Radiation and Chest CT Scan Examinations

CHEST

Special Features CHEST IMAGING

Radiation and Chest CT Scan Examinations What Do We Know? Asha Sarma, MD; Marta E. Heilbrun, MD; Karen E. Conner, MD; Scott M. Stevens, MD; Scott C. Woller, MD; and C. Gregory Elliott, MD, FCCP

In the past 3 decades, the total number of CT scans performed has grown exponentially. In 2007, . 70 million CT scans were performed in the United States. CT scan studies of the chest comprise a large portion of the CT scans performed today because the technology has transformed the management of common chest diseases, including pulmonary embolism and coronary artery disease. As the number of studies performed yearly increases, a growing fraction of the population is exposed to low-dose ionizing radiation from CT scan. Data extrapolated from atomic bomb survivors and other populations exposed to low-dose ionizing radiation suggest that CT scanassociated radiation may increase an individual’s lifetime risk of developing cancer. This finding, however, is not incontrovertible. Because this topic has recently attracted the attention of both the scientific community and the general public, it has become increasingly important for physicians to understand the cancer risk associated with CT scan and be capable of engaging in productive dialogue with patients. This article reviews the current literature on the public health debate surrounding CT scan and cancer risk, quantifies radiation doses associated with specific studies, and describes efforts to reduce population-wide CT scan-associated radiation exposure. CT scan examinations of the chest, including CT scan pulmonary and coronary angiography, high-resolution CT scan, low-dose lung cancer screening, and triple rule-out CT scan, are specifically considered. CHEST 2012; 142(3):750–760 Abbreviations: CTCA 5 CT scan coronary angiography; CTPA 5 CT scan pulmonary angiography; HRCT 5 high-resolution CT; LET 5 linear energy transfer; TROCT 5 triple rule-out CT

risks of medical imaging, particularly CT scan, Health have captured the attention of the medical community and the public. Medical providers and patients often are underinformed about the potential adverse health effects of radiation exposure from medical imaging.1-6 Estimates suggest that 6,800 future cancers

Manuscript received November 9, 2011; revision accepted February 13, 2012. Affiliations: From the Department of Medicine (Dr Sarma); Department of Radiology (Dr Conner); and Division of Pulmonary and Critical Care Medicine (Dr Elliott) and Division of General Internal Medicine (Drs Stevens and Woller), Department of Medicine, Intermountain Medical Center, Murray, UT; and Department of Radiology (Dr Heilbrun) and Department of Internal Medicine (Drs Stevens, Woller, and Elliott), University of Utah School of Medicine, Salt Lake City, UT. Correspondence to: Asha Sarma, MD, c/o Cami Bills, Transitional Year Residency Program, 5121 S Cottonwood St, Ste 303, Murray, UT 84107; e-mail: [email protected] © 2012 American College of Chest Physicians. Reproduction of this article is prohibited without written permission from the American College of Chest Physicians. See online for more details. DOI: 10.1378/chest.11-2863

may be attributable to chest CT scan examinations performed in 2007 alone7 and that 0.7% to 2% of all future cancers in the United States may be caused by radiation from CT scan.8,9 As society begins to recognize the potential public health impact of imagingassociated radiation,10-12 it is important for medical providers to become familiar with radiation dose For editorial comment see page 549 reduction strategies and gain comfort in discussing risks with patients. This review focuses on chest CT scan, including CT scan pulmonary angiography (CTPA); CT scan coronary angiography (CTCA); high-resolution CT (HRCT) scan; low-dose screening for lung cancer; and triple rule-out CT (TROCT) scan, a modality that simultaneously assesses the aorta and the pulmonary and coronary vascular beds in patients with nonspecific acute chest pain. We provide relevant definitions

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Table 1—Definitions of Relevant Terms Term Absorbed dose Effective dose

Organ dose

Excess relative risk

Low-linear energy transfer radiation

Stochastic effects

Deterministic effects

Definition

Unit

Amount of energy deposited in tissue (J/kg); does not account for variable radiosensitivity of tissues. Estimate of the amount of whole-body radiation equivalent to a partial-body exposure, such as a CT scan; accounts for tissue-specific weighting factors. Not useful for assessing individual risk but for comparing studies and exposure types. Estimate of the amount of radiation deposited in a specific organ by a radiologic procedure; accounts for tissue-specific weighting, patient, and technical factors based on direct measurements of radiation absorbed by anthropomorphic phantoms with components representing human organs. Better metric for individual risk than effective dose. The risk added to population baseline cancer risk from an exposure such as ionizing radiation. For example, if baseline risk is 42% and excess relative risk of radiation-induced cancer from a particular study is 1%, then an individual undergoing that study carries a total lifetime cancer risk of 43%. Radiation doses under about 100 mSv, such as those associated with medical imaging studies, that are believed to result in stochastic effects on human tissues, most significantly, malignancy. Effects of radiation on tissues that are random and unpredictable. Believed to result from low linear-energy transfer radiation, stochastic effects manifest after long lag periods. The linear-no-threshold model describes these effects, which are seen even at minimal doses. The probability, though not the severity, of stochastic effects is believed to correlate with individual cumulative radiation dose. The most medically significant stochastic effect is malignancy. Effects of radiation on tissues that are predictable and occur within hours to days after acute exposure to large doses of radiation (. 100 mSv). These doses may be incurred from radiotherapy and nuclear accidents. Severity of resultant widespread cell death correlates with dose, and there is a minimum threshold dose required to observe these effects. Examples include pulmonary fibrosis and GI necrosis.

Gy, mGy Sv, mSva

Gy, mGy or Sv, mSv

…

…

…

…

In the context of whole-body, uniform exposures to x-radiation, the gray and the sievert are equivalent.

a

(Table 1) and review evidence for the association between medical radiation and cancer. In addition, we review published radiation dose estimates associated with specific study types and discuss methodologies for estimating and reducing radiation dose. Radiation Effects of Chest CT Scans on the Individual Patient Pioneers of radiation science discovered that ionizing radiation from x-rays causes damaging physical effects.13 Electrons are liberated when x-rays traverse living cells. Free electrons may mutate DNA directly or ionize water molecules to form harmful reactive oxygen species. Most damage is readily repaired, though persistent DNA damage may lead to cellular loss of function, necrosis, or malignancy.8,12,14 The term stochastic effect refers to tissue damage from low doses of radiation that is unpredictable and random in nature. This occurs after long latency periods, 5 to ⱖ 20 years after exposure.12 Doses used for medical imaging, referred to as low linear energy transfer (LET) radiation, generally are , 100 mSv and result in stochastic effects, the most significant of which is the development of cancer.8,15 The risk of journal.publications.chestnet.org

heritable defects from damage to germ cells is negligible.16 Generally, younger female patients are at higher risk for developing cancers from low-LET radiation exposure because they possess a greater proportion of actively dividing cells than older male patients and have longer remaining life spans in which malignant transformation could occur.15 Acute exposure to high doses (. 100 mSv) of radiation, such as those used for radiotherapy, results in what is termed “deterministic effect.” This is defined as widespread cell death in affected tissues, generally within days or weeks of exposure, that results in clinical manifestations such as burn-like dermatitis, radiation pneumonitis, pulmonary fibrosis,17 and GI illness.12,16,18 Such nonneoplastic tissue damage does not result from low doses associated with medical imaging. National and international bodies have concluded from current evidence that for low-LET radiation, the relationship between dose and cancer risk is linear, with some risk even at minimal doses (the linear, no-threshold model).5,11,12,15,18-20 Study of atomic bomb survivors, nuclear plant workers, and other groups exposed to low-LET radiation yields sufficient data to conclude that organ doses between 5 and 125 mSv cause a small, but statistically significant increase in CHEST / 142 / 3 / SEPTEMBER 2012

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cancer risk.8,15,20 The mean effective dose in atomic bomb survivors was 40 mSv,8 a dose similar to that incurred from five to six routine chest CT scans.21 Although there is active discourse on whether the random risks associated with stochastic effects on tissues are strictly additive probabilities (a core assumption of the linear, no-threshold model), experts agree that each exposure carries incremental risk, increasing the cumulative probability of developing a malignancy with repeated exposures.5,19,22 The effect of interval length between low-dose imaging studies on cancer risk is unresolved partly because of the undefined influence of cell repair mechanisms.23-25 Overall, however, the radiation risk from two CT scans is believed to be roughly twice the risk of a single scan, irrespective of the time interval between the two.26 There may be logical error in extrapolating risks of medical radiation from epidemiologic studies of Japanese atomic bomb survivors and other cohorts exposed to low-LET radiation that is dissimilar from the acute, partial-body exposure of a CT scan.27 Estimating the risk of cancer from a given effective dose is associated with statistical limitations inherent in assumptions for the calculation of risk.28,29 Furthermore, existing data may not definitively establish that the cancer risk associated with exposure to small doses of ionizing radiation adds significantly to human high baseline lifetime risk (25%-42%).9,12,27-30 Large studies designed to conclusively define the relationship between imaging-associated radiation and cancer are underway, including a UK-based longitudinal study of about 250,000 patients.11 A clinician may appropriately become more liberal in ordering CT scans of most body regions as his or her patients age because of declining risk of carcinogenesis; however, this may not be a prudent strategy for chest CT scan. Perhaps counterintuitively, an older age (50-60 years) at the time of exposure correlates with increased respiratory tract cancer risk in atomic bomb survivors.15,31-33 Therefore, lung cancer risk is likely higher when radiation exposure occurs at an older age. This is in contrast to other forms of cancer, such as lymphoma and breast cancer, where the risk of developing cancer is higher with exposure at a younger age. Patients with lung cancer often are especially vulnerable to radiation-induced carcinogenesis because of both advanced age and an established synergistic carcinogenic effect of radiation and tobacco smoke.34 The benefits of imaging surveillance studies in patients with lung cancer should be weighed carefully against the risks.

CT scanning with patients. It may be important to avoid citing numeric values for which patients have no frame of reference. A comparison with natural background radiation exposure may be better understood. In 1 year, individuals receive slightly less than one-half the dose associated with a routine chest CT scan from background sources, including cosmic radiation and radon gas (3 mSv).19 Another comparator is risk from exposure to normal activities. For example, driving 2,000 miles carries a risk of death of one in 10,000 (from a motor vehicle accident), which is comparable to the added risk of cancer fatality associated with an exposure of 1 to 10 mSv of ionizing radiation. As another alternative, one may express CT scan-radiation dose in relation to the dose from a chest radiograph, where radiation from a chest CT scan is 100 to 400 times higher than from a twoview chest radiograph.5,35 In discussions with patients, it is worth emphasizing the compounded risk of repeated CT scans and encouraging personal dose recording and reporting to all providing clinicians.9,20 Public Health Effects of Chest CT Scans CT scanning has revolutionized the management of many diseases.5,11 Its cost, availability, convenience, and versatility have made it one of the most used and fastest growing imaging technologies.11,15 In 1993, about 18 million CT scans were performed in the United States, and in 2007, this number had increased to . 70 million.5,7 During this period, CT scan utilization grew at 10 times the rate of US population growth.14 Diseases of the chest are major public health concerns, and advances in CT scan technology have revolutionized the management of many pulmonary and cardiac conditions. Of the 67 million CT scans performed in the United States in 2006, 11.6 million (17.4%) included the chest. Because the thyroid, breast, and lungs are among the most cancer-susceptible organs in the body and are included in chest CT scan, the large scale of use may have epidemiologic significance.14,36 Efforts to Reduce Radiation Dose Efforts have been made to decrease CT scanrelated radiation dose on the population and individual levels.9,11,28 Dose reduction strategies are summarized in Table 2. Population-Wide Dose Reduction Initiatives

Discussing Risks With Patients Given the complex nature of the subject, it can be challenging to discuss the radiation risks of

In the United States, medical radiation has replaced background radiation as the primary source of population-wide exposure.18,29 The threshold for

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Table 2—Dose Reduction Strategies Dose Reduction Strategy Determine whether imaging is indicated

Appropriate study selection

Dose reduction during study

Reporting and recording

Available Tools Guidelines Appropriateness criteria Decision support Radiology consultation Guidelines Patient’s past imaging record/exposure Appropriateness criteria Decision support Radiology consultation Alternative modalities (eg, ultrasound, MRI) Refer to accredited imaging facilities Optimized user-set parameters (see Table 3) Bismuth shielding Automated exposure control ECG gating, iterative reconstruction Patient medical record Dose calculating/recording software or websites Reporting to appropriate agencies as necessary

ordering CT scan has lowered even in younger, healthier patients for whom risks may outweigh benefits.5,15,30 Risk-benefit analyses are difficult to perform, and available decision aids often are underused in practice.11 It has been estimated that 26% to 44% of CT scans are ordered inappropriately.5,11,15,37 To counter this, appropriateness criteria and decision support software are increasingly being incorporated into computerized physician order entry systems.38 In addition, focused educational programs have been shown to increase appropriate use.39,40 Professional organizations have initiated national campaigns (Image Gently,41,42 Step Lightly,41,42 and Image Wisely43) to educate the general public and professionals about dose reduction measures. Technical shortfalls represent another source of excessive exposure. Wide dose variation exists within and across institutions; therefore, it has been proposed that CT scan protocols be optimized and standardized across sites.5 Quality control recommendations, such as facility accreditation offered by the American College of Radiology,11 have been put forth to ensure that patients are exposed to doses that are as low as reasonably achievable. Some patient populations, including those with chronic conditions such as cystic fibrosis and screeningeligible patients at risk for various diseases, are often exposed to high cumulative radiation doses from repeated CT scans.9,34,44 To date, recording of individuals’ cumulative CT scan radiation doses has been limited to mobile device applications and websites journal.publications.chestnet.org

where motivated and informed patients can enter their own data.5,9,20 Recently, there has been a movement toward institutionalized dose recording and reporting of unintended exposures. California recently adopted Senate Bill 1237, requiring facility accreditation, recording of individual cumulative radiation doses, and reporting of accidents to a state agency.35 It is likely that other institutional bodies will adopt similar measures. The US Food and Drug Administration recently recommended that CT scanners be manufactured with the capability of alerting technicians if safe doses are exceeded. Additional authorization would then be required to proceed with the study, and finally, data would be transmitted to automatically initiate an audit.45 Tracking an individual patient’s cumulative radiation dose remains difficult, particularly because patients commonly receive care from unaffiliated providers. Ideally, after protocols are standardized, consensus is reached on how to accurately measure administered doses, and the majority of institutions gain technical dose recording ability, reporting to centralized registries will be implemented. Although such a registry for radiation dose does not currently exist, registries founded on similar principles for public health measures, such as childhood vaccination, have been instituted.46 Reducing Radiation Dose From Individual Studies Reducing the dose administered during each study decreases individual radiation burden from CT scan. Evidence-based decision-making identifies patients for whom a CT scan study would provide clear net benefit and situations in which imaging tools that do not deliver radiation (eg, MRI, ultrasonography) are appropriate.5,15 Once the decision for a CT scan has been made, it is important to optimize technical parameters to minimize risk and maximize diagnostic utility. Dose reduction strategies have been variably incorporated into CT scanning protocols. In understanding these technologies, it is important to consider a fundamental concept: Higher doses of radiation yield clearer CT images. Therefore, with all dose reduction measures, dose and image clarity must be balanced.15 Several factors determine the amount of radiation delivered during CT scan, including patient size, body habitus, and anatomic area of interest. User-set parameters are important determinants of radiation dose.8 The relationship of various factors with delivered dose is presented in Table 3. Image quality for a given dose depends on the volume and density of a given slice of tissue. Smaller, less-dense slices require less radiation to generate images of sufficient clarity. Small CHEST / 142 / 3 / SEPTEMBER 2012

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Table 3—Relationships Between Test and Patient Properties and Radiation Dose From CT Scan Examinations Test and Patient Property

Proportionality to Dose

Patient body habitus Maximum tube potential Tube current Table speed Gantry rotation time Scan length

Directa Exponential Directa Inverse Directa Direct

Increased doses necessary for acceptable image quality with increasing BMI.

a

adult patients and children, therefore, require smaller doses than larger adults. Automated exposure control (automatic tube current modulation), which has been shown to reduce dose by 40% to 70%, is a feature based on this principle.11,47 Because different parts of the body have different size and density characteristics, data from a CT scan scout image may be used to determine how much radiation must be delivered to each region. The well-aerated chest requires less radiation for a clear image than the denser, larger abdomen. During a study that evaluates both the chest and the abdomen, automated exposure control allows a smaller x-ray tube current to be used on the chest, protecting sensitive chest structures from the larger current required for abdominal imaging. Another software-based approach applies mathematical models called noise reconstruction algorithms, enhancing the performance characteristics of lowerdose scans by retrospectively subtracting artifact from CT images.48 A newer processing innovation known as model-based iterative reconstruction may provide chest CT images of similar quality to those obtained with traditional methods using doses similar to a chest radiograph.49 The protection of radiosensitive tissues, such as the breast and thyroid, with bismuth shields is another method being used for dose reduction.48,50 Although traditional shielding (eg, of the gonads during pelvic radiography) is meant to block virtually the entire x-ray beam, bismuth shielding allows partial penetration.48 Bismuth breast shields have been associated with dose reductions of 29% to 57% and have potential to become ubiquitous public health tools.8,48,51 Experts disagree about the importance of image artifacts produced by bismuth breast shields. Although the practice does affect image quality, it does not seem to impair diagnostic discrimination, especially when shields are spaced 1 to 2 cm from the body.52 Technical strategies such as organ-based angular tube current modulation (not currently commercially available), which decreases tube current as the tube passes closest to specific organs, decrease dose throughout

the breast, rather than just at the shielded surface, without increasing image noise or producing the artifacts associated with bismuth shielding.53 ECG gating in cardiac CT scan eliminates the artifact produced by movement of the heart. Synchronizing CT scan data with simultaneous ECG recordings allows image reconstruction from scan data obtained from the portion of the cardiac cycle where the heart is most still (diastole).11,14,48,54 In typical (retrospective) ECG gating, the heart is imaged throughout its cycle, and systolic data are subtracted afterward. Prospective ECG gating activates the x-ray beam only during diastole and may allow radiation dose reductions of . 90%.55

Radiation, Cancer Risk, and Specific Chest CT Scan Modalities Dose estimates from various chest CT scan studies are summarized in Table 4. CT Scan Coronary Angiography CTCA has emerged as a noninvasive alternative to traditional coronary angiography. An additional application of CTCA is coronary artery calcium screening.14 Although CTCA once required high doses of radiation, developments such as prospective ECG gating and reconstruction techniques have led to significant dose reductions.14,48,50,54 Concern over radiation risk from CTCA arises from lack of standardization of protocols and increasingly widespread use for screening.14 As in other CT scan examinations of the chest, the lungs and breasts absorb the highest radiation doses and are at greatest risk for developing cancers. Einstein et al75 estimated lifetime attributable risk of cancer in a 20-year-old woman from a single, standard CTCA without dose reduction measures at 0.70%. Using different methods, Hurwitz et al56 determined that, for a 25-year-old woman undergoing CTCA, excess relative risk for breast cancer was 1.4% to 2.6% and for lung cancer, 2.4% to 3.8%. It is important to note that older individuals at high risk for coronary artery disease but at lower risk for most radiation-induced cancers (except respiratory tract cancers) are much more likely to undergo cardiac CT scan in clinical practice. Reflecting this, in a cohort referred for CTCA by clinicians, an estimate of the average lifetime radiation-induced cancer risk was only 0.13%.26 Another study estimated that 21 to 23 individuals per 100,000 62-year-old individuals undergoing one low-dose CTCA study would develop a radiation-induced cancer; nonetheless, the lower-risk characteristics of actual screening populations do not

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CHEST

Special Features CHEST IMAGING

Radiation and Chest CT Scan Examinations What Do We Know? Asha Sarma, MD; Marta E. Heilbrun, MD; Karen E. Conner, MD; Scott M. Stevens, MD; Scott C. Woller, MD; and C. Gregory Elliott, MD, FCCP

In the past 3 decades, the total number of CT scans performed has grown exponentially. In 2007, . 70 million CT scans were performed in the United States. CT scan studies of the chest comprise a large portion of the CT scans performed today because the technology has transformed the management of common chest diseases, including pulmonary embolism and coronary artery disease. As the number of studies performed yearly increases, a growing fraction of the population is exposed to low-dose ionizing radiation from CT scan. Data extrapolated from atomic bomb survivors and other populations exposed to low-dose ionizing radiation suggest that CT scanassociated radiation may increase an individual’s lifetime risk of developing cancer. This finding, however, is not incontrovertible. Because this topic has recently attracted the attention of both the scientific community and the general public, it has become increasingly important for physicians to understand the cancer risk associated with CT scan and be capable of engaging in productive dialogue with patients. This article reviews the current literature on the public health debate surrounding CT scan and cancer risk, quantifies radiation doses associated with specific studies, and describes efforts to reduce population-wide CT scan-associated radiation exposure. CT scan examinations of the chest, including CT scan pulmonary and coronary angiography, high-resolution CT scan, low-dose lung cancer screening, and triple rule-out CT scan, are specifically considered. CHEST 2012; 142(3):750–760 Abbreviations: CTCA 5 CT scan coronary angiography; CTPA 5 CT scan pulmonary angiography; HRCT 5 high-resolution CT; LET 5 linear energy transfer; TROCT 5 triple rule-out CT

risks of medical imaging, particularly CT scan, Health have captured the attention of the medical community and the public. Medical providers and patients often are underinformed about the potential adverse health effects of radiation exposure from medical imaging.1-6 Estimates suggest that 6,800 future cancers

Manuscript received November 9, 2011; revision accepted February 13, 2012. Affiliations: From the Department of Medicine (Dr Sarma); Department of Radiology (Dr Conner); and Division of Pulmonary and Critical Care Medicine (Dr Elliott) and Division of General Internal Medicine (Drs Stevens and Woller), Department of Medicine, Intermountain Medical Center, Murray, UT; and Department of Radiology (Dr Heilbrun) and Department of Internal Medicine (Drs Stevens, Woller, and Elliott), University of Utah School of Medicine, Salt Lake City, UT. Correspondence to: Asha Sarma, MD, c/o Cami Bills, Transitional Year Residency Program, 5121 S Cottonwood St, Ste 303, Murray, UT 84107; e-mail: [email protected] © 2012 American College of Chest Physicians. Reproduction of this article is prohibited without written permission from the American College of Chest Physicians. See online for more details. DOI: 10.1378/chest.11-2863

may be attributable to chest CT scan examinations performed in 2007 alone7 and that 0.7% to 2% of all future cancers in the United States may be caused by radiation from CT scan.8,9 As society begins to recognize the potential public health impact of imagingassociated radiation,10-12 it is important for medical providers to become familiar with radiation dose For editorial comment see page 549 reduction strategies and gain comfort in discussing risks with patients. This review focuses on chest CT scan, including CT scan pulmonary angiography (CTPA); CT scan coronary angiography (CTCA); high-resolution CT (HRCT) scan; low-dose screening for lung cancer; and triple rule-out CT (TROCT) scan, a modality that simultaneously assesses the aorta and the pulmonary and coronary vascular beds in patients with nonspecific acute chest pain. We provide relevant definitions

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completely negate the potential public health impact of CTCA because of the scale on which screening studies may be performed. Hall and Brenner15 estimated that about 7,000 deaths would result from radiation-induced lung cancer if all 61 million eligible Americans were screened for coronary artery calcium. CT Scan Pulmonary Angiography CTPA has replaced ventilation-perfusion scintigraphy and conventional pulmonary angiography as the imaging study of choice for suspected pulmonary embolism.14,76 CTPA is widely available, relatively noninvasive, and has higher rates of diagnostic resolution and interobserver agreement than ventilationperfusion scintigraphy.14,76 Because of the epidemiologic characteristics of pulmonary embolism, CTPA is used to evaluate patients of all ages.14 Over a 2-year period at one institution, 60% of CTPA studies were performed in women. Nearly 30% of those women were aged , 40 years.57 Because CTPA use is common, and because radiationassociated risks to younger patients and women are higher, increasing use may elevate the populationwide risk for breast and other cancers.76 Use of CTPA and the resultant cancer risk could be reduced by closer adherence to emerging appropriate-use criteria. Use of radiation-sparing modalities, such as ventilationperfusion scintigraphy and ultrasound of the lower extremities, remains the approach of choice in certain situations.77 For example, lower-extremity venous ultrasonography has been put forth as the first-line study for a stable, female patient of reproductive age with a high clinical probability of pulmonary embolism and an elevated D-dimer. In addition, nearly one-third of PIOPED II (Prospective Investigation of Pulmonary Embolism Diagnosis) investigators favor ventilationperfusion scanning over multidetector CTPA in evaluating for pulmonary embolism.78 As with other CT scan examinations of the chest, strategies such as automated exposure control, decreased tube voltage, and limited-range scanning that excludes nonessential anatomic structures have resulted in decreased doses without detriment to diagnostic performance.14 The average effective dose range for CTPA has been estimated to be 1.4 to 15 mSv.21,58,59 For comparison, the average dose incurred from conventional pulmonary angiography is about 7.1 mSv (range, 3.3-17.3 mSv) and from ventilation-perfusion scintigraphy, 1.2 mSv.58,59 The average organ-absorbed dose delivered per breast to an average 60-kg woman undergoing CTPA is 20 mGy (by contrast, the average organ dose per breast from ventilation-perfusion scintigraphy is 0.28 mGy).57 A 20-year-old woman receiving a breast-absorbed dose of 40 mGy from a single CTPA study has been estimated to be at

68% greater risk for breast cancer by age 35 years than a 20-year-old woman without such exposure.79 Low-Dose CT Scan Screening for Lung Cancer The landmark National Lung Screening Trial80 showed that annual screening with low-dose CT scanning for 3 years reduced all-cause mortality by 6.7% and death from lung cancer by 20.0% compared with screening by conventional radiography. Similarly, the Italung-CT trial32 showed a 10% to 30% reduction in mortality risk from low-dose CT screening. The risk of radiation-induced cancer was not assessed by these studies because of insufficient length of follow-up.80 Estimates of average effective dose from screening chest CT scans range from 0.6 to 1.1 mSv per study.14 The large population eligible for lung cancer screening is vulnerable to radiation-induced lung cancer because of advanced age and smoking status (in contrast to other types of cancer, as discussed previously). Brenner34 estimated that if 50% of current and former smokers in the United States underwent annual CT scan screening from age 50 to 75 years, associated radiation-induced lung cancer would increase the total lung cancer burden by 1.8%. Without accounting for annual repeat or follow-up examinations, the International Commission on Radiologic Protection60 determined that per 100,000 screened, three to six cases of radiation-induced cancer would occur over a 15- to 20-year period. From Italung-CT trial data, it is estimated that 4-year effective doses of 3.3 to 5.8 mSv would be incurred, accounting for additional follow-up and interventional CT scans for indeterminate, suspicious nodules.32 This exposure is predicted to result in 11.7 to 20.5 radiation-induced fatal cancers per 100,000 50- to 70-year-old subjects screened. Despite this, the measured benefits of a 4-year screening program outweigh the risks for both female and male smokers. Similarly, Brenner34 suggested that a ⱖ 5% reduction in overall mortality from CT scan screening would outweigh radiation risks. The results of both Italung-CT trial and National Lung Screening Trial far exceed this threshold. HRCT Scan of the Lung HRCT scan, a technique used to obtain detailed images of the lung parenchyma and interstitium, is useful in evaluating diseases that affect the lung diffusely, such as bronchiectasis and interstitial lung disease. One-millimeter slices, spaced 1 to 2 cm apart, are imaged. Technical hallmarks of this technique include thin collimation, in which the peripheral, nonuseful portion of the x-ray beam is eliminated for improved resolution, and a specialized highfrequency spatial mathematical algorithm for image reconstruction.61-63

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HRCT scan does not image the entire lung and, therefore, cannot be used to evaluate focal pathology; thus, it is often used in conjunction with standard, volumetric CT scan. In the past, these protocols were performed separately. With the advent of multidetector CT scanners, it became possible to perform the studies together in a single breath hold. HRCT images are reconstructed from data obtained during the volumetric scan, sparing the patient the radiation dose associated with a separate HRCT scan.62 The effective dose range for a single HRCT scan alone is 0.2 to 0.98 mSv, and the mean effective dose from a volumetric multidetector CT scan is 3.8 to 5.5 mSv.61-63 The individual or population-wide radiation burden or cancer risk from these studies is unpublished. TROCT Scan Studies The TROCT scan, or comprehensive chest pain protocol, is designed to evaluate patients with nonspecific chest pain who are at low to intermediate risk for acute coronary syndrome.54 This tailored, ECG-gated study evaluates for aortic dissection, acute coronary syndrome, and pulmonary embolism.81 Compared with CTCA and CTPA in combination, TROCT scan requires less contrast material and can be performed in ⱕ 15 s.64 In a study of 175 patients, TROCT scan was found to eliminate the need for further diagnostic testing in 76% of patients.82 A 2008 survey indicated that about 18% of EDs use TROCT scanning.83 Resistance to adoption has been attributed to limited reimbursement, difficult-toexecute protocols, requirement for large contrast doses, and low-image quality. Advances, however, have mitigated technical limitations.54 Numerous authors have identified the need for randomized controlled trials designed to determine important outcomes before widely implementing TROCT scanning.54,84 Careful patient selection is warranted, and it has not yet been proven which patients benefit from TROCT scanning.64 As TROCT scanning and other broad-based approaches are adopted, attention should be paid to the pretest probability of disease in patients being considered for imaging because low pretest probability reduces positive predictive value. Protocols that are more specific than TROCT scanning, such as dedicated CTPA, may be more appropriate in many patients.85 TROCT scanning delivers an effective dose between 4 and 31.8 mSv, depending on the protocol and dose reduction strategies selected.54,64,86 It is estimated that a TROCT scan could deliver an organ dose as high as 91 mGy to the lung and 80 mGy to the breast and that the lifetime risk of cancer from one cardiac CT scan study could range from , 0.02% (80-year-old man, journal.publications.chestnet.org

ECG-gated cardiac CT scan) to 1% (20-year-old woman, nongated TROCT scan).75 Even with dose reduction, the lifetime cancer risk from one study could be 30 times greater for a young woman than for an elderly man, highlighting the importance of careful risk stratification.75

Conclusions Even if single CT scans increase the individual risk of malignancy minutely, expanding use amplifies population-wide risk.5,8,14,15 There is still scientific uncertainty surrounding the risk and likelihood of developing a radiation-induced malignancy from CT scan. However, at our current level of understanding, it appears unwise to assume that there is no increased risk and, thereby, to expose patients to doses that future study may reveal to be critical.26 Chest physicians can minimize patient exposure to imaging-associated radiation through (1) self and patient education; (2) use of evidence-based guidelines, appropriateness criteria, and decision support to eliminate unnecessary studies; (3) mindfulness of the number and type of CT scans each patient has received; (4) referral of patients to accredited imaging facilities that use current dose reduction strategies; and (5) recording and reporting of radiation doses.

Acknowledgments Financial/nonfinancial disclosures: The authors have reported to CHEST that no potential conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article. Other contributions: Jana Johnson provided additional editorial and formatting assistance. We thank Julie Felice, CPM, and Ulrich Rassner, MD, for their advice on the topic of medical physics.

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