CT radiation dose: current controversies and dose reduction strategies.

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 • R ev i ew Costello et al. CT Dose Reduction

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Medical Physics and Informatics Review

CT Radiation Dose: Current Controversies and Dose Reduction Strategies Justin E. Costello1 Nathan D. Cecava Jonathan E. Tucker Jennifer L. Bau Costello JE, Cecava ND, Tucker JE, Bau JL

OBJECTIVE. The purpose of this article is to use clinical scenarios to explore aspects of ionizing radiation imparted to patients undergoing CT examinations. Examination appropriateness, effective doses, cancer risks, and pertinent dose reduction strategies are reviewed. CONCLUSION. CT accounts for the majority of radiation exposure related to medical imaging. Medical professionals should have a working knowledge of the benefits and risks of medical radiation and an understanding of strategies for reducing CT radiation dose.

A

Keywords: CT, dose reduction strategies, radiation dose DOI:10.2214/AJR.12.9720 Received July 31, 2012; accepted after revision December 4, 2012. 1

All authors: Department of Radiology, Wilford Hall Ambulatory Surgical Center, San Antonio Military Medical Center, 3851 Roger Brooke Dr, San Antonio, TX 78234. Address correspondence to J. E. Costello ([email protected]).

CME/SAM This article is available for CME/SAM credit. AJR 2013; 201:1283–1290 0361–803X/13/2016–1283 © American Roentgen Ray Society

ttention to radiation dose is becoming more frequent in professional journals [1] and the general press [2]. Radiologists will likely be increasingly confronted with clinical questions related to CT radiation exposure. Since the early 1990s, use of diagnostic CT in the United States has risen nearly 20-fold [3]. Once accounting for 20% of the overall per capita effective radiation dose (ERD), medical imaging now accounts for more than 50% of radiation exposure in the United States, half of which is related to CT scanning [4]. The rapid availability of CT, along with its diagnostic accuracy, has led to dramatically increased use in acute care. Early disease detection, reduction in mean hospitalizations, and lowered health care costs have all been attributed to greater use of CT [5–7]. With the current expansion of CT in medical practice, an increased understanding of cancer risks and strategies for reducing radiation dose is of utmost importance. Cancer Risks From CT Determining cancer risks associated with CT examinations is controversial. The biologic effects of low-dose radiation exposure have been notoriously difficult to study because of the large number of patients needed to show statistical significance among a substantial background (25–33%) of naturally occurring cancers in the population [8]. The most widely accepted cancer risk models are outlined in the 2006 Biologic Effects of Ionizing Radiation (BEIR) VII report [9]. BEIR VII assumes

a linear no-threshold correlation between radiation exposure of Hiroshima and Nagasaki atomic bomb survivors and the low-dose radiation encountered in medical imaging (Fig. 1). Thus with this model, even the lowest radiation dose imparts some cancer risk. The linear no-threshold model is disputed by several health organizations, including the American Association of Physicists in Medicine and the Health Physics Society, both of which concluded that cancer risk estimation should be limited to doses greater than 50 mSv [10, 11]. Both organizations state that risks from doses below this threshold are too small to be detectable and may be nonexistent. In addition, some scientists argue that low doses of radiation may provide some health benefits. Although many studies have shown the harmful DNA effects of radiation at low doses, various laboratory experiments have shown that radiation hormesis does occur and is theorized to protect against spontaneous cancers in humans through acquired DNA repair mechanisms [12]. In response to a rising concern about radiation-induced cancer, the American College of Radiology (ACR) position statement recognizes that low doses of CT radiation may cause harm. However, the ACR does acknowledge some uncertainties in comparing an atomic bomb to CT radiation. Atomic bomb survivors experienced instantaneous whole-body exposure. Most CT examinations are limited to small portions of the body. In addition, although CT radiation is composed solely of x-rays, atomic radiation

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included x-rays, particulate radiations, neutrons, and other radioactive materials [13]. The U.S. Food and Drug Administration [14] has stated that compared with the natural incidence of fatal cancer in the United States (≈ 1 chance in 5), an effective CT radiation dose of 10 mSv may be associated with the possibility of fatal cancer in approximately 1 in 2000 patients. This is similar to the overall BEIR VII cancer estimate of 1 in 1000 per 10 mSv. Other health organizations that support the linear no-threshold model include the United Nations Scientific Committee on the Effects of Atomic Radiation [15] and the International Committee on Radiologic Protection [16]. In a retrospective study published in The Lancet that included the largest patient cohort to date, Pearce et al. [17] documented a three times increased relative risk of leukemia among children who had received a cumulative radiation dose of at least 30 mSv. These findings appear to be consistent with the linear no-threshold model, though the American Association of Physicists in Medicine has responded by emphasizing the low incidence of leukemia in children [18], which can exaggerate relative risk. For elderly patients, estimates made with BEIR VII models revealed a minimally increased attributable risk (0.03– 0.04%) for development of cancer related to ionizing radiation [19]. The goals of this article are to answer caseformatted questions (Table 1) regarding cancer risks associated with CT examinations and to review radiation dose reduction strategies as they pertain to the patients who appear to have the highest risk of radiation-induced cancers: children (10-year-old Timothy) and

young adults (30-year-old Amy). Multiple CT scans for one patient and multiphase examinations are also discussed (Dr. Smith). Biologic Effects of Ionizing Radiation VII Cancer Risk Estimates The Committee to Assess Health Risks From Exposure to Low Levels of Ionizing Radiation, in its BEIR VII report, listed its preferred estimates of lifetime incidence and mortality for all solid cancers and for leukemia. Excluding leukemia, and assuming approximately equal numbers of male and female individuals in the population, BEIR VII predicted that 19,800 of every 100,000 individuals who have not been exposed to ionizing radiation (other than naturally occurring background radiation) will die of cancer. For every 100,000 individuals who have received an excess ionizing radiation dose of 100 mGy, BEIR VII predicted that 20,310 will die of cancer [9]. Both circumstances represent absolute risks. The lifetime attributable risk (LAR) for the exposed population is merely the difference between the two quantities, that is, 510 per 100,000 individuals. This correlates with a 5 in 1000 LAR for fatal cancer among individuals exposed to 100 mGy. The LAR for overall cancer incidence (fatal and nonfatal cancers) is approximately double this value. Thus by similar calculations, this same group of individuals would have an LAR of 1 in 100 for overall solid cancer incidence. When the radiation dose is decreased from 100 to 10 mGy, the LAR for overall solid cancer incidence is reduced to approximately 1 in 1000, the standard BEIR VII cancer estimation.

Risk Radiation-Related Cancer Risk


Costello et al.

Threshold

Radiation Dose Linear no-threshold (high dose rate) Linear no-threshold (low dose rate) Linear quadratic model Linear model with a threshold

Fig. 1—Graph shows results of comparison of linear no-threshold model with other models of effects of ionizing radiation. Some authors believe radiationinduced cancers do not occur until threshold level is reached. Others argue that cancers occur exponentially in response to higher radiation doses. Biologic Effects of Ionizing Radiation VII committee has adopted linear no-threshold model.

More complex calculations are available to account for patient age and sex. We chose to implement these in our case scenarios using Microsoft Excel software (version 14). Calculated effective dose in CT is, at best, a rough estimate due to many factors, such as variations in patient size and the limitations of dose measurement and calculation methods [20]. Consequently, the effective doses we present are, in Timothy’s case, based on published CT dose data adjusted for irradiated length, patient age, tube current reduction recommended by Image Gently, and ef-

TABLE 1: Clinical Cases Clinical Scenario

Questions

Timothy 10-year-old boy presents with fever, abdominal pain, and leukocytosis

Mother asks,

Abdominal ultrasound findings are equivocal for acute appendicitis

How much radiation will my son be exposed to?

Pediatric surgeons request abdominopelvic CT

Does this increase his risk for cancer in the future?

Amy 30-year-old woman presents with new-onset chest pain for past week

Amy asks,

d-dimer laboratory test result is positive, and pulmonary CT angiography is

How much radiation will I be exposed to?

Patient has a family history of cancer and is concerned with risks of CT

Is there anything you can do to reduce my risk of cancer from CT?

ordered to rule out pulmonary embolism

Dr. Smith Has patient in his emergency department with abdominal pain consistent with renal colic

Dr. Smith asks,

Patient has a known history of chronic nephrolithiasis

What is the typical dose for abdominopelvic CT?

Imaging review reveals eight previous CT examinations for similar presentations How will multiple CT studies affect my patient’s risk of cancer development?

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CT Dose Reduction TABLE 2: Average Radiation Doses for Typical Adult CT Examinations [23] Average Effective Dose (mSv)

Values Reported in Literature (mSv)

Head

2

0.9–4.0

Chest

7

4.0–18.0

Chest for pulmonary embolism

15

13–40

Abdomen

8

3.5–25

Pelvis

6

3.3–10

Spine

6

1.5–10

fective dose per dose-length product [21–23]. In Amy’s case, the doses are from published CT dose data only [24] (Table 2). The associated risk estimates in BEIR VII are for illustration only. In Timothy’s case, the effective dose for an abdominopelvic CT examination was 6.5 mSv (Fig. 2). According to BEIR VII, of 100,000 male patients who receive an effective dose of 6.5 mSv at age 10, 94 individuals (0.094%) in this group would be expected to eventually have cancer, which would be fatal in 46 cases (0.046%). These are expressions of LAR, adjusted for age and sex. For comparison, 44.8% of all men and boys in the United States are expected to eventually have cancer and 23.1% to die of it over their lifetimes [25]. In Amy’s case, the average effective dose for a chest pulmonary CT angiographic examination is 15 mSv [24]. According to BEIR VII, of 100,000 women who receive an effective dose of 15 mSv when they are 30 years old, 160 (0.160%) in this group would be expected to eventually have cancer, 81 cases (0.081%) of which would be fatal. For comparison, 38.2% of all women and girls in the United States are expected to eventually have cancer and 19.4% to die of it over their lifetimes [26]. 14.0 Effective Dose (mSv)


CT Examination

12.0 10.0

Dose Reduction Strategies To avoid unnecessary radiation exposure, the as low as reasonable achievable (ALARA) principle should always be applied. CT should be ordered only when the results are expected to affect patient care. Nonionizing alternatives, such as ultrasound and MRI, must also be considered [27, 28]. The ACR Appropriateness Criteria are specific evidence-based recommendations for assisting providers in making the most appropriate imaging decision based on the given clinical situation [29]. After application of the ACR Appropriateness Criteria to identify CT as the best clinical approach, the best measures that can be implemented to optimize ERD involve decreasing tube potential, automatic current modulation, and CT postprocessing. Basic CT Parameters Key CT parameters with which radiologists should be familiar include scan length and helical pitch. Scan length seems readily apparent and is often overlooked in a busy clinical setting; however, its effect on patient dose can be significant. Campbell et al. [30] found that nearly 100% of chest CT examinations performed over a 2-week period at a large medical institution included additional

supraapical and infrapulmonary images that were not clinically important, leading to unnecessary radiation exposure. Protocols must include the minimum body length needed to derive an accurate diagnosis, a concept that should be routinely discussed with technologists. Helical pitch (Fig. 3) is defined as the ratio of table feed per gantry rotation to the width of the x-ray beam [31]. Increases in table speed or beam collimation result in higher helical pitch, which reduces exposure time and decreases radiation dose. Higher pitch also results in increased production of noise such as helical artifacts and decreased spatial resolution [32]. Therefore, most modern MDCT scanners are automatically equipped with helical pitch settings that optimize dose and noise based on each body region. Tube Potential Lowering tube potential (x-ray beam energy, kilovoltage, peak kilovoltage) is particularly advantageous in two subsets of patients: slim patients with a small body mass index (BMI) and patients undergoing CT examinations with IV iodinated contrast medium. However, increased noise and variation in tissue contrast accompanied by a decrease in tube potential limit widespread application of this technique [32]. In Amy’s case, iodinated contrast medium is better enhanced with lower kilovoltage [33]. Studies [34, 35] have shown similar diagnostic accuracy with the implementation of low-tube-potential (80 and 100 kVp) contrastenhanced CT protocols for the detection of pulmonary embolism. Schueller-Weidekamm et al. [35] reported improved subsegmental pulmonary artery visualization using 100 kVp as opposed to 140 kVp and attributed the improvement to better visualization of IV con-

Neonate 1 year old 5 year old 10 year old Adult

8.0 6.0 4.0 2.0 0.0

Head

Chest

Abdomen

Pelvis

Abdomen and Pelvis

Fig. 2—Graph shows adult and pediatric effective doses for selection of common CT examinations. Adult effective doses are from [23]. Adult effective doses are modified as described for Timothy’s abdominopelvic CT examination according to [21–23] for estimation of pediatric effective doses.

Fig. 3—Diagram shows helical pitch. Each dash represents position of x-ray tube in xy plane around patient in 5° increments as patient couch moves through gantry.


Automatic Tube Current Modulation Lowering tube current (photon fluence, measured in milliampere seconds) is another method of reducing Amy’s ERD. This can be done by use of a low fixed tube current or by automatic tube current modulation (ATCM), which is included in most modern CT scanners. ATCM is similar to the autoexposure control used in conventional radiographic systems [32]. Integrated CT software automatically adjusts tube current on the basis of differences in tissue attenuation. Therefore, regions of the body that require fewer x-rays for adequate data acquisition receive lower tube current. This results in improved dose efficiency and lowers overall ERD while maintaining an acceptable level of quantum noise [32]. ATCM can be applied with either angular (x- and y-axis) or z-axis modes of modulation. Angular modulation adjusts current as a function of the projection angle [37]. For low-attenuation projection angles in the anteroposterior direction, current is reduced, and for high-attenuation projection angles in the lateral direction, current is increased (Fig. 4). The efficacy of angular modulation is greater in areas of the body with more pronounced asymmetry. For example, in chest CT, the attenuation in the shoulder region is much less

1286

tor y

100 mA

ajec

Fig. 4—Diagram shows angular current modulation. Source emits x-rays (dashed arrows) toward detector. Current is reduced in anteroposterior direction (100 mA) and increased in lateral direction (200 mA), resulting in dose reduction to dimensions of body that require fewer x-rays for adequate data acquisition.

X-ray tube travel

200 mA

y tr

trast medium. Those authors also concluded that noise in 100-kVp protocols for chest CT did not subjectively affect image quality for diagnostic evaluation of findings other than pulmonary embolism. This finding likely reflects the lower absorption characteristics of aerated lung tissue that make the chest region less susceptible to noise associated with a decrease in tube potential. Because ERD is proportional to the square of kilovoltage, considerable decreases in dose can be achieved with relatively small decrements in tube potential [36]. A reduction from 140 to 100 kVp would result in a greater than 50% overall reduction in Amy’s ERD. Lowering tube potential is an important tool for decreasing CT radiation exposure; however, many CT scanners have limited selections of tube potential settings (e.g., 80, 100, 120, or 140 kVp). This restricts the ability to achieve delicate modifications in radiation dose compared with tube current methods [31]. The capability of finer tube potential modulation may allow further application in imaging of larger patients and in other body regions of adults, such as the abdomen and pelvis, where higher tube voltage settings are needed to limit image noise.

X-ra


Costello et al.

Patient

200 mA

Detector travel

100 mA

in the anteroposterior than in the lateral projection, which can result in up to 90% local current reduction with the use of angular modulation [38]. Z-axis modulation adjusts current to maintain a specified quantum noise level (noise index). Although the terminology varies between CT scanner manufacturers [39], the user can select noise index and current range before the CT examination. When differences in tissue density are detected, current is altered to achieve the desired noise index. Z-axis modulation is used to generate all CT images with similar noise, irrespective of patient size or anatomy [39]. Novel research in angular ATCM has shown substantial decreases in ERD by decreasing anterior current with respect to posterior current during CT gantry revolution. When lower current is delivered in the anterior aspect, critical organs such as the cornea, thyroid, and breast are spared radiation dose. This method has shown a possible 27–50% dose savings to the radiosensitive anterior organs [40]. Similar dose reductions have been reported for zaxis modulation. Depending on the noise index, one study showed a 16–26% reduction in tube current compared with low fixed tube current techniques [41]. Kalra et al. [42] highlighted outstanding ERD reduction with z-axis modulation but also reported a mean increase of 17 mAs in 13% (8/62) of patients. All eight patients were found to be much heavier (p = 0.03) than those for whom z-axis modulation reduced ERD, emphasizing the importance of modulation settings in imaging of patients with a greater BMI. If Amy had been a heavier patient, she may have benefited from a lower modulation range or increased noise index. Some CT scanners are equipped with ATCM software that includes aspects of both angular

and z-axis modulation, known as combined or x-y-z current modulation. Greater ERD reductions are achieved with combined current modulation versus angular or z-axis methods alone because both cross-sectional asymmetries and differences in tissue attenuation are accounted for [43, 44]. Overall, ATCM is an effective method of radiation dose reduction, and because of the direct relation between current and ERD, any decrease in current will result in a similar decrease in Amy’s ERD. Further dose reductions with ATCM are expected with the continued development of CT postprocessing software, which could allow ATCM to operate at a higher noise index. Iterative Reconstruction Iterative reconstruction is a computationally complex method of CT postprocessing that was used when CT was introduced [45]. However, because of limitations related to long computing time, filtered back projection (FBP) has been the primary reconstruction algorithm since approximately 2000. FBP is useful at reducing noise but does not produce consistent diagnostic-quality images if tube current is substantially reduced [46]. With advances in computer technology, iterative reconstruction software is again being implemented in many CT scanners and has proved efficacious in reducing noise associated with low-dose CT protocols. Studies of adaptive statistical iterative reconstruction (ASIR) technique have shown it is possible to use the technique to perform diagnostic chest CT examinations with a considerably lower tube current–exposure time setting than would be required with FBP [46–48]. In Amy’s case, ASIR could result in greater than 30% overall ERD reduction.



CT Dose Reduction Bismuth Breast Shielding With the advent of ATCM the use of bismuth breast shields became controversial. Initial studies performed near the time breast shields were introduced in 1997 revealed substantial dose reductions to breast tissue [49], making the shields an attractive option for chest CT of young women, such as Amy. However, it has been clearly shown that similar dose reductions to anterior-surface radiosensitive organs can be attained with ATCM [40]. In addition, breast shields are known to increase noise [50], causing streak and beam artifacts that can make CT images difficult to interpret. Authors also argue that breast shields are an inefficient use of radiation dose, because x-rays are also attenuated in the posteroanterior direction while providing no additional radioprotection [51]. The combined use of ATCM and breast shields can lead to unpredictable and potentially undesirable dose and image quality [51]. When sensing the breast shield, ATCM responds by increasing current, potentially causing a higher radiation dose. Some ATCM systems can exclude an interaction with the bismuth breast shield, as found in a 2009 study [52]; however, this can result in increased noise related to an inappropriately low radiation dose over the shielded breast region. Thus in Amy’s case, breast shields would not be advised, because she would receive a more efficient and global dose reduction to the entire body through an ATCM or low-voltage approach. Pediatric CT Protocols Because of greater organ radiosensitivity and a longer lifespan to potentially develop radiation-induced cancer [53, 54], children arguably are the subset of patients most affected by CT radiation. Although largely determined by attenuation characteristics of each body region [52], an overall decrease in x-ray absorption related to a smaller BMI allows substantial decreases in tube current while maintaining diagnostic image quality. This can be similarly achieved with a low fixed tube current [23, 55] or use of ATCM techniques [38, 56], though modulation parameters, such as current range, must be adjusted to account for small pediatric BMI. Low fixed tube current protocols based on body size and region are outlined at the Image Gently website [57] (established by the Alliance for Radiation Safety in Pediatric Imaging to promote radiation safety in children) and by Frush et al. [58], who used a color-coded Broselow-Luten scheme. Children also benefit from low

tube voltage protocols, because patients of slim habitus are less affected by image noise produced from lowering tube current. Singh et al. [59] found effective use of 80- to 120kVp protocols for imaging of pediatric patients, depending on patient weight. Because of the lower absorption characteristics of aerated lung tissue, the pediatric chest is able to tolerate even larger decreases in current or voltage without the degree of associated noise production that might be encountered in the abdomen and pelvis. Another pediatric dose reduction strategy is the use of a pediatric beam-shaping filter, or bowtie filter [60]. In the CT beam, photons at the lateral aspects of the patient typically have less tissue to travel through and are therefore less attenuated. The filter serves to attenuate these lateral photons to decrease photon scatter and dose to the skin. These bowtie filters are usually sized for adults. However, pediatric filters may be available in pediatrics-specific scanners, or CT vendors may offer adjustable filters that can be optimized to the size of the patient. Timothy represents a patient who could greatly benefit from a decrease in radiation dose. Strategies to reduce his ERD include lowering tube current or tube potential and the use of bowtie filters. Standardizing dose reduction methods to pediatric size and shape are key to avoiding unnecessary radiation exposure. Repeat or Multiple CT Scans Repetitive CT scanning is being recognized as a developing concern for patients presenting to the emergency department with persistent disorders. When multiple CT examinations are performed on the same patient, the resulting cumulative ERD can approach or exceed that sustained by atomic bomb survivors (50–100 mSv) [61]. Numerous studies have shown a trend for repeat CT use among certain patient subsets, the most common being abdominal CT for patients with recurrent abdominal pain [62]. Griffey and Sodickson [63] found that 2% of patients who underwent CT in the emergency department had undergone three or more CT examinations within the past 7 years, leading to a mean cumulative ERD of approximately 120 mSv. A commonly repeated CT examination in the emergency department, illustrated by the case involving Dr. Smith, is unenhanced abdominopelvic CT of patients with chronic renal colic. On the basis of known ERD ranges from a typical single CT examination of the abdo-

men and pelvis (Table 2), it would be reasonable to assume that Dr. Smith’s patient may have received an ERD similar to the mean value of 120 mSv, as reported by Griffey and Sodickson [63]. According to the BEIR VII LAR of 1 in 1000 cases of cancer per 10 mSv, a cumulative ERD of 120 mSv would increase the patient’s LAR to approximately 1 in 82. On the basis of this significant, albeit theoretic, increased risk of cancer development, some authors have proposed a new radiologic approach to patients with chronic renal colic. Because of these patients’ high pretest probability of having nephrolithiasis, and therefore decreased likelihood for missed diagnoses, Katz et al. [64] recommend radiography of the kidney, ureter, and bladder (KUB) with sonography in the initial evaluation of patients known to have chronic stone formation. The combination of KUB and ultrasound has been reported to have sensitivity and negative predictive value comparable to those of CT with no clinically important misdiagnoses [65]. Katz et al. recommend CT use only for patients with normal or equivocal KUB and ultrasound findings and highlight the high efficacy of low-dose protocols in the detection of renal stones when CT is indicated [66, 67]. Awareness of the increase in cancer risk associated with multiple CT examinations along with consideration for alternative nonionizing imaging modalities can lead to substantial ERD reduction in Dr. Smith’s patient. Multiphase CT Studies Results of a 2011 study by Guite et al. [68] suggest that many multiphase abdominal CT examinations are unnecessarily performed. Those authors reported that over a 4-month period at a tertiary care medical center, one third of all abdominal phases were unwarranted according to the ACR Appropriateness Criteria, resulting in a mean excess ERD of approximately 13 mSv. Guite et al. also recognized the potential high radiation doses that can be incurred in multiphase examinations: Approximately 20% of patients were exposed to ERDs greater than 50 mSv. One specific clinical situation in which the value of multiphase examinations has come under scrutiny is CT urography used to evaluate hematuria in young adults. Because of the rare incidence of renal or urothelial malignancy in patients younger than 40 years, some authors argue that not enough cases of cancer are detected with CT urography to validate its high radiation exposure. The use of unenhanced CT



Costello et al. alone [69] or in combination with a split-bolus protocol [70] that incorporates nephrographic and excretory phases into one examination has been suggested as an alternative approach to imaging of young adults. The European Society of Urogenital Radiology supports the use of split-bolus CT urograms for young patients with hematuria, though its efficacy versus separate nephrographic and excretory phases has not been clearly delineated [71]. Professional Education Dose reduction strategies include limiting the use of medical radiation to answer clinically relevant questions. This model depends on the imaging competence of the ordering physician, the radiologist, and the technologist preforming the imaging. These competencies have gained the public and professional spotlight as the long-term effects of medical radiation are debated. In addition to regularly updating its imaging appropriateness criteria, the ACR has also published white paper initiatives [72, 73], which are largely directed at increasing physician and technologist imaging expertise. Case Summaries Chest CT of Young Adult Amy is a young adult with chest pain and suspected pulmonary embolism. “How much radiation will I be exposed to?” The average ERD for an adult pulmonary CT angiographic examination is 15 mSv. “Is there anything you can do to reduce my risk of cancer from a CT examination?” Considerable decreases in radiation dose can be achieved by use of lower tube potential or current and with CT postprocessing software (iterative reconstruction). Other key points to consider include the following. Lowering tube potential is particularly useful in contrast-enhanced CT examinations. The chest can tolerate larger decreases in tube current and tube potential because of its inherent low absorption properties. The dose reduction with ATCM is similar to that with bismuth breast shields without streak or beam artifact. Pediatric CT Timothy is a child with abdominal pain and suspected appendicitis. “How much radiation will my son be exposed to?” The average ERD for a pediatric abdominopelvic CT is 6–7 mSv. “Does this increase his risk of cancer in the future?”

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According to BEIR VII, Timothy’s lifetime risk of development of fatal cancer from a single CT examination of the abdomen and pelvis would be near 0.08%. This could potentially increase Timothy’s overall risk of cancer development from 44.8% to 44.9%. Other key points to consider include the following. Children may be more susceptible to the effects of radiation because of increased radiosensitivity and a longer lifespan. Because of the smaller BMI of children, substantial decreases in tube potential and current can be applied to decrease ERD in children. Dose reduction strategies must be standardized to pediatric size to avoid unnecessary radiation. Repetitive CT Scanning Dr. Smith is treating a young adult with recurrent renal colic who has undergone multiple previous CT examinations. “What is the typical dose range a patient should expect from a CT examination of the abdomen and pelvis?” The average effective radiation dose for an adult abdominopelvic CT examination is 14 mSv. “How will multiple CT examinations affect my patient’s risk of cancer development?” Multiple CT examinations can lead to high cumulative ERDs (50–200 mSv). Compared with the dose from a single CT examination, a cumulative dose of 120 mSv (approximately eight CT examinations) can increase lifetime risk of cancer development from 1 in 1000 to 1 in 82. Other key points to consider include the following. Sonography with KUB can be an effective diagnostic tool in the initial evaluation of patients with recurrent renal colic. The radiation dose effect of multiphase CT examinations can be similar to that of multiple examinations; therefore, caution should be exercised in the use of multiphase CT, especially for younger patients. Conclusion CT use constitutes the majority of medical radiation exposure. Cancer induction by medical radiation at low doses is controversial, with the most widely accepted estimates being outlined in the 2006 BEIR VII report. Radiologists should be prepared to answer questions related to radiation dose and cancer risks associated with CT. References 1. Berrington de González A, Mahesh M, Kim KP, et al. Projected cancer risks from computed tomographic scans performed in the United States in

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