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Review Article

Lung Cancer Screening

Computed Tomography Radiation and Protocols

Murugan, Venkatesh A. MD; Kalra, Mannudeep K. MD; Rehani, Madan PhD; Digumarthy, Subba R. MD

Author Information
doi: 10.1097/RTI.0000000000000150
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Lung cancer is the third most common cancer in the United States but has the highest mortality rate among cancers.1 Although it contributes to only 13.5% of estimated new cancer cases, it contributes to about 27% of all cancer-related deaths in the United States.2 The 5-year survival rate from lung cancer is 16.8%, which is dismal compared with that of breast cancer (89.2%) and colon cancer (64.7%), 2 other common cancers.2–4 An important contributing factor to this high mortality is that most lung cancers are in advanced stages at the time of diagnosis. Most (85%) lung cancers are non–small cell lung cancers.5 The 5-year survival of non–small cell lung cancer varies from 49% when detected at stage IA to 1% when detected at stage IV.6 Hence, early diagnosis of lung cancer would be expected to reduce mortality from this disease. This concept was reinforced by the successful reduction in breast and colon cancer mortality by screening programs.7,8

Several studies have assessed ways to screen the at-risk population for lung cancer using methods such as chest radiography, sputum analysis, and low-dose computed tomography (LDCT). Initial studies using chest radiography and/or sputum analysis at varying intervals of time were not successful in reducing lung cancer-specific mortality.9,10 Revolutionary advances in CT technology and use of low-radiation dose protocols for chest CT revived interest in using CT for lung cancer screening. Several studies utilizing LDCT for lung cancer screening demonstrated its feasibility as a lung cancer-screening tool. The National Lung Screening Trial (NLST) reported a 20% reduction in lung cancer mortality using LDCT for lung cancer screening.11 Recent studies have also documented promising results regarding the cost-effectiveness of lung cancer screening along with smoking cessation.12–15


Although CT constitutes approximately 17% of all diagnostic radiology procedures in the United States, it contributes to around half of the radiation dose from medical imaging.16 This is of particular concern, as increased population radiation dose from CT could limit its use as a large-scale public health diagnostic tool. The Early Lung Cancer Action Project (ELCAP) has reported that LDCT was effective in the early detection of malignant lung nodules.17 This led to increased interest in exploring the use of LDCT as a screening tool for high-risk populations as well as to optimizing the LDCT protocol for screening of lung cancer. Recent trials have used non-contrast-enhanced LDCT for lung cancer screening using multidetector-row CT (MDCT) scanners.18–21

Given the fact that LDCT is being promoted for use in screening for early detection of lung cancer, special attention should be given to the justification and optimization of radiation dose. The foremost aspect is justification or appropriateness of CT. Therefore, selecting high-risk populations for CT scanning according to available/recommended guidelines for use of LDCT in screening for lung cancer is important. The Centers for Medicare and Medicaid Services (CMS) have announced eligibility criteria for implementation of LDCT lung cancer screening in health care institutions in their recent proposal (Table 1).22

CMS Eligibility Criteria for Lung Cancer Screening (All the Criteria Must be Met to Qualify a Patient for LDCT Lung Cancer Screening)

As justification of CT for this indication has already been addressed by a number of key stakeholder organizations and individuals, in this review article we focus solely on radiation dose optimization aspects of CT scanning.


The most widely reported radiation dose indices are volume CT dose index (CTDIvol), dose-length product (DLP), size-specific dose estimate (SSDE), and effective dose (ED). CTDI is the radiation absorbed along the length of a 16 or 32 cm phantom in a duration that produces a CT image. As the radiation absorbed along the length of the phantom is not uniform, a measure to account for this difference called weighted CTDI (CTDIw) was introduced. The currently used descriptor, CTDIvol (represented in milli-Grey or mGy), is basically CTDIw normalized for pitch for helical CT scanners (CTDIvol=CTDIw/pitch [mGy]). The DLP is the product of CTDIvol and the length of the scanned phantom.

As the above-mentioned dose metrics are specific to 16 and 32 cm phantoms, the American Association of Physicists in Medicine (AAPM) has recommended the use of SSDE to account for size variations among patients. It is important to note that CTDIvol, DLP, and SSDE are not patient-specific doses but represent scanner output or phantom doses.

The estimated ED (represented in milli-Sievert or mSv) is the product of DLP and a conversion factor (0.014 for chest CT), which is derived on the basis of the attenuation characteristics of each region (or organ) scanned. Several methods can be used to estimate the ED and can provide a wide range of different values for the same CT examination.

All CT manufacturers are required to display CTDIvol and DLP but not the estimated EDs. The American College of Radiology-Society of Thoracic Radiology collaboration (ACR-STR) practice parameters (released in 2014) set a maximum limit of up to 3 mGy for LDCT lung cancer screening in average-sized patients. However, we believe that both the ACR-STR and the CMS guidelines are quite conservative. In fact, most modern MDCT scanners should enable submilli-Sievert (or sub-mGy) radiation doses for LDCT lung cancer-screening protocols in most small and average-sized patients. With recently available iterative reconstruction techniques, it is possible to reduce radiation dose to well under 1.0 mSv or 2 mGy. We believe that the recommendations from the ACR-STR and the CMS should be taken as exceptional maximum allowable limits of radiation doses for LDCT lung cancer-screening protocol and that users should target a much lower radiation dose in most patients. In fact, submilli-Sievert scanning of the chest is slowly becoming the standard for chest CT imaging at many institutions. A recent study evaluating radiation dose in chest imaging using spectral filtration (tin filters) has reported CTDIvol and DLP values as low as 0.46 mGy and 16 mGy cm, respectively.23


Tube Current

Radiation output from the x-ray tube is directly proportional to the tube current when all other scanning parameters are held constant. Different studies on screening LDCT for lung cancer have used low tube currents (≤50 mAs at ≤140 kV), using either fixed manually selected low tube current or lower automatic exposure control (AEC).19,21 Modern MDCT scanners offer several techniques to adjust radiation dose.24–26 Fortunately, the lungs are high–inherent contrast organs, which implies that lesions can be seen at a relatively lower level of radiation dose even in the presence of high image noise. Lower attenuation of x-ray beam through the lungs also results in lower image noise at lower dose as compared with more solid or dense organs such as the liver or the brain. Use of fixed tube current requires manual adjustment of mA based on patient size, weight, or body mass index (BMI). Most of the screening LDCTs for lung cancer have been performed with the use of the fixed tube current technique.20,27–30 AEC modulates the tube current to adjust for the attenuation as scanning progresses around (angular modulation) and/or along the length (z-modulation) of the patient. Several studies have shown that AEC can help to reduce radiation dose.31–34 The ACR-STR lung cancer-screening guidelines recommend the use of AEC techniques, although manual adjustment of tube current can also be used.

When selecting the “right” tube current manually for screening LDCT, users must target a specific CTDIvol in order to obtain a CT examination that is able to provide adequate image quality for desired diagnostic information.35,36 Then, with changing patient body habitus, the AEC techniques will automatically adapt the tube current to obtain a constant image quality regardless of patient size.

When using AEC techniques, users must keep in mind that there are large variations in the extent of change in tube current for different patient sizes among AEC techniques from different CT vendors. At a constant image quality requirement (ie, noise index or standard deviation), some AEC techniques can increase the tube current substantially for large patients as compared with an average-sized patient. Such proportionate increase in tube current for large patients may not be desirable for LDCT examinations and can be avoided by either setting limits to the maximum-allowed tube current or by using fixed tube current. Another differentiating point between the use of AEC techniques for LDCT and routine body CT examination is that, for LDCT, AEC techniques should be allowed to go to the minimum possible tube current (about 10 mA). As the applied tube current and radiation dose is linked to the input image quality as well as to prospective section thickness for some vendors (GE Healthcare and Toshiba), thicker prospective section thickness (eg, 5 mm) should be coupled with high noise index or standard deviation. For other vendors (Siemens Healthcare), users must adapt the strength of AEC modulation to allow maximum decrease (strong decrease in mA) in tube current for smaller subjects and minimal increase in tube current for larger subjects (weak increase in mA) undergoing LDCT for lung cancer screening.

The other caveat with the ACR-STR-recommended CTDIvol is the scanner type. Certain wide area detector CT systems (≥4 rows) may provide reasonable diagnostic quality at a radiation dose well under 3 mGy for standard-sized patients. Likewise, with the use of iterative reconstruction techniques it may be possible to obtain reasonable quality at much lower doses as well.

Availability of a reference dose level of ≤3 mGy should make it easy for the users to adapt their tube current and other scanning parameters for standard-sized patients. AEC techniques can then automatically adapt the dose depending upon the patient size. Although users do not need to select image quality settings on the basis of patient size for AEC, to achieve lower-dose CT for lung cancer screening compared with routine chest CT examination, users will need to specify higher noise or lower tube current requirements. Such values can be set initially to ensure that the radiation dose is ≤3 mGy (CTDIvol) for screening LDCT. Subsequently, depending on user experience, noise tolerance, and CT technology, radiation dose can be well under 1 mSv.

To our knowledge, the use of organ-based tube current modulation has not been assessed for lung cancer-screening purposes, and such techniques may enable dose reduction for organs such as thyroid and breast. However, these techniques are available on only some CT equipment, and their use can increase radiation dose to the lung.

Tube Voltage

Prior studies on screening LDCT have used tube voltages between 80 and 120 kV for most patients and 140 kV for obese patients. The ACR technical specifications for lung cancer screening recommend the use of 100 to 140 kV for standard-sized patients.36 Initially, the tube potential should be set in combination with the tube current to meet the recommended CTDIvol specifications. Subsequently, the users can attain substantially lower values of radiation dose (<0.5 mSv or approximately 1 mGy) by altering the tube parameters in small decrements.

A fixed kV can be set manually. In our experience, 100 to 120 kV is generally sufficient in most patients regardless of body habitus. An extremely large body habitus may necessitate the use of 140 kV. Alternatively, automatic kV selection techniques such as Care kV (Siemens Healthcare) can be used with a reference kV of 100. This technique can then automatically select a kV on the basis of patient body habitus.

A recent study has reported the use of 100 kV with a tin filter for detection of lung nodules with a radiation dose under 0.1 mSv.37 However, further validation in a lung cancer-screening population is awaited.

Gantry Rotation Speed

In general, faster gantry rotation is associated with less motion artifacts. As the speed of rotation increases, the radiation dose to the patient decreases, if all other scanning parameters are kept constant. A rotation time of <0.5 seconds is recommended by the ACR.35,36 The intent of using a faster rotation time is to minimize motion artifacts by reducing the total scan duration to <15 seconds.

Detector Collimation

On older-generation MDCT (<16 slice), detector collimation should be set to enable reconstruction of images ≤1 mm, although section thickness of up to 1.5 mm is acceptable according to the ACR guidelines.35,36 The use of thinner detector configuration on some MDCTs (≤4 slice) may increase the radiation dose and scan duration and should certainly be avoided in patients who cannot hold their breath very well. On more advanced CT systems (≥16 slice), a wider beam collimation is preferable to reduce the radiation dose and the scan duration. Reconstruction section interval (intersection gap) should be equal to or less than the reconstructed section thickness.


Pitch is the ratio of the table feed per gantry rotation to the beam collimation.38 For some scanners (such as Siemens and Philips), there is generally no change in the radiation dose with change in pitch, as the tube current is automatically changed to maintain a constant dose with a change in pitch. In other scanners (such as GE Healthcare), higher pitch is associated with lower dose as compared with lower pitch. The ACR technical specifications call for the use of a pitch between 0.7 and 1.5:1, which is similar to the pitch used in prior screening trials.36

Generally, a higher nonoverlapping pitch is preferable (>1:1) to minimize the scan duration and the probability of motion artifacts. In patients with exceptionally large body habitus, on older scanners, a lower pitch (<0.75 to 1:1) may be suitable to reduce artifacts and image noise.

A caveat to the above-mentioned guidelines on pitch should be noted for dual-source CT scanners in which a very high pitch of ≥1.6:1 (up to 3.2:1) is associated with a lower radiation dose and extremely short scanning duration (typically <1 to 2 s). However, use of this very high pitch has not been specifically assessed for lung cancer screening. This technique may help in reducing the radiation dose and scan time in patients who cannot hold their breath.


Reconstruction techniques help to convert acquired projection raw data into CT images. Filtered back projection (FBP) techniques have been used traditionally owing to the speed of generating images. Several CT vendors have recently introduced iterative reconstruction techniques. These techniques calculate the projection data from the reconstructed image, taking into account the scanner properties, and compare it with the measured projection data in an iterative manner.

Most LDCT screening studies have used the FBP technique for image reconstruction. Iterative reconstruction algorithms such as adaptive statistical iterative reconstruction (ASIR-GE Healthcare, Waukesha, WI), sinogram-affirmed iterative reconstruction (SAFIRE-Siemens Healthcare, Forchheim, Germany), adaptive iterative dose reduction (AIDR 3D-Toshiba American Medical Systems, Tustin, CA), and iDose (Philips Healthcare, Cleveland, OH) have been shown to improve image quality while reducing radiation dose39–46 (Fig. 1). With radiation dose reduction, image noise increases. Noise reduction filters are software programs that can reduce image noise. With the availability of advanced iterative reconstruction techniques, the use of noise reduction filters has declined.

LDCT of the chest comparing iterative reconstruction with standard FBP. All images have been acquired at estimated EDs <0.3 mSv with AQ4 CT scanners from different vendors. A–C, Noncalcified 4mm lung in the right lower lobe (black arrows) in a 69-year-old man (90 kg, 30 kg/m2) is well seen on CT images (CTDIvol=0.46 mGy) reconstructed with FBP (A), ASIR 50% (B), and MBIR (C) (all images from GE Discovery 750 HD). D and E, Noncalcified 4mm lung in the right lower lobe (black arrows) in a 61-year-old man (95 kg, 32 kg/m2) is well seen on CT images (CTDIvol=0.4 mGy) reconstructed with FBP (D) and Safire (E) (all images from Siemens Definition Flash). Although image quality is better with iterative reconstruction techniques, lung nodules can be seen in FBP images as well.

Although prior studies on screening LDCT have not used iterative reconstruction techniques, we believe that use of modern iterative reconstruction techniques can allow substantial radiation dose reduction as compared with FBP techniques. Furthermore, iterative reconstruction techniques have lower image noise and lesser artifacts as compared with FBP images at low radiation dose levels.39,47–54


Body Habitus

The patient’s body habitus plays a very important role in the designing of CT protocols. Scanning parameters such as tube current and tube voltage should be increased for larger patients in order to attain acceptable image quality. The ACR guidelines specify that the CTDIvol for lung cancer screening in average-sized patients (170 cm, 70 kg, BMI of 24.1 kg/m2) should be ≤3 mGy.35,36 For larger patients, higher radiation dose may be required to obtain images of acceptable quality. For smaller-sized patients, corresponding images can be achieved at lower radiation dose. Such adjustments in radiation dose can be automatically achieved with the use of AEC as discussed above. When fixed current is used for scanning, a weight or BMI-based approach should be adopted. Generally, 100 to 120 kV can provide acceptable diagnostic information regardless of patient size with appropriate selection of tube current. The National Comprehensive Cancer Care Network guidelines have suggested an ED of ≤3 mSv for patients with BMI≤30 kg/m2 and ≤5 mSv for patients with BMI>30 kg/m2.50 As mentioned previously, we believe that these recommendations are quite conservative, and, in practice, LDCT chest images can be routinely acquired at significantly lower values of radiation dose (typically <0.5 mSv even in obese patients) irrespective of body habitus. Recent LDCT lung cancer-screening trials have included patient weight-based adjustments of tube parameters.18–21 As EDs are not displayed on the current CT scanners, it can be relatively difficult for users to back-calculate the CTDIvol or DLP values (currently displayed values on CT user interface) because of variations in patient lengths and in converting EDs to CTDIvol.

Positioning and Centering

Minor variations in the position of the patient from the scanner isocenter can have significant effects on radiation dose and image quality.55–59 Therefore, attention should be paid to appropriately centering the patient in the scanner isocenter. Positioning of the limbs in relation to the body is also known to affect the image quality and radiation dose. Previous studies on arm positioning in polytrauma patients demonstrated reduced radiation dose when the arms were positioned vertically relative to the chest.60–62 With AEC techniques, radiation dose increases when arms are placed by the side of the body.61 If clinical constraints dictate or the patient experiences pain, arms should be placed vertically in relation to the chest to avoid beam-hardening artifacts, poor image quality, and higher radiation dose. The NLST protocol utilized vertical positioning of the arm in relation to the chest for obtaining screening images.20

Length of the Scanned Segment

Scanning should be limited to only those areas that might contribute to diagnostic information. The recent ACR-STR guidelines for lung cancer screening specify that “the study must include axial images from the lung apices to the costophrenic sulci.”35,36 Routine extension of scan length to include the upper abdomen and adrenal glands should be avoided.

Protocols for LDCT

Various LDCT protocols for lung cancer screening in major studies have been summarized in Table 2. Table 3 illustrates the guidelines for LDCT for lung cancer screening used in our institution.

LDCT Lung Cancer Screening Protocols Used by Selected Studies
Guiding Principles for Designing LDCT Lung Cancer Screening Protocols at Our Institution


Radiologists must use appropriate scanning strategies to reduce and optimize the radiation dose for lung cancer screening LDCT protocols. Fortunately, recent developments offer a good opportunity to achieve appropriate dose utilization for screening LDCT. Recent advances in CT scanning technology have facilitated the ability to acquire chest CT images at much lower radiation dose when compared with previous scanners. The CT scanning protocols at all institutions should be constantly updated to adapt to these rapid improvements in technology.


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lung cancer screening protocols; low-dose computed tomography; CT radiation dose

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