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The Proceedings of the American Thoracic Society 4:316-320 (2007)
© 2007 The American Thoracic Society
doi: 10.1513/pats.200611-172HT
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Radiation Doses and Risks in Chest Computed Tomography Examinations

Walter Huda1

1 Department of Radiology, State University of New York Upstate Medical University, Syracuse, New York

Correspondence and requests for reprints should be addressed to Walter Huda, Ph.D., 750 East Adams Street, Syracuse, NY 13210. E-mail: hudaw{at}upstate.edu

ABSTRACT

Effective doses, and the corresponding risks of radiation-induced cancers, are presented for patients undergoing chest computed tomography (CT) examinations. Patient dose calculations were based on the characteristics of 16-slice CT scanner from 4 imaging equipment vendors. The dose–length product (DLP) was used to quantify the amount of radiation used to perform chest CT examinations. Values of DLP were converted into a corresponding effective dose (E) using age-dependent E/DLP conversion coefficients applicable to chest CT examinations. Calculations of effective doses were performed for a typical chest CT examination, as well as for a low-dose protocol for patients with cystic fibrosis. Effective doses were used to estimate nominal cancer risks based on data in Report VII of the Committee of the Biological Effects of Ionizing Radiation. Patient effective doses in standard chest CT examinations range from approximately 1.7 millisieverts (mSv) in newborns to approximately 5.4 mSv in adults. The effective dose to a 5-year-old patient with cystic fibrosis using a low-dose protocol is approximately 0.55 mSv, which is about a factor of four lower than a standard chest CT examination. An effective dose of 0.55 mSv for a 5-year-old patient corresponds to a nominal excess risk of carcinogenesis of approximately 1.5 cancers per 10,000 individuals, with half of these being fatal. It is concluded that patients undergoing chest CT examinations should have a benefit that exceeds the (small) radiation risk.

Key Words: computed tomography • cystic fibrosis • pediatric • radiation doses • radiation risks

When a given patient is exposed to X-ray radiation, the amount of energy absorbed per unit mass of a specified tissue is the absorbed dose, which is expressed in grays (Gy). For example, if the liver absorbs 1 J/kg of tissue, the absorbed dose is 1 Gy. The radiation risk to the tissue is related to the magnitude of the corresponding absorbed dose, which is of two types: (1) deterministic risks, which occur at high doses and are therefore extremely unlikely to occur in computed tomography (CT) scanning; and (2) stochastic risks, which are the principal concerns when adult and pediatric patients undergo CT scanning.

Deterministic radiation risks, such as the induction of skin burns and epilation, are associated with a threshold dose (1, 2). For a given radiologic examination, deterministic effects will only occur if the most highly irradiated tissue exceeds this threshold dose, currently taken to be at least 2 Gy (3). The most frequent deterministic effects are skin burns and epilation that may occur in high-dose interventional procedures (3, 4). In CT examinations, the entrance skin doses are approximately 40 mGy for head examinations and approximately 20 mGy for body examinations (5). Accordingly, deterministic effects are not expected for any patient undergoing a standard diagnostic CT examination.

Below the threshold for the induction of deterministic effects, the principal concern of any radiation exposure is the induction of stochastic (random) risks (1, 6). In diagnostic radiology, these stochastic radiation risks are carcinogenesis and genetic effects that would appear in the offspring of an irradiated individual. Stochastic radiation risks do not appear to have threshold doses, and carcinogenesis is currently deemed to be much more important than genetic effects (1, 4, 7). The principal concern for any patient undergoing a diagnostic chest CT examination is the risk of developing a radiation-induced cancer, which may be fatal or nonfatal (8). The total patient risk is related to the effective dose (see below), which depends on the dose to each organ, as well as its radiosensitivity, and is measured in sieverts (Sv). At the (low) doses associated with diagnostic radiologic examinations, the radiation risk is generally taken to be proportional to the cumulative organ dose. The radiation risk from two CT scans, for example, would be approximately twice the risk of a single scan, irrespective of the time interval between the two CT scans.

Patient doses (and risks) in CT need to be weighed against the anticipated patient benefits from the diagnostic information that is obtained (9). Use of any diagnostic test requires a net patient benefit, with the benefit being greater than any risk(s). In addition, it is also important to ensure that patient doses are kept as low as reasonably achievable to ensure that any patient risks are minimized (10, 11). In this article, we outline a method for estimating radiation doses, and the corresponding patient risk, in CT. This method is applied to generate doses (and risks) to patients undergoing standard chest CT examinations, as well as for patients with cystic fibrosis (CF) scanned using a low–radiation dose protocol.

METHODS

CT Scanning
Dose and image quality in CT generally depend on the choice of technique factors that are used to perform a chest CT examination (12). The most important of the parameters that are under the control of the CT operator are as follows:

  1. X-ray tube voltage. The choice of X-ray tube voltage (kV) in CT scanning ranges from 80 to 140 kV. Increasing the X-ray tube voltage will increase the amount of radiation used in the exam, and will also increase the average photon energy. As a result, high voltages reduce image contrast (13), as well as reducing the amount of noise (mottle) (14). In addition, use of high kV values may also reduce artifacts, such as beam hardening (15).
  2. Tube current/exposure time. The product of the X-ray tube current (mA) and scan time (s) is known as the mAs, which is a measure of the amount of radiation that is used to generate any radiographic or CT image (16). Because pediatric patients are smaller, and therefore easier to penetrate, the CT mAs used to scan pediatric patients is generally reduced relative to those used for adults (Table 1) (17–19).
  3. Pitch ratio. In helical CT, the pitch ratio (P) is given by the table increment distance per 360° rotation of the X-ray tube divided by the X-ray beam width (20). A pitch of 1 corresponds to performing axial scans with contiguous slices. For a fixed scan length, increasing the pitch will spread the radiation energy over a larger patient volume, decreasing patient doses, and vice versa.
Table 1 shows typical values of technique factors that are normally used in chest CT scanning for pediatric patients and a normal-sized adult (21). The scan length increases for older children because of the increase in patient size with increasing age.


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  		TABLE 1. TECHNIQUE FACTORS USED TO PERFORM STANDARD CHEST COMPUTED TOMOGRAPHY EXAMINATIONS IN PATIENTS RANGING FROM NEWBORN TO ADULT (120 kV AND PITCH RATIO 1.5)

 
CT Dosimetry
Radiation dosimetry is currently based on the Computed Tomography Dose Index (CTDI) (22, 23). CTDI doses are measured in cylindrical acrylic phantoms with a diameter of either 16 cm (head) or 32 cm (body), and may be measured at the center of the phantom (CTDIc), or at the phantom periphery (CTDIp). When the CT X-ray tube rotates through 360° around the acrylic dosimetry phantom, the pattern of energy deposition will result in a dose profile that is relatively high in the directly irradiated region, with long scatter tails outside the directly irradiated region. CTDI measurements generally include both of these components; the resultant CTDI value is interpreted as the average dose in the directly irradiated region when the dosimetry phantom is scanned contiguously.

For a given phantom, it is customary to define a weighted CTDI (CTDIw) as

Formula 1((1))
which takes into account (any) differences in radiation dose throughout the CT dosimetry phantom. Table 2 shows values of CTDIw for 16-slice CT scanners from four vendors, operated at X-ray tube voltages ranging from 80 to 140 kV (24). CTDIw data in Table 2 are taken to be representative CTDIw in a 16-cm acrylic phantom for current generation CT scanners. Figure 1 shows how the vendor average CTDIw varies with X-ray tube voltage; increasing the X-ray tube voltage from 80 to 140 kV will increase patient doses by a factor of about 3.5.


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  		TABLE 2. WEIGHTED COMPUTED TOMOGRAPHY DOSE INDEX FOR THE 16-CM-DIAMETER COMPUTED TOMOGRAPHY DOSIMETRY PHANTOM

 

Figure 1
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  		Figure 1. Weighted computed tomography (CT) dose index (CTDIw) per unit X-ray tube current (mA) and scan time (s) (mAs) as a function of X-ray tube voltage (average of 4 commercial 16-slice CT scanners).

 
Modern CT scanners generally display two dose quantities on the operator's console, the volume CTDI (CTDIvol) and the dose–length product (DLP). These two dose quantities are defined as

Formula 2((2))
and

Formula 3((3))
The CTDIvol value relates to dose measurements in acrylic dosimetry phantom, and cannot be interpreted as a patient dose, because CTDIvol does not take into account the physical characteristics of the patient.

Patient Radiation Risks
In diagnostic radiology, the patient effective dose, expressed in sieverts, is given by

Formula 4((4))
where E is the effective dose, Di is the dose to organ i, and wi is the relative radiosensitivity of this organ, as given in Table 3 (1). The summation in Equation 4 includes all the organs that were irradiated for any specified examination. The effective dose in mSv is related to the total patient (stochastic) radiation risk, whereas the organ dose in mGy may be used to predict the risks for a single exposed tissue.


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  		TABLE 3. ORGAN WEIGHTING FACTORS SPECIFIED BY THE INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION

 
The patient effective dose from a chest CT examination is obtained using

Formula 5((5))
where (E/DLP)age is an age-specific conversion factor for children undergoing CT chest examinations. The effective dose is currently taken as the most appropriate dose descriptor that is directly related to the patient stochastic radiation risk (25, 26). The E/DLP data published by Chapple and colleagues (27) (Figure 2) permit CT DLP data obtained for 16-cm-diameter dosimetry phantoms to be converted into the corresponding patient effective doses. Also included in Figure 2 is a conversion factor for adult patients undergoing chest CT examinations. The adult (i.e., 18 yr old) datum in Figure 2 was obtained by converting the value of 0.017 mSv/mGy-cm based on a 32-cm dosimetry phantom (28) to one based on a 16-cm dosimetry phantom. The adult E/DLP value of 0.017 was multiplied by 2.4, which is the ratio of 16 to 32 cm CTDIw obtained for 16-slice scanners operated at 120 kV (24).


Figure 2
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  		Figure 2. Effective dose (E) per unit dose–length product (DLP) in a 16-cm acrylic phantom as a function of patient age. CT = computed tomography.

 
The recent Biological Effects of Ionizing Radiation VII report estimates of approximately 1 radiation-induced cancer per 10,000 individuals with an age distribution similar to that of the entire U.S. population exposed to 1 mSv, of which half would be fatal (7). For comparison purposes, in the general population, it is expected that there would be approximately 4,200 cancer cases in 10,000 individuals over a lifetime (42% incidence), with about half of these being fatal. It is important to note that children have a nominal radiation risk that is about a factor of three higher than a population average, whereas patients who are approaching retirement from work would be allocated a nominal radiation risk that is about a factor of three lower than a population-averaged radiation risk (8).

RESULTS AND DISCUSSION

Standard Chest CT
Figure 3 shows the DLP for normal patients undergoing chest CT examinations. The DLP values increase from approximately 50 mGy-cm for newborns to approximately 760 mGy-cm for normal-sized adults weighing 70 kg. The increase in DLP with patient age (size) is partly due to the use of longer scan lengths, and partly to the need for more radiation (i.e., mAs) to penetrate larger patients (see Table 1). It is important to note that all DLP values shown in Figure 3 are based on measurements in a 16-cm dosimetry phantom; DLP measurements based on a 32-cm-diameter body phantom, as is normally shown for adult scans, would be a factor of 2.4 lower.


Figure 3
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  		Figure 3. Values of dose–length product (DLP) for standard chest computed tomography (CT) examinations as a function of patient age.

 
Figure 4 shows how patient effective dose varies with age for patients undergoing standard chest CT examinations. Patient effective doses range from approximately 1.7 mSv in newborns to approximately 5.4 mSv in a normal-sized adult. These patient effective doses may be compared to effective doses encountered in daily life, as summarized in Table 4. A typical chest X-ray examination has an effective dose of 0.05 mSv, and a 6-hour trans-Atlantic flight will result in passengers receiving an additional 0.03 mSv from the increased cosmic radiation at an elevation of about 10,000 m. Patient doses in chest CT examinations are much higher than those associated with conventional chest X-rays or any single flight on a commercial jet, but are of the same magnitude as effective doses from average natural background doses (including radon exposure) each year (i.e., ~ 3 mSv/yr) (4).


Figure 4
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  		Figure 4. Values of patient effective dose (E) for standard chest computed tomography (CT) examination as a function of age.

 

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  		TABLE 4. EFFECTIVE DOSES FROM NATURAL BACKGROUND AND USED IN REGULATORY DOSE LIMITS

 
Patients with CF
In radiology, it is normal practice to modify radiographic techniques to take into account patient characteristics, as well as the diagnostic task at hand (17, 19). Table 5 summarizes the technique used in a protocol that has been recommended for scanning patients with CF (F. Long, M.D., personal communication). The effective dose to a 5-year-old patient with CF undergoing a chest CT examination is about 0.55 mSv, a factor of four lower than a standard pediatric chest CT study.


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  		TABLE 5. TECHNIQUE AND DOSE COMPARISON OF A CONVENTIONAL CHEST COMPUTED TOMOGRAPHY SCAN WITH THE LOW-DOSE PROTOCOL FOR SCANNING 5-YEAR-OLD PATIENTS WITH CYSTIC FIBROSIS

 
A radiation effective dose of 0.55 mSv in a 5-year-old patient corresponds to a nominal radiation risk of approximately 1.5 radiation-induced cancers per 10,000 individuals undergoing this type of low-dose chest CT examination, half of which would be expected to be fatal. It is important to note that there are large uncertainties regarding radiation risks at the low doses encountered in diagnostic CT examinations (29–31). Estimating a radiation risk in patients with CF is made more difficult because little is known of (any) differences in radiosensitivity that this group of patients may possess, as well as uncertainties in the life expectancy for this group of patients. In this article, radiation risks to patients undergoing chest CT examinations have been called nominal; this emphasizes the fact that such risk estimates must not be interpreted as being definitive. This approach is helpful in that it explicitly acknowledges the large uncertainties of these risk estimates for patients with CF undergoing chest CT examinations.

Dealing with scientific uncertainties is inherently a political (not scientific) process. Given the current level of scientific uncertainty regarding radiation risks at low doses, it is appropriate to act on the assumption that such risks are real, as this conservative approach is unlikely to underestimate patient risks (32). The alternative strategy of assuming that there is no radiation risk would clearly carry the danger that future studies may prove this assumption erroneous. Assuming that low-dose radiation risks exist is the current recommendation made by all the leading scientific authorities (1, 4, 7), and implicitly requires that: (1) patients should not be exposed unless there is a net benefit (i.e., the benefit is greater than the risk); and (2) no more radiation should be used in any examination than what is required to achieve a satisfactory diagnosis (33). This approach to a judicious use of X-rays for radiologic examinations will permit patients to obtain the undoubted benefits from CT imaging for any indicated examination (34, 35).

FOOTNOTES

Conflict of Interest Statement: W.H. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

(Received in original form November 23, 2006; accepted in final form March 6, 2007)

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