRP publication 105: radiation protection in medicine. Ann ICRP 2007; 37:1–63 14. Sierzenski PR, Linton OW, Amis ES Jr, et al. Applications of justification and optimization in
AJR:204, January 2015
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