- Email: [email protected]
Low-Dose CT for Lung Cancer Screening
RADIATION SENSIBILITIES RICHARD L. MORIN, PHD, DONALD P. FRUSH, MD
Low-Dose CT for Lung Cancer Screening Mannudeep K. Kalra, MD Low-dose CT (LDCT) for lung cancer screening has been recently approved for subjects who meet specific criteria that are associated with increased risk for lung cancer [1]. LDCT is recommended for those older than 55 years, to limit radiation exposure to younger subjects. Strict adherence to inclusion and exclusion criteria for lung cancer screening is critical, ensuring appropriate use and limiting radiation exposure to otherwise asymptomatic subjects [2]. Because of the high prevalence of incidental pulmonary nodules detected on LDCT, follow-up CT should be triggered on the basis of specific LungRad guidelines [3]. The ACR and the Society of Thoracic Radiology have recommend scan and radiation dose parameters for LDCT, including a maximum CT dose index volume (CTDIvol) of 3 mGy in standard-sized subjects weighing up to 160 lb and 5 feet 7 inches in height [4]. Most modern multidetector CT (MDCT) scanners can easily enable LDCT at a substantially lower radiation dose than the recommended CTDIvol of 3 mGy without compromising evaluation of solid and subsolid pulmonary nodules [5]. It is essential to build scannerspecific LDCT protocols for lung cancer screening. The ACR and Society of Thoracic Radiology practice guidelines recommend MDCT scanners with 16 or more detector rows to enable scanning in a single
breath hold [4]. These scanners are more dose efficient compared with the older MDCT scanners with fewer than 16 detector rows. Attention to details such as demonstration of the breath-hold requirement for LDCT helps avoid suboptimal studies caused by respiratory motion artifacts. The patient must be centered in the gantry isocenter to ensure appropriate application of automatic exposure control (AEC) and avoid asymmetric distribution of image noise and artifacts. When possible, patients must be asked to place their arms above their shoulders to avoid artifacts as well as an increase in radiation dose from AEC. To minimize extra radiation dose, scan length should include the lungs only and should not extend to cover the adrenal glands. Tube current and tube potential are the most important scan parameters for optimizing radiation dose from LDCT. Techniques such as AEC and automatic tube potential selection must be preferred over manual or fixed prescription for tube current and tube potential. These techniques must be adjusted to ensure reduced radiation dose compared with other chest CT protocols. For example, a lower image quality requirement (such as noise index, standard deviation, or quality reference tube current–time product) must be set for LDCT compared with other chest CT protocols. These image quality parameters for AEC must be set to
ª 2017 American College of Radiology 1546-1440/17/$36.00 n http://dx.doi.org/10.1016/j.jacr.2017.01.052
initially meet but eventually beat the aforementioned CTDIvol criterion. A tube potential of 100 kV must be set as the reference tube voltage for automatic tube potential selection techniques. When using fixed tube voltage, generally 80 to 100 kV suffices for most patients. The widest possible detector configuration allowing reconstruction of 1-mm section thickness should be used to maximize scanner dose efficiency. Unfortunately, MDCT scanners with 16 or fewer detector rows allow 1-mm sections only on thinner detector configurations (such as 16 0.625 mm or 16 0.75 mm instead of 16 1.25 mm or 16 1.5 mm), which are less dose efficient. Most scanners adapt the tube current with change in gantry rotation times so that there is little change in radiation dose except at the lowest possible tube current, at which reduction in gantry rotation times will reduce radiation dose. Faster gantry rotation times of 0.5 seconds help reduce scan time and breath-hold duration. Likewise, a nonoverlapping pitch greater than 1:1 should be used for LDCT to minimize scan duration and the possibility of motion artifacts. On scanners from certain vendors (GE and Toshiba), a higher pitch is associated with lower radiation dose, but on others (Philips and Siemens), dose does not change with pitch. Prospective section thickness also has a similar effect on radiation dose with AEC per the CT vendor.
719
Last, several iterative reconstruction (IR) techniques from CT vendors can help with additional reduction in radiation dose for LDCT compared with the conventional filtered back projection [5]. When available, IR techniques must be used with suitable reductions in tube current and tube potential to reduce radiation dose. Several studies have demonstrated that IR techniques help improve image quality and retain diagnostic value of LDCT at substantially reduced radiation dose [6-8]. In summary, LDCT for lung cancer screening can be performed
at substantially lower radiation dose, often below the recommended CTDIvol of 3 mGy, with suitable modifications in scan parameters.
5.
6.
REFERENCES 1. Allen B Jr. Lung cancer screening: Medicare got it right; now it’s our turn. J Am Coll Radiol 2015;12:121-2. 2. Bach PB, Mirkin JN, Oliver TK, et al. Benefits and harms of CT screening for lung cancer: a systematic review. JAMA 2012;307:2418-29. 3. Fintelmann FJ, Bernheim A, Digumarthy SR, et al. The 10 pillars of lung cancer screening: rationale and logistics of a lung cancer screening program. Radiographics 2015;35:1893-908. 4. Kazerooni EA, Austin JH, Black WC, et al. ACR-STR practice parameter for the performance and reporting of lung cancer screening thoracic computed tomography
7.
8.
(CT): 2014 (Resolution 4). J Thorac Imaging 2014;29:310-6. Murugan VA, Kalra MK, Rehani M, Digumarthy SR. Lung cancer screening: computed tomography radiation and protocols. J Thorac Imaging 2015;30: 283-9. Mathieu KB, Ai H, Fox PS, et al. Radiation dose reduction for CT lung cancer screening using ASIR and MBIR: a phantom study. J Appl Clin Med Phys 2014;15: 4515. Minehiro K, Takata T, Hayashi H, et al. [Phantom study on dose reduction using iterative reconstruction in low-dose computed tomography for lung cancer screening] [article in Japanese]. Nihon Hoshasen Gijutsu Gakkai Zasshi 2015;71: 1201-8. Sakai N, Yabuuchi H, Kondo M, et al. Lowdose CT screening using hybrid iterative reconstruction: confidence ratings of diagnoses of simulated lesions other than lung cancer. Br J Radiol 2015;88:20150159.
Dr Kalra has received research grants from Siemens and Toshiba and consultation fees from Bracco that are unrelated to this article. Mannudeep K. Kalra, MD: Massachusetts General Hospital, Webster Center for Quality and Safety, Divisions of Thoracic and CardioVascular Imaging, Department of Imaging, 55 Fruit Street, Boston MA 02114; e-mail: [email protected].
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Journal of the American College of Radiology Volume 14 n Number 5 n May 2017
Low-Dose CT for Lung Cancer Screening Mannudeep K. Kalra, MD Low-dose CT (LDCT) for lung cancer screening has been recently approved for subjects who meet specific criteria that are associated with increased risk for lung cancer [1]. LDCT is recommended for those older than 55 years, to limit radiation exposure to younger subjects. Strict adherence to inclusion and exclusion criteria for lung cancer screening is critical, ensuring appropriate use and limiting radiation exposure to otherwise asymptomatic subjects [2]. Because of the high prevalence of incidental pulmonary nodules detected on LDCT, follow-up CT should be triggered on the basis of specific LungRad guidelines [3]. The ACR and the Society of Thoracic Radiology have recommend scan and radiation dose parameters for LDCT, including a maximum CT dose index volume (CTDIvol) of 3 mGy in standard-sized subjects weighing up to 160 lb and 5 feet 7 inches in height [4]. Most modern multidetector CT (MDCT) scanners can easily enable LDCT at a substantially lower radiation dose than the recommended CTDIvol of 3 mGy without compromising evaluation of solid and subsolid pulmonary nodules [5]. It is essential to build scannerspecific LDCT protocols for lung cancer screening. The ACR and Society of Thoracic Radiology practice guidelines recommend MDCT scanners with 16 or more detector rows to enable scanning in a single
breath hold [4]. These scanners are more dose efficient compared with the older MDCT scanners with fewer than 16 detector rows. Attention to details such as demonstration of the breath-hold requirement for LDCT helps avoid suboptimal studies caused by respiratory motion artifacts. The patient must be centered in the gantry isocenter to ensure appropriate application of automatic exposure control (AEC) and avoid asymmetric distribution of image noise and artifacts. When possible, patients must be asked to place their arms above their shoulders to avoid artifacts as well as an increase in radiation dose from AEC. To minimize extra radiation dose, scan length should include the lungs only and should not extend to cover the adrenal glands. Tube current and tube potential are the most important scan parameters for optimizing radiation dose from LDCT. Techniques such as AEC and automatic tube potential selection must be preferred over manual or fixed prescription for tube current and tube potential. These techniques must be adjusted to ensure reduced radiation dose compared with other chest CT protocols. For example, a lower image quality requirement (such as noise index, standard deviation, or quality reference tube current–time product) must be set for LDCT compared with other chest CT protocols. These image quality parameters for AEC must be set to
ª 2017 American College of Radiology 1546-1440/17/$36.00 n http://dx.doi.org/10.1016/j.jacr.2017.01.052
initially meet but eventually beat the aforementioned CTDIvol criterion. A tube potential of 100 kV must be set as the reference tube voltage for automatic tube potential selection techniques. When using fixed tube voltage, generally 80 to 100 kV suffices for most patients. The widest possible detector configuration allowing reconstruction of 1-mm section thickness should be used to maximize scanner dose efficiency. Unfortunately, MDCT scanners with 16 or fewer detector rows allow 1-mm sections only on thinner detector configurations (such as 16 0.625 mm or 16 0.75 mm instead of 16 1.25 mm or 16 1.5 mm), which are less dose efficient. Most scanners adapt the tube current with change in gantry rotation times so that there is little change in radiation dose except at the lowest possible tube current, at which reduction in gantry rotation times will reduce radiation dose. Faster gantry rotation times of 0.5 seconds help reduce scan time and breath-hold duration. Likewise, a nonoverlapping pitch greater than 1:1 should be used for LDCT to minimize scan duration and the possibility of motion artifacts. On scanners from certain vendors (GE and Toshiba), a higher pitch is associated with lower radiation dose, but on others (Philips and Siemens), dose does not change with pitch. Prospective section thickness also has a similar effect on radiation dose with AEC per the CT vendor.
719
Last, several iterative reconstruction (IR) techniques from CT vendors can help with additional reduction in radiation dose for LDCT compared with the conventional filtered back projection [5]. When available, IR techniques must be used with suitable reductions in tube current and tube potential to reduce radiation dose. Several studies have demonstrated that IR techniques help improve image quality and retain diagnostic value of LDCT at substantially reduced radiation dose [6-8]. In summary, LDCT for lung cancer screening can be performed
at substantially lower radiation dose, often below the recommended CTDIvol of 3 mGy, with suitable modifications in scan parameters.
5.
6.
REFERENCES 1. Allen B Jr. Lung cancer screening: Medicare got it right; now it’s our turn. J Am Coll Radiol 2015;12:121-2. 2. Bach PB, Mirkin JN, Oliver TK, et al. Benefits and harms of CT screening for lung cancer: a systematic review. JAMA 2012;307:2418-29. 3. Fintelmann FJ, Bernheim A, Digumarthy SR, et al. The 10 pillars of lung cancer screening: rationale and logistics of a lung cancer screening program. Radiographics 2015;35:1893-908. 4. Kazerooni EA, Austin JH, Black WC, et al. ACR-STR practice parameter for the performance and reporting of lung cancer screening thoracic computed tomography
7.
8.
(CT): 2014 (Resolution 4). J Thorac Imaging 2014;29:310-6. Murugan VA, Kalra MK, Rehani M, Digumarthy SR. Lung cancer screening: computed tomography radiation and protocols. J Thorac Imaging 2015;30: 283-9. Mathieu KB, Ai H, Fox PS, et al. Radiation dose reduction for CT lung cancer screening using ASIR and MBIR: a phantom study. J Appl Clin Med Phys 2014;15: 4515. Minehiro K, Takata T, Hayashi H, et al. [Phantom study on dose reduction using iterative reconstruction in low-dose computed tomography for lung cancer screening] [article in Japanese]. Nihon Hoshasen Gijutsu Gakkai Zasshi 2015;71: 1201-8. Sakai N, Yabuuchi H, Kondo M, et al. Lowdose CT screening using hybrid iterative reconstruction: confidence ratings of diagnoses of simulated lesions other than lung cancer. Br J Radiol 2015;88:20150159.
Dr Kalra has received research grants from Siemens and Toshiba and consultation fees from Bracco that are unrelated to this article. Mannudeep K. Kalra, MD: Massachusetts General Hospital, Webster Center for Quality and Safety, Divisions of Thoracic and CardioVascular Imaging, Department of Imaging, 55 Fruit Street, Boston MA 02114; e-mail: [email protected].
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Journal of the American College of Radiology Volume 14 n Number 5 n May 2017