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Report No. 293 - Size Specific Dose Estimate (SSDE) for Head CT
The Report of AAPM Task Group 293 (2019)

Category: Reports

The CT dose index value reported on a CT scanner during a patient exam (i.e., CTDIvol) does not represent the absorbed dose to the patient on which the scan was performed.1 To address the desire by the medical community to estimate actual patient dose, AAPM Report 204, published in 2011, described methods by which the CTDIvol could be multiplied by a patient-size-specific conversion factor to estimate the absorbed dose to a patient of a given size, a quantity referred to as the Size-Specific Dose Estimate (SSDE).2 Since the publication of AAPM Report 204, the SSDE concept and the associated size-specific conversion factors have been validated in a number of phantom, cadaver, and Monte Carlo studies.3–6 However, wide-scale clinical adoption of SSDE remains dependent on manufacturer implementation of methods to automatically calculate and report the SSDE, a task that will be greatly facilitated by the recently completed IEC standard on the calculation and validation of SSDE. However, the size-dependent conversion factors provided in AAPM Report 204 were developed specifically for CT imaging of the abdomen and pelvis, although use for CT scans of the thorax was deemed acceptable since the errors were anticipated to be below 20%. AAPM Report 204 used effective diameter, Deff, which is defined as the diameter of a circle of equivalent area to the patient’s crosssectional area at a given position along z, to describe the size of the patient. Methods for the estimation of the water-equivalent diameter, Dw, were subsequently standardized and are described in detail in AAPM Report 2207—where Dw takes into consideration tissue attenuation, as assessed by the CT scanning process, to more accurately estimate SSDE in body regions that contain a range of tissue densities and compositions, such as the thorax. The current document—Report of AAPM TG-293—is an extension of AAPM Report 204 and focuses on the development of SSDE conversion factors appropriate for CT examinations of the head.

As in AAPM Report 204, SSDE represents the absorbed dose to the center section along the z-axis of a typical clinical CT scan, although in this report, the CT examination under consideration is for the head. The overall length (in z) of such a scan increases somewhat with increasing age because of the increasing size of the head with increased age, although the effect is small as the size of the skull grows very little beyond about seven years of age. The absorbed dose in this thin center section of the head includes dose from scattered radiation from the tissues above (superior to) and below (inferior to) the center section.8,9 It is, therefore, recognized that the dose to the center section (along z) of the CT scan represents a near-maximum absorbed dose to the tissues within the scan volume and, consequently, the average absorbed dose to the entire scan volume will be lower in virtually all cases. The principal outcome of this report—a set of CTDIvol,16-to-SSDE conversion factors for CT examinations of the head as a function of water-equivalent diameter (Dw)—is a result of independent determinations of SSDE by four groups of investigators: two sets of physical measurements made using physical phantoms and ionization chambers and two sets of data determined from Monte Carlo simulations. One of the sets of physical measurements from St. Jude Children’s Research Hospital (SJCRH) provided extensive physical measurements that were used to validate the Monte Carlo simulation results performed at the University of California–Davis (UCD). The variable f H16 is used to represent the CTDIvol,16-to-SSDE conversion factors for CT examinations of the head, where H indicates that these conversion factors are for the head and 16 indicates that the conversion factors are for use with CTDIvol,16 values, which are acquired with a 16-cm CTDI phantom.

ISBN: 978-1-936366-71-2

Keywords: Computed tomography, CT, CTDIvol, Size specific dose estimate, SSDE, Radiation dose, Pediatric imaging
Task Group No. 293 - Task Group on Size Specific - Dose Estimate (SSDE) for Head CT (TG293)

John M. Boone, Keith J. Strauss, Andrew M. Hernandez, Anthony J. Hardy, Kimberly E Applegate, Nathan S Artz, Samuel L Brady, Dianna D. Cody, Nima Kasraie, Cynthia H. McCollough, Michael F. McNitt-Gray

Committee Responsible: Computed Tomography Subcommittee
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