Radiation Risks	Choose Top of pageABSTRACTRadiation Risks <<Imaging PolicyClinical PracticeConclusionAcknowledgmentsReferencesCITING ARTICLES

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:

UNSCEAR [7] stated the following in a 2006 report:

The BEIR Committee, in BEIR VII [8], concluded the following:

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	Choose Top of pageABSTRACTRadiation RisksImaging Policy <<Clinical PracticeConclusionAcknowledgmentsReferencesCITING ARTICLES

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], 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 no-threshold 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 radiation 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].

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Fig. 1 —Scatterplot shows radiation risk estimates per 100,000 individuals exposed to uniform whole-body–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.

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 information, 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	Choose Top of pageABSTRACTRadiation RisksImaging PolicyClinical Practice <<ConclusionAcknowledgmentsReferencesCITING ARTICLES

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 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	Choose Top of pageABSTRACTRadiation RisksImaging PolicyClinical PracticeConclusion <<AcknowledgmentsReferencesCITING ARTICLES

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.