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AAPM REPORT NO. 293

Size-Specific DoseEstimate (SSDE)

for Head CT

The Report of AAPMTask Group 293

July 2019

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The AAPM does not endorse any products, manufac-turers, or suppliers. Nothing in this publication shouldbe interpreted as implying such endorsement.

© 2019 by American Association of Physicists in Medicine

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Size-Specific DoseEstimate (SSDE)

for Head CT

The Report of AAPM Task Group 293

John M. Boone1, Chair; Keith J. Strauss2, Vice Chair; Andrew M. Hernandez1;

Anthony Hardy3; Kimberly E. Applegate4; Nathan S. Artz5;

Samual L. Brady2; Dianna D. Cody6; Nima Kasraie7;

Cynthia H. McCollough8; Michael McNitt-Gray3

1University of California–Davis, Davis, CA2Cincinnati Children’s Hospital Medical Center, Cincinnati, OH3University of California–Los Angles, Los Angeles, CA4Indianapolis, IN5St. Jude Children’s Research Hospital, Memphis, TN6MD Anderson Cancer Center, Houston, TX7Children’s Mercy Hospital, Kansas City, MO8Mayo Clinic, Rochester, MN

DISCLAIMER: This publication is based on sources and information believed to be reliable,but the AAPM, the authors, and the publisher disclaim any warranty or liability

based on or relating to the contents of this publication.

The AAPM does not endorse any products, manufacturers, or suppliers. Nothing in thispublication should be interpreted as implying such endorsement.

ISBN: 978-1-936366-71-2ISSN: 0271-7344

© 2019 by American Association of Physicists in Medicine

All rights reserved

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American Association of Physicists in Medicine1631 Prince Street

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THE REPORT OF AAPM TASK GROUP 293:Size-Specific Dose Estimate (SSDE) for Head CT

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1 St. Jude Children’s Research Hospital (Memphis, TN): Physical Measurements inTissue-Equivalent Head Phantoms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Mayo Clinic: Physical Measurements in Tissue-Equivalent Head Phantoms . . . . . . . . . . . . . . . . . . . . . . . 92.3 University of California–Los Angeles (UCLA): Monte Carlo Estimations in Voxelized

Head 170 Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.4 UC Davis: Monte Carlo Estimation in Virtual Head CT Phantoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.1 Mayo Clinic: Measured f H16 Conversion Factors in Tissue-Equivalent Head Phantoms . . . . . . . . . . . . 133.2 UCLA: Simulated f H16 Conversion Factors in Voxelized Patient Models . . . . . . . . . . . . . . . . . . . . . . . . 133.3 UC–Davis Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.3.1 Monte Carlo Validation using St. Jude Children’s Research Hospital 298 Measured Data . . . . 143.3.2 Monte Carlo f H16 Conversion Factors for Tissue-Equivalent Head Phantoms . . . . . . . . . . . . . 14

3.4 Comprehensive Assessment of SSDE for Head CT Examinations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4. Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165. Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197. Report Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Appendix 1: Tube-Potential-Dependent Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Appendix 2: UCD Monte Carlo Estimation of Dose to Bone Marrow in the Calvarium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23


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1. Introduction

The CT dose index value reported on a CT scanner during a patient exam (i.e., CTDIvol) does not rep-resent the absorbed dose to the patient on which the scan was performed.1 To address the desire by themedical community to estimate actual patient dose, AAPM Report 204, published in 2011, describedmethods by which the CTDIvol could be multiplied by a patient-size-specific conversion factor to esti-mate the absorbed dose to a patient of a given size, a quantity referred to as the Size-Specific DoseEstimate (SSDE).2 Since the publication of AAPM Report 204, the SSDE concept and the associatedsize-specific conversion factors have been validated in a number of phantom, cadaver, and MonteCarlo studies.3–6 However, wide-scale clinical adoption of SSDE remains dependent on manufacturerimplementation of methods to automatically calculate and report the SSDE, a task that will be greatlyfacilitated 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 developedspecifically for CT imaging of the abdomen and pelvis, although use for CT scans of the thorax wasdeemed acceptable since the errors were anticipated to be below 20%. AAPM Report 204 used effec-tive diameter, Deff, which is defined as the diameter of a circle of equivalent area to the patient’s cross-sectional area at a given position along z, to describe the size of the patient. Methods for the estima-tion of the water-equivalent diameter, Dw, were subsequently standardized and are described in detailin AAPM Report 2207—where Dw takes into consideration tissue attenuation, as assessed by the CTscanning process, to more accurately estimate SSDE in body regions that contain a range of tissuedensities and compositions, such as the thorax. The current document—Report of AAPM TG-293—isan extension of AAPM Report 204 and focuses on the development of SSDE conversion factorsappropriate for CT examinations of the head.

As in AAPM Report 204, SSDE represents the absorbed dose to the center section along thez-axis of a typical clinical CT scan, although in this report, the CT examination under consideration isfor the head. The overall length (in z) of such a scan increases somewhat with increasing age becauseof the increasing size of the head with increased age, although the effect is small as the size of theskull grows very little beyond about seven years of age. The absorbed dose in this thin center sectionof 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 (alongz) 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 CTexaminations of the head as a function of water-equivalent diameter (Dw)—is a result of independentdeterminations of SSDE by four groups of investigators: two sets of physical measurements madeusing physical phantoms and ionization chambers and two sets of data determined from Monte Carlosimulations. 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 simu-lation results performed at the University of California–Davis (UCD). The variable f H16 is used to rep-resent the CTDIvol,16-to-SSDE conversion factors for CT examinations of the head, where H indicatesthat these conversion factors are for the head and 16 indicates that the conversion factors are for usewith CTDIvol,16 values, which are acquired with a 16-cm CTDI phantom.

2. MethodsMonte Carlo estimations in virtual head CT phantoms (at UCD) and in voxelized patient models (atUCLA) were combined with physical measurements of air kerma (where air kerma absorbed dose toair at these x-ray energies) in head CT phantoms (at SJCRH and Mayo Clinic, Rochester) to develop


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the f H16 CTDIvol,16-to-SSDE conversion factors as a function of Dw. The following sections describethe methods used by each group.

2.1 St. Jude Children’s Research Hospital (Memphis, Tennessee): Physical Measurements in Tissue-Equivalent Head PhantomsValidation data for the Monte Carlo simulations performed at UCD by Boone and Hernandez weremeasured at SJCRH by Artz, Brady, and Strauss. Two CT systems from the same manufacturer wereused for both the physical measurements and the simulations: Volume CT (VCT) and Revolution CT(RCT) scanners, GE Healthcare, Waukesha WI. Data were acquired using a standard 16-cm-diameterpolymethyl methacrylate (PMMA) CTDI head phantom (Figure 1), and four tissue-equivalent headphantoms of different sizes (model numbers 007TE-21, 007TE-22, 007TE-23, and 007TE-27, CIRSInc., Norfolk, VA), as shown in Figure 2.

The tissue-equivalent phantoms were 15 cm long, approximately elliptical in shape, and designedto mimic age-specific head size, shape, and tissue/bone composition of a newborn, 1-year-old, 5-year-old, and medium-sized adult. Phantom characteristics are summarized in Table 1. These were the very

Figure 2. Validation data acquired using tissue-equivalent head phantoms, which mimic age-specific head size andcomposition.

Figure 1. Validation data acquired using a head CTDI phantom. CTDIvol,16 measurements were made using a cylin-drical 16-cm-diameter, 15-cm-long PMMA CTDI phantom. This phantom also has a 10 cm inner diameter insert.Data were collected with the phantoms placed in the head holder and on the patient table.


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same phantoms used at Mayo (described in section 2.2); they were shipped from Minnesota to Mem-phis for these studies. Each tissue-equivalent phantom has cylindrical holes along the entire length ofthe phantom to allow insertion of the standard CTDI ionization chamber in a manner similar to theCTDI phantom. The holes were located at the phantom center and at the 3, 6, 9, and 12 o’clock periph-eral positions, which were 1.3 cm in diameter and centered 1.0 cm from the edge of the phantom. Allholes were filled with tissue-equivalent rods when not occupied by an ionization chamber.

Both CT scanners were operated at 200 mAs, and a single axial scan was performed at 80, 100,120, and 140 kV. A 10-cm-long CTDI ionization chamber (Fluke 500–100, Cleveland, OH) and elec-trometer (Keithley 35050A, Cleveland, OH) were used with the 16-cm CTDI phantom to acquireCTDI100 measurements at the center, 3, 6, and 12 o’clock positions. On the VCT, CTDIvol,16 was deter-mined for a nominal beam collimation of 2.0 cm. CTDI100-like measurements were also made usingall four tissue-equivalent phantoms with a 2.0 cm nominal beam collimation. On the RCT, CTDIvol,16values were determined for nominal beam collimation values of 0.5, 12, and 16 cm. CTDI-like mea-surements were also made for all four tissue-equivalent phantoms with a 0.5 cm nominal beam colli-mation. Measurements in the CTDI phantom were made with the phantom placed both in thescanner’s head holder and directly on the patient table. For the CTDI-like measurements made in thetissue-equivalent phantoms, the three phantoms simulating pediatric head sizes were positioneddirectly on the patient table, reflecting the clinical scenario for young children, while the adult headphantom was placed in the scanner’s head holder.

The weighted dose-to-air (2/3 peripheral dose-to-air + 1/3 center dose-to-air) was converted todose to brain tissue by multiplying the weighted dose-to-air by the ratio of the mass energy-absorptioncoefficient of brain tissue to that of air, 1.08, which varies by only a few percent for the effective ener-gies of the tube potential values evaluated in this study, which ranged from 80 to 140 kV. The dose-to-brain data measured at SJCRH in the tissue-equivalent phantoms were used only for validating theMonte Carlo simulations performed at UCD (section 2.4), because the single-rotation CT acquisitiondid not cover the entire 15-cm length of the phantom and was thus not representative of the absorbeddose for a complete head CT examination.

To build the Monte Carlo model for the VCT and RCT scanners for use in Section 2.4, half valuelayer (HVL) and free-in-air (i.e., with no phantom) air kerma measurements were made at the scannerisocenter. HVL measurements were made using a solid-state multi-sensor detector (AGMS-D+, Rad-cal, Monrovia CA), and air kerma measurements were made with the 10-cm-long CTDI ionizationchamber. For both scanners, the x-ray tube was stationary at the 12 o’clock position (i.e., the top of the

AP = anterior-posterior dimension, LAT = lateral dimensionThe brain tissue is made from a solid plastic designed to emulate the linear attenuation and scattering properties of water to within 1% at diagnostic energies.

Its density was 1.029 g/cc for all age groups

Table 1. Tissue-equivalent head phantom specifications. These phantoms simulate solid brain tissue witha bounding layer of simulated bone. As the phantoms “age” from newborn to medium adult, they

get larger and have a thicker bone layer of higher density, as described in the table. All unitsare in centimeters unless noted otherwise. Dw values were calculated by Mayo. Reported

values are the average and coefficient of variation (CV) for tube potential valuesof 70, 90, 110, 120, 130, and 150 kV.

Phantom AP LAT Dw (CV) Cranium Thickness

Cranium Density (g/cc)

Newborn 12 9.5 11.1 (1.5%) 0.25 1.41

1-year-old 16 12.0 14.9 (1.4%) 0.30 1.45

5-year-old 17 13.5 16.3 (1.4%) 0.35 1.52

Medium Adult 19 14.5 18.1 (0.9%) 0.50 1.60


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gantry) for the irradiation. The multi sensor and ionization chamber were separately exposed with anominal beam collimation of 0.5 cm at 40 mAs for each of four tube potential settings (80, 100, 120,and 140 kV).

2.2 Mayo Clinic: Physical Measurements in Tissue-Equivalent Head Phantoms The very same tissue-equivalent head phantoms used at SJCRH (Table 1) were used for physical mea-surements at the Mayo Clinic. The head phantom of interest (e.g., 5-year-old) was placed on thepatient table in a “head first” orientation, and the corresponding (e.g., also 5-year-old) tissue-equiva-lent chest phantom from the same manufacturer was placed inferiorly to the head phantom to providescattering media below the head (Figure 3). At the top of the head, one of the other head phantomswas placed vertically, adjacent to the top of the head phantom, to provide a more realistic scatteringmedium (i.e., bone) at the top of the skull. The 1-year-old head phantom was used for this purposewith the newborn head phantom and the 5-year-old head phantom. The newborn head phantom wasused as the top-of-the-head surrogate for the 1-year-old head phantom, and the 5-year-old head phan-tom was used as the top-of-the-skull surrogate for the adult head phantom.

All Mayo data were acquired by McCollough using a third-generation dual-source CT scanner(SOMATOM Force, Siemens Healthcare, Germany). Data were acquired using a 192 0.6 mm(115.2 mm) nominal beam collimation and the routine head CT scan protocol (1 s rotation time) fortube potential values between 70 and 150 kV, in steps of 10 kV. Automatic exposure control (AEC)was turned on (Care Dose 4D, Siemens Healthcare) to determine the CTDIvol,16 for each phantom sizewhen the Quality Reference mAs was set to 220 mAs. (The Quality Reference mAs is the automaticexposure control (AEC) parameter used on Siemens CT scanners to establish the desired clinicalimage quality.) AEC was then turned off, and the effective mAs was adjusted to obtain the size-adapted CTDIvol,16 determined with use of the AEC system. This approach produced a “right-sized”CTDIvol,16 for each phantom size, but did not require the use of tube current modulation, which is nottypically used for head CT scanning at the Mayo Clinic. The scanner-reported CTDIvol,16 was recordedfor each scan, which was confirmed to be accurate compared to the values measured during qualitycontrol testing. Absorbed doses to the center (along z) of the phantoms at the center, 12, and 3 o’clockpositions were acquired using a 0.6-cc ionization chamber (10 5-3CT, Radcal, Monrovia, CA) and

Figure 3. Experimental setup for CIRS head CT phantom measurements at Mayo Clinic.


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electrometer (9015, Radcal, Monrovia, CA) using a helical acquisition. That is, with the ionizationchamber centered longitudinally in the phantom, a helical scan was performed as if the phantom werean actual patient, and the accumulated dose at the location of the ionization chamber was measured.Pitch values were adjusted slightly (from a nominal value of 1.0, range 0.94 to 1.02) to reduce dosevariations at the periphery, which was verified using self-developing Gafchromic film (XR-CT2, Ash-land Advanced Materials, Bridgewater, NJ). It is important to note that the peripheral holes where theionization chamber was placed were within the solid water portion of the phantom; the holes do notextend into the thin bone-mimicking shell on the exterior of the phantom. Thus, the measuredabsorbed dose values to air were made within solid water that was residing within a shell of bone (i.e.,brain parenchyma), and does not represent absorbed dose to the bone surrounding the brain (i.e., thecalvarium).

Cranial-caudal scan lengths were retrospectively measured in a cohort of 120 patients who hadreceived clinically indicated routine head CT exams, which are used to image primarily the brain andthus irradiate only cranial anatomy. The cranial-caudal scan length was calculated, and average valueswere computed for newborns, 1-year-olds, 5-year-olds, and adults (13, 14, 16, and 16 cm, respec-tively). For each phantom size, tube potential setting, and chamber position (center, 12, and3 o’clock), three scans were acquired from the bone surrogate at the top of the head (the vertically ori-ented head phantom); these scans extended 13, 14, or 16 cm inferiorly based on the phantom beingscanned, and three cumulative absorbed dose-to-air values were recorded and averaged. The 12 and3 o’clock data were averaged to estimate dose to the periphery.

The average weighted absorbed dose-to-air (2/3 peripheral dose-to-air + 1/3 center dose-to-air) was converted to absorbed dose to brain by multiplying the weighted absorbed dose-to-air valueby the ratio of the mass energy-absorption coefficient of brain to that of air, 1.08, which varies by onlya few percent for the effective energies of the tube potential values evaluated in this study, whichranged from 70 to 150 kV. These values represented the weighted dose to brain at the center section(along z) of a clinically realistic scan. Dw was calculated for each phantom size at each tube potentialsetting using axial images of the phantoms and the formula given in AAPM Report 220.7 A CV of 1%to 1.5% was found to exist across the four phantom sizes (Table 1), indicating that the energy depen-dence of Dw is very small for these phantoms. The absorbed dose values measured in the CIRS phan-toms were divided by the reported CTDIvol,16 values to calculate the CTDIvol,16-to-SSDE conversionfactors (f H16) for each CIRS phantom, as reported below.

2.3 University of California–Los Angeles (UCLA): Monte Carlo Estimations in Voxelized Head Models

At UCLA, Hardy and McNitt-Gray used Monte Carlo simulation techniques to determine the averageabsorbed dose to brain at the center (along z) of a scan volume corresponding to routine head CTscans.10 Simulations were performed with an equivalent source model11 of a 64-slice CT system(SOMATOM Sensation 64, Siemens Healthcare) using scan acquisition parameters taken from theAAPM’s Routine Head CT protocol recommendations for Siemens scanners.12 The voxelized headmodels used included 10 voxelized head models—eight from the GSF family of patient models13 andtwo reference ICRP patient models14—which together represented a range of patient sizes. The GSFfamily of patient models includes two pediatric patients, one of which is seven weeks old and one ofwhich is five years old (Figure 4). To increase the number of voxelized pediatric models, five addi-tional voxelized patient models were retrospectively generated from previously obtained routine headCT scans15 of pediatric patients ranging in age from seven weeks to two years. An axial CT image of a23-month-old pediatric head CT scan is shown in Figure 5 (top) along with the voxelized representa-tion used in the Monte Carlo simulations (bottom). Simulated helical head scans were performed onall 15 voxelized head models using scan lengths that included all the cranial anatomy for each model.


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The CT dose simulations were performed with a previously described method16 built on a modi-fied version of MCNPX (Monte Carlo N-Particle eXtended version 2.7.a) radiation transport code.17–19

Voxelized dose distributions of the entire head of each voxelized patient model were produced usingmesh tally configurations within MCPNX. For each patient model, absorbed dose to the brain paren-chyma was calculated from the mesh tallies. Using these methods, Hardy et al. further report absorbeddose to cortical bone and a mass-weighted absorbed dose of both the brain and cortical bone compart-ments.10

All estimates of absorbed dose to brain tissue were divided by the corresponding CTDIvol,16 valuesto compute the f H16 CTDIvol,16-to-SSDE conversion factors, which mimics the Mayo measurements.Dw was used as the size metric for these models, and all diameter estimates were made at the center ofthe scan volume (along z). For the GSF and ICRP phantom models, Dw estimates were obtained indi-rectly from a correlation between effective diameter and Dw.20 For the remaining five phantom modelsproduced from image data, Dw was calculated directly from the CT image at the center slice.

2.4 UC Davis: Monte Carlo Estimation in Virtual Head CT PhantomsThe UCD group used a validated Monte Carlo transport code (MCNP6)21 to estimate absorbed dose atthe center (along z) of the scan volume from routine head CT exams using a virtual version of the tis-sue-equivalent head phantoms used by SJCRH and Mayo (Table 1). The VCT and RCT scanners usedfor the SJCRH measurements were modeled in the UCD simulations using measured bowtie filter pro-files22, the half value layers measured at SJCRH, the nominal beam collimation values used for theSJCRH measurements, and geometrical specifications were obtained from scanner manufacturer’stechnical manuals. The Monte Carlo models of these scanners were validated against the physicaldose measurements performed at SJCRH using the tissue-equivalent phantoms (section 2.1.).

Figure 4. The entire family of GSF voxelized patientmodels, which were developed using CT scans ofcadavers.13,14

Figure 5. Top: pediatric head CT axialimage for a routine CT head exam. Bottom:voxelized Monte Carlo representation ofthe patient produced using a CT number-based look-up table.


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The tissue-equivalent head CT phantoms were modeled as elliptical cylinders using the dimen-sions and material densities given in Table 1. For the purpose of validating the Monte Carlo simula-tions, the air kerma delivered in a single rotation of the x-ray source was simulated in a 10-cm-longCTDI ionization chamber at the central and four peripheral holes of the phantom for 80, 100, 120, and140 kV—analogous to the physical measurements performed at SJCRH (see section 2.1.). Theweighted air kerma estimation (2/3 peripheral dose-to-air + 1/3 center dose-to-air) was calculatedfor each tube potential (at these x-ray energies air kerma absorbed dose to air) and converted toaverage absorbed dose to brain by multiplying the average absorbed dose to air by 1.08 (as was donein sections 2.1 and 2.2). The resultant value was then divided by the simulated CTDIvol,16 and com-pared against the SJCRH measurements presented in section 2.1, solely for the purpose of MonteCarlo validation. It is important to note that the air kerma was measured using the 10-cm-long CTDIionization chamber, a range of nominal beam collimations, and a single rotation of the source; they donot represent the average absorbed dose at the center (along z) of the scan volume. Thus, the normal-ized dose data acquired at SJCRH were not directly used to estimate f H16.

After validating the Monte Carlo simulation methods, absorbed dose to brain in the virtual tissue-equivalent head phantoms at the center (along z) of the scan volume was estimated for head CT examson the VCT and RCT scanners. For the RCT scanner, a single axial rotation with nominal beam colli-mation values of 12, 14, 14, and 16 cm was used to simulate clinical CT acquisitions for the newborn,1-year-old, 5-year-old, and adult tissue-equivalent head phantoms, respectively. These scan lengthswere used to simulate the scan range (along z) of real patients and are very similar to those used atMayo. The VCT head CT protocol was simulated as a helical acquisition with a pitch of 0.516 and20 mm nominal beam collimation. The same phantom-dependent scan lengths were used in the RCTsimulations to allow direct comparisons. In order to mimic continuous scatter media encountered in aclinical CT examination, a 10 cm extrusion of soft tissue was added to the inferior side of the head(corresponding to the neck) by matching the age-specific phantom sizes. A bone layer was added tothe superior side of each phantom as a surrogate for the top of the calvarium—again emulating thephysical measurements made at Mayo.

Average absorbed dose-to-brain for a 0.5 cm thick slab of solid water at the center (along z) of thescan volume was estimated in all simulations by tallying the energy fluence within the solid water por-tion of the central slab, thereby excluding the thin bone-mimicking cylinder around the periphery ofthe phantom. The tallied energy fluence was multiplied by the mass energy absorption coefficient ofwater, and the resultant value, which excluded dose to bone, was used to estimate the absorbed dose tothe brain. Appendix 2 does provide data to assess the absorbed dose to bone marrow. This averageabsorbed dose to brain was divided by the simulated CTDIvol,16cm to calculate the CTDIvol,16cm -to-SSDE conversation factors (f H16) for each phantom size. The use of a 0.5-cm-thick slab at the center(along z) of the simulated CT scan is consistent with the Mayo data, where absorbed dose was mea-sured using a short ionization chamber at the center of the scan volume. Dw was computed numeri-cally using the known dimensions and composition of the tissue-equivalent head phantoms (Table 1)and x-ray spectra matching those of the VCT and RCT. X-ray spectra were modeled using TAS-MICS23 by matching the measured HVLs and known x-ray tube potentials. The modeled spectra weremathematically projected (as monoenergetic x-ray beams) through the phantoms to estimate the effec-tive linear attenuation coefficient (µeff) for the phantom materials (solid water and bone surrogate) ateach x-ray tube potential. The CT number (in HU) for each material was determined and used to com-pute Dw for each phantom size and x-ray tube potential using the formulae given in AAPM Report220.7


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3. Results3.1 Mayo Clinic: Measured f H16Conversion Factors in Tissue-Equivalent Head phantomsPhysical measurements from Mayo of f H16 in the tissue-equivalent head phantoms are shown in Figure6. The f H16 values (and coefficient of variation, CV) were 1.19 (2.2%), 1.00 (5.6%), 0.95 (6.3%), and0.87 (8.8%) for the newborn, 1-year-old, 5-year-old, and adult head phantoms, respectively. Dw valuesare reported in Table 1 and ranged from 11.1 cm for the newborn phantom to 18.1 cm for the adultphantom. The larger CV in f H16 for the adult phantom is the result of significantly more beam harden-ing in the larger phantom across the range of tube potentials evaluated (70 to 150 kV).

3.2 UCLA: Simulated f H16 Conversion Factors in Voxelized Patient ModelsMonte Carlo estimations of f H16 performed at UCLA are shown in Figure 7. Resulting values rangefrom 1.18 for a 7-week-old pediatric patient to 0.76 for the GSF 48-year-old male “Frank” patientmodel. Dw values ranged from 10.6 cm for the 7-week-old to 20.2 cm for the “Rex” ICRP referenceman model. The results are in close agreement to the Mayo measured data.

3.3 UC Davis ResultsThe UCD results are separated into two sections: Monte Carlo validation against SJCRH measuredresults (section 3.3.1) and Monte Carlo computation of CTDIvol,16-to-SSDE conversion factors (f H16)

Figure 7. CTDIvol,16-to-SSDE conversion factors (f H16)derived from Monte Carlo simulations performed atUCLA using GSF, ICRP, and five pediatric voxelizedpatient models and simulated on a Siemens Sensation64 CT scanner. The SSDE was estimated as the cumula-tive dose deposited to a 0.5 cm central slab of the brainparenchyma within the voxelized models for a clinicalhead CT examination. The data shown demonstrateindividual Monte Carlo measurements on each of the15 phantoms used in this study.

Figure 6. CTDIvol,16-to-SSDE conversion factors (f H16)measured at the Mayo Clinic in the newborn, 1-year-old, 5-year-old, and adult tissue-equivalent phantoms ona Siemens FORCE CT Scanner. The error bars repre-sent the variation due to different tube potentials (70 to150 kV). The horizontal error bars reflect the variationin the calculation of water-equivalent diameter (Dw) asa function of tube potential.


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for virtual phantoms emulating the tissue-equivalent physical phantoms used at SJCRH and Mayo(section 3.3.2).

3.3.1 Monte Carlo Validation using St. Jude Children’s Research Hospital Measured DataQuantitative agreement between the absorbed dose simulated by UCD and measured by SJCRH, for asingle rotation of the x-ray tube, is shown in Figure 8. The mean absolute difference was used to com-pare these values across phantom sizes (newborn, 1-year-old, 5-year-old, and adult) and across a rangeof tube potentials (80, 100, 120, and 140 kV). The mean (range) absolute difference was 1.45% (0.05–4.33) and 0.64% (0.01–2.86) for the RCT and VCT scanners, respectively. These results indicate goodcorrespondence between brain CT dose values obtained through simulations in MCNP6 (UCD) andthose physically measured in tissue-equivalent head phantoms (SJCRH).

3.3.2 Monte Carlo f H16 Conversion Factors for Tissue-Equivalent Head PhantomsCTDIvol,16-to-SSDE conversion factors (f H16) for the solid-water portion of the tissue-equivalent headphantoms used by SJCRH and Mayo are shown in Figure 9. The f H16 factors (CV) were 1.14 (3.4%),0.99 (6.1%), 0.91 (7.6%), and 0.80 (10.4%) for the newborn, 1-year-old, 5-year-old, and adult headphantoms, respectively, for the VCT scanner. The f H16 factors were 1.08 (3.3%), 0.97 (5.8%), 0.90(7.2%), and 0.80 (9.9%) for the RCT scanner. Consistent with the results observed by the MayoClinic, the larger variation in f H16 values for the adult phantom (18 cm Dw) is the result of significantlymore beam hardening in the larger phantom size across the range of tube potentials (80 to 140 kV)used in these simulations. Dose to bone marrow (BM) was simulated in a comprehensive manner,however the results demonstrated that dose to BM had very little effect (<1%) due to its small massrelative to the soft tissue mass. Nevertheless, the BM results are included in the fit parameters shownin Figure 10 and in Equation 2. Because of the very small impact of including dose to BM, the com-plete methods and results for BM dose computation are described in Appendix 2.

Figure 8. UCD Monte Carlo validation of integrated dose values against the physical measurements acquired atSJCRH in the newborn, 1-year-old, 5-year-old, and adult head phantoms. The results shown are for tube potentialsof 80, 100, 120, and 140 kV on the GE VCT and GE Revolution scanners. Nominal beam collimation values of 2.0and 0.5 cm were used for the VCT and Revolution scanner validations, respectively.


15

3.4 Comprehensive Assessment of SSDE for Head CT ExaminationsA plot of all f H16 conversion factors versus Dw is shown in Figure 10, along with the associated expo-nential fit. The large variation in conversion factors for similar Dw values is primarily a result of alarge variation in tube potentials investigated by both the Mayo Clinic and UCD.

Figure 9. CTDIvol,16-to-SSDE conversion factors (f H16)estimated with Monte Carlo simulations at UCD for thenewborn, 1-year-old, 5-year-old, and adult head phan-toms for the GE Revolution and GE VCT CT scanners.The brain dose was estimated as the dose deposited to a0.5 cm central slab of the solid water portion of the tis-sue-equivalent phantoms. Error bars correspond toone standard deviation of the average values acrosstube potentials ranging from 80 to 140 kV.

Figure 10. CTDIvol,16-to-SSDEconversion factors (f H16) from allgroups as a function of water-equivalent diameter (Dw). Themultiple data points for eachphantom size for the UCDresults correspond to tubepotentials of 80, 100, 120, and140 kV. Dose to bone marrow(BM) results are included in thisfit (see Appendix 2). The multipledata points for each phantom sizefor the Mayo results correspondto tube potentials of 90, 110, 120,and 130 kV (70 and 150 kV wereexcluded). The exponential fit ofall data is shown along with the fitcoefficients and coefficient ofdetermination. Dw values (fromTable 1) corresponding to thenewborn, 1- year-old, 5-year-old,and adult tissue-equivalent CIRShead phantoms are also shownfor reference PT, patient; BM,bone marrow.


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Figure 11 shows the best fit curves of f H16 vs. Dw for (a) all data combined across the 80, 100, 120,and 140 kV tube potential settings and (b) data at individual tube potential values. The use of datafrom the 100 or 120 kV fits results in less than a 5% error when compared against the curve for thecombined tube potential data for water-equivalent diameters of 12, 14, and 16 cm. An overestimationof SSDE of 3.8%, 6.7%, and 9.7% was observed when using the combined data f H16 fit (see Figure10) compared to the individual tube potential f H16 fit at 80 kV for Dw values of 12, 14, and 16 cm.These differences are all below the 20% limits of uncertainty in SSDE anticipated by AAPM Report204. Differences for the adult head (18 cm Dw) at 80 kV are not reported as 80 kV is unlikely to beused for an adult head CT examination due to increased beam hardening from the calvarium. Anunderestimation of 3.2%, 4.5%, 5.7%, and 7.0% was observed when using the combined data f H16 fitcompared to the individual tube potential f H16 fit at 140 kV for Dw values of 12, 14, 16, and 18 cm.However, it is extremely unlikely that such a high tube potential setting would be used in children,especially for imaging of the brain, because of the poorer gray-white matter differentiation at highertube potential settings. These results indicate that use of a single fit across all tube potentials results inless than a maximum error of 5% for tube potentials of 100 and 120 kV and a maximum error of 9.2%at 80 kV, assuming that an 80 kV tube potential would only be used for children less than about threeyears old, after which the calcification of the cranium becomes very close to that of an adult.24

4. Discussion

The f H16 conversion factors shown in Figure 10 represent the combination of data measured by fourdifferent groups using four different CT scanner models, two from each of two manufacturers. Physi-cal measurements were performed at Mayo and SJCRH, one set of which was used to validate theMonte Carlo data produced at UCD. All three of these sites used the same tissue-equivalent headphantoms, either in physical or virtual form. Monte Carlo data were also produced at UCLA usingvoxelized head models obtained from CT scans of human heads. By combining measurements from

Figure 11. Exponential fit results separated by tube potential (80, 100, 120, and 140 kV) and the single best fitcurve using all data.


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these different groups, both physical and those based on Monte Carlo, a consensus on the head SSDEconversion factors emerges.

While it is tempting to adopt the x-ray tube-potential-dependent conversion factors as shown inFigure A1 (Appendix 1), where the correlation coefficients range from 0.91 to 0.95, the additionalcomplexity of including tube potential dependency reduces the simplicity of the head SSDE metric.Furthermore, at a given tube potential, the conversion factors determined here may differ somewhatthan for what might be measured on other CT systems because x-ray beam quality is dependent uponbeam filter shape, thickness, and composition, which vary by CT manufacturer and model. Hence, therecommendation of this task group is to make use of the simpler combined fit, as shown in Figure 10,despite the slightly lower correlation coefficient (R2 = 0.83). The mathematical fit to the curve shownin Figure 10 is:

where: = 1.9852 [absorbed dose to tissue (mGy) / CTDIvol,16 (mGy)] = 0.0486 (cm–1)Dw = water-equivalent diameter (in cm)The issue of absorbed dose to bone versus absorbed dose to brain was discussed at length during

task group deliberations. Because of the importance of the blood-forming tissues in the calvarium ofthe pediatric head, the decision was made to proceed with an evaluation of absorbed dose to bone mar-row separate from that of absorbed dose to brain. UCD performed a detailed assessment of absorbeddose to the red and yellow bone marrow located in the calvarium’s spongiosa (Appendix 2). Thesedata demonstrated that while the absorbed dose to spongiosa differs from the average absorbed dose tobrain, the tissue mass of the blood-forming elements in the bone was so small that the mass-weightedaverage absorbed dose summed over the brain and marrow compartments—which can be consideredthe average biologically relevant absorbed dose at the center of the scan volume (along z)—waswithin 1% of the average absorbed dose to the brain alone. Despite this negligible difference, thisexercise was illustrative and the results were included in the formulation of the combined fit (Figure10 and Eq 1). Appendix 2 also provides information for the estimation of the absorbed dose to thebone marrow of the calvarium.

As discussed in section 1, even for the same Dw, the spatial distribution of absorbed dose deposi-tion to the brain is considerably different than the absorbed dose deposition to the body, primarily dueto the fact that the head is encased in a shell of relatively dense bone. Comparing the f 16 conversionfactors between the body (AAPM Report 204, f B16) and those of the head (this report, f H16), it is seenin Figure 12 that while the shapes of the curves are quite similar, the SSDE conversion factors forhead CT are consistently lower, most likely due to the attenuation of the skull and the more aggressivebeam shaping (i.e., bow tie) filters used in head CT scan protocols. These results are consistent withthe physics of radiation dose deposition in the rotational geometry of CT. Assessment of the twocurves shown in Figure 12 suggests that these differences lead to a lower average absorbed dose tobrain of about 9% for the same applied CTDIvol,16.

The dose to the center section (along z) of the scan volume represents a maximum absorbed doseto tissues within the scan volume under most conditions, due to the decreased amount of scatteredradiation included at the cranial and caudal ends of the scan volume (due to the absence of scatteredradiation emanating from these regions due to the lack of primary irradiation of tissues outside of thescan volume). This assumes that x-ray tube current modulation is either not used in routine head CT,which appears to be a very common situation, or is not significant in terms of the overall averageabsorbed dose, as has been shown in the body when regional values of CTDIvol are used to calculateSSDE.25

(Eq. 1)f e DWH16


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5. Nomenclature

CTDIvol measurements for the head, for both adults and children, do not use the 32-cm PMMA phan-toms. Rather, international standards (IEC 60601-2-44 Edition 3.2, 2016) require use of the 16-cm-diameter PMMA CTDI phantom. The concept of the size-specific dose estimate (SSDE) was intro-duced in AAPM Report 204, “Size Specific Dose Estimates in Pediatric and Adult Body CT Examina-tions,” however in terms of nomenclature, that document did not anticipate the existence of headSSDE conversion factors. In addition, virtually all calculations of SSDE are made using automatedsoftware to determine Dw, making the “X” in the f 32X nomenclature introduced in AAPM Report 204(page 18) obsolete, as does the IEC standard on determination of Dw and SSDE. This “X” referred tothe metric used to reflect patient size in AAPM Report 204, namely the sum of the lateral and anterior-posterior linear dimensions of the torso, the lateral dimension alone, the anterior-posterior dimensionalone, or the effective diameter. It is the recommendation of this report that the nomenclature shownbelow replace the nomenclature recommended in AAPM Report 204:

For Body CT (AAPM Report 204), the “B32” nomenclature should be used in the superscript ofthe conversion factor f when the 32-cm PMMA CTDI phantom was used for the body CTDIvol mea-surement, and 32 added to the subscript of CTDIvol :

For Body CT (AAPM Report 204), the “B16” nomenclature should be used in the superscript of theconversion factor f when the 16-cm PMMA CTDI phantom was used for the body CTDIvol measure-ment (typically for pediatric body metrics), and 16 added to the subscript of CTDIvol:

(Eq. 2a)SSDE f CTDIvol B3232,

(Eq. 2b)SSDE f CTDIvol B16,16

Figure 12. The f B16 conversion factors for the body from AAPM Report 204 and f H16 conversion factors for thehead derived in this report, plotted over the same Dw range. The head conversion factors, and hence the SSDE val-ues for the same value of CTDIvol,16, are lower by about 8.7% compared to the corresponding values for the body.


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For Head CT (TG-293), the “H16” nomenclature should be used in the superscript of the conversionfactor f when the 16-cm PMMA CTDI phantom was used for the Head CTDIvol measurement, and 16added to the subscript of CTDIvol:

6. Summary

This report provides CTDIvol-to-SSDE conversion factors for the calculation of SSDE for head CTexaminations. The reported f H16 conversion factors represent a fit derived from data provided by fourdifferent groups—two groups that made physical measurements (albeit one group’s data were usedonly for validation of the UCD Monte Carlo simulations and did not directly measure f H16), and twogroups that made dose calculations based upon Monte Carlo modeling. While a separate determina-tion of the absorbed dose to bone marrow was made, the absorbed dose to bone marrow was found tobe negligible relative to the average absorbed dose to the entire brain. When compared to the f B16 val-ues reported for body CT in AAPM Report 204, the head SSDE conversion coefficients f H16 weresomewhat smaller (by about 9%), which is physically consistent with the attenuation of the skull thatreduces dose to the brain. Readers are reminded that the computation of head SSDE conversion fac-tors specifically calculate the average absorbed dose to brain for the central section (along z) of thescanned volume for routine head CT examinations. While data were included for multiple tube poten-tial values, it is recommended that only the equation derived from the combined data at all tube poten-tial values, as described in Figure 10 (Equation 1), be used for clinical head SSDE calculations; datafor individual tube potentials, which are provided in Appendix 1 (Figure A1), should be used forresearch purposes only. Furthermore, as stated in AAPM Report 204, it is recommended that theSSDE not be used for the computation of effective dose using published K-factors and reported dose-length-product (DLP) values, as sufficient data are not yet available to justify an extension of theSSDE concept to the estimation of effective dose.

7. Report Recommendations

1. A single fit of the data, which uses data measured over a wide range of tube potentials, pro-vides a single set of CTDIvol-to-SSDE conversion factors for the head (f H16) for clinical use that is independent of tube potential setting and will simplify work flow, while keeping errors below ±20%. (Equation 1).

2. Replace the nomenclature recommended in AAPM Report 204 with the nomenclature described in section 5.

3. SSDE should not be used for the computation of effective dose using published K-factors and reported dose-length-product (DLP) values.

8. References

1. McCollough, C. H., S. Leng, L. Yu, D. D. Cody, J. M. Boone, and M. F. McNitt-Gray. (2011.) “CT dose index and patient dose: they are not the same thing.” Radiology 259(2):311–16.

2. Boone, J. M., K. J. Strauss, D. D. Cody, C. H. McCollough, M. F. McNitt-Gray, and T. L. Toth. “Size-Specific Dose Estimates (SSDE) in Pediatric and Adult Body CT Examinations: The Report of AAPM Task Group 204.” AAPM, 2011.

(Eq. 2c)SSDE f CTDIvol H16,16


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3. Brady, S. L. and R. A. Kaufman. (2012.) “Investigation of American Association of Physicists in Medicine Report 204 Size-specific Dose Estimates for Pediatric CT Implementation.” Radiology 265(3):832–40.

4. Zhang, D., A. Padole, X. Li, et al. (2014.) “In vitro dose measurements in a human cadaver with abdomen/pelvis CT scans.” Med Phys. 41(9):091911.

5. Sinclair, L., T. M. Griglock, A. Mench, et al. (2015.) “Determining Organ Doses from CT with Direct Measurements in Postmortem Subjects: Part 2-Correlations with Patient-specific Parame-ters.” Radiology 277(2):471–76.

6. Franck, C., C. Vandevoorde, I. Goethals, et al. (2016.) “The role of Size-Specific Dose Estimate (SSDE) in patient-specific organ dose and cancer risk estimation in paediatric chest and abdom-inopelvic CT examinations.” Eur. Radiol. 26(8):2646–55.

7. McCollough, C., D. M. Bakalyar, M. Bostani, et al. “Use of Water Equivalent Diameter for Cal-culating Patient Size and Size-Specific Dose Estimates (SSDE) in CT: The Report of AAPM Task Group 220.” AAPM, 2014.

8. Boone, J. M. (2007.) “The trouble with CTD100.” Med. Phys. 34(4):1364–71. 9. Dixon, R. L, J. A. Anderson, D. M. Bakalyar, et al. “Comprehensive Methodology for the Eval-

uation of Radiation Dose in X-ray Computed Tomography: The Report of AAPM Task Group 111.” AAPM, 2010.

10. Hardy, A. J., M. Bostani, A. M. Hernandez, et al. (2019.) “Estimating a size-specific dose for helical head CT examinations using Monte Carlo simulation methods. Med. Phys. 46(2):902–12.

11. Turner, A. C., D. Zhang, H. J. Kim, et al. (2009.) “A method to generate equivalent energy spec-tra and filtration models based on measurement for multidetector CT Monte Carlo dosimetry simulations.” Med. Phys. 36(6):2154–64.

12. AAPM. Adult Routine Head CT Protocols Version 2.0. 2016. 13. Petoussi-Henss, N., M. Zanki, U. Fill, and D. Regulla. (2002.) “The GSF family of voxel phan-

toms.” Phys. Med. Biol. 47(1):89–106. 14. Zankl, M., K. F. Eckerman, and W. E. Bolch. (2007.) “Voxel-based models representing the male

and female ICRP reference adult—the skeleton.” Radiat. Prot. Dosimetry 127(1–4):174–86. 15. DeMarco, J. J., T. D. Solberg, and J. B. Smathers. (1998.) “A CT-based Monte Carlo simulation

tool for dosimetry planning and analysis.” Med. Phys. 25(1):1–11. 16. DeMarco, J. J., C. H. Cagnon, D. D. Cody, et al. (2005.) “A Monte Carlo based method to esti-

mate radiation dose from multidetector CT (MDCT): cylindrical and anthropomorphic phan-toms.” Phys. Med. Biol. 50(17):3989–4004.

17. Bostani, M., J. W. Mueller, K. McMillan, et al. (2015.) “Accuracy of Monte Carlo simulations compared to in-vivo MDCT dosimetry.” Med. Phys. 42(2):1080–86.

18. Sechopoulos, I., E. S. Ali, A. Badal, et al. (2015.) “Monte Carlo reference data sets for imaging research: Executive summary of the report of AAPM Research Committee Task Group 195.” Med. Phys. 42(10):5679–91.

19. Waters, L. S., G. W. McKinney, J. W. Durkee, et al. The MCNPX Monte Carlo radiation trans-port code. Paper presented at: AIP Conference Proceedings 2007.

20. McMillan, K., M. Bostani, C. Cagnon, M. Zankl, A. R. Sepahdari, and M. McNitt-Gray. (2014.) “Size-specific, scanner-independent organ dose estimates in contiguous axial and helical head CT examinations.” Med. Phys. 41(12):121909.

21. Pelowitz, D. MCNP6 User’s Manual. Los Alamos National Laboratory. LACP-00634. May, 2013.

22. Yang, K., X. Li, Xu X. George, and B. Liu. (2017.) “Direct and fast measurement of CT beam filter profiles with simultaneous geometrical calibration.” Med. Phys. 44(1):57–70.


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23. Hernandez, A. M. and J. M. Boone. (2014.) “Tungsten anode spectral model using interpolating cubic splines: unfiltered x-ray spectra from 20 kV to 640 kV.” Med. Phys. 41(4):042101.

24. Delye, H., T. Clijmans, M. Y. Mommaerts, J. V. Sloten, and J. Goffin. (2015.) “Creating a nor-mative database of age-specific 3D geometrical data, bone density, and bone thickness of the developing skull: a pilot study.” J. Neurosurg. Pediatr. 16(6):687–702.

25. Khatonabadi, M., H. J. Kim, P. Lu, et al. (2013.) “The feasibility of a regional CTDIvol to esti-mate organ dose from tube current modulated CT exams.” Med. Phys. 40(5):051903.

26. Kleinman, P. L., K. J. Strauss, D. Zurakowski, K. S. Buckley, and G. A. Taylor. (2010.) “Patient size measured on CT images as a function of age at a tertiary care Children’s Research Hospi-tal.” AJR Am. J. Roentgenol. 194(6):1611–19.

27. ICRP. (1995.) Publication 70: Basic anatomical and physiological data: The skeleton. Ann. ICRP 25(2):1–80.

28. Lillie, E. M., J. E. Urban, S. K. Lynch, A. A. Weaver, and J. D. Stitzel. (2016.) “Evaluation of Skull Cortical Thickness Changes With Age and Sex From Computed Tomography Scans.” J. Bone Miner Res. 31(2):299–307.

29. Menzel, H., C. Clement, and P. DeLuca. (2009.) “ICRP Publication 110. Realistic reference phantoms: an ICRP/ICRU joint effort. A report of adult reference computational phantoms.” Ann. ICRP. 39(2):1.

30. Johnson, P. B., A. A. Bahadori, K. F. Eckerman, C. Lee, and W. E. Bolch. (2011.) “Response functions for computing absorbed dose to skeletal tissues from photon irradiation—an update.” Phys. Med. Biol. 56(8):2347–65.

31. Zhang, J., Y. H. Na, P. F. Caracappa, and X. G. Xu. (2009.) “RPI-AM and RPI-AF, a pair of mesh-based, size-adjustable adult male and female computational phantoms using ICRP-89 parameters and their calculations for organ doses from monoenergetic photon beams.” Phys. Med. Biol. 54(19):5885–5908.


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Appendix 1. Tube-Potential-Dependent CurvesFigure A1 shows f H16 results separately for 80, 100, 120, and 140 kV. These results demonstrate avery good fit to an exponential function when grouped by tube potential. However, as was the case forAAPM Report 204, this task group sought to provide a single functional form for the CTDIvol-to-SSDE conversion factors as a function of Dw and felt that a single set of conversion factors would notresult in errors more than ±20%. This was achieved using Equation 1 of the main report (data shownin Figure 10).

Figure A1. CTDIvol,16-to-SSDE conversion factors (f H16) plotted separately for (A) 80 kV, (B) 100 kV, (C) 120 kV, and(D) 140 kV. The 80, 100, and 140 kV data points for the results from Mayo Clinic were interpolated from data pointsat 70, 90, 110, 130, and 150 kV. The exponential fit for each kV is shown along with the fit coefficients and coefficientof determination (R2).


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Appendix 2. UCD Monte Carlo Estimation of Dose to Bone Marrowin the Calvarium

A series of mathematical head phantoms were designed in silico to represent age-dependent differ-ences in the dimensions, composition, and marrow cellularity of the brain parenchyma and craniummicrostructure. These mathematical phantoms were modeled as 15-cm-long elliptical cylindersdesigned to represent 0-, 1-, 5-, and 21-year-old patients. Age-dependent anterior-posterior and lateralhead dimensions of the elliptical cylinders were interpolated from a data set of 336 head CT examina-tions of patients ranging in age from 0 to 21 years old.26 The age-dependent geometry and composi-tion of the cranium surrounding the brain parenchyma were interpolated from previously reportedmeasurements using bone surface modeling of 172 head CT examinations of patients ranging in agefrom 0 to 20 years old.24 Table A2.1 outlines the modeled mathematical head phantom dimensions.

The cranium is composed of inner and outer walls of cortical bone that enclose the spongiosa, asshown in the micro-CT image of the cross-section of a portion of the calvarium in Figure A2.1. Withinthe spongiosa microstructure is the trabecular bone, and marrow cavities containing both red (active)bone marrow (RBM) and yellow (inactive) bone marrow. Given the present lack of robust, age-depen-dent values for the volume fraction of spongiosa (SVF), the four mathematical phantoms weredesigned to have equivalent thicknesses for the inner cortical wall, the spongiosa, and the outer corti-cal wall, representing a 50% SVF for the cranium. Age-specific trabecular bone volume fractions(TBVF) within the spongiosa were interpolated from data published in ICRP 70.27 The volume frac-tion of red bone marrow within the total marrow (red + yellow), known as marrow cellularity, wasinterpolated from age-dependent cranium data published in ICRP 70. The age-dependent elementalcomposition of the brain tissue and cortical bone were obtained from ICRU 46,30 and the bone marrowcompositions were obtained from ICRU 89.31

All four age-specific mathematical phantoms were used to estimate absorbed dose to the brainparenchyma, red bone marrow (RBM), and shallow marrow. Dose deposited to the RBM is of interestfor assessing leukemia risk. The shallow marrow—defined by ICRP 110 as a 50 m thick layer cover-ing the surfaces of the trabecular bone within the spongiosa29—contains osteoprogenitor cells and is,therefore, also of interest for assessing the risk of radiogenic bone cancer. Dose to the brain paren-chyma was estimated in MCNP6 by multiplying the energy fluence within the brain by the energy-dependent mass energy absorption coefficient of the brain parenchyma. Estimations of dose to theRBM and shallow marrow were estimated using the three-factor method adopted from the work ofJohnson et al.30 The three-factor method utilizes previously reported dose enhancement factors forboth the shallow and RBM that account for secondary electron marrow dose deposition. The imple-mentation of this method in MCNP6 follows the methodologies employed by Zhang et al.31

Table A2.1. Mathematical phantom dimensions derived from ICRP 70 and other published dataregarding head dimensions. All units are in centimeters. The water-equivalent diameter, Dw ,

was calculated by UCD for x-ray spectra with tube potentials of 80, 100, 120, and 104 kV.Reported Dw values are the average and coefficient of variation (CV)

across these tube potentials.

Age(yrs)

PA LateralDw

(CV)CraniumThickness

0 11.6 9.6 11.6 (1.1%) 0.34

1 16.4 13.2 15.8 (0.9%) 0.41

5 18.4 14.6 17.7 (1.0%) 0.52

21 20.6 16.2 20.0 (1.2%) 0.69


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The average absorbed dose to the radiosensitive tissues of the head (brain parenchyma, shallowmarrow, and RBM) was calculated as a mass-weighted average of the absorbed doses to the brainparenchyma, shallow marrow, and RBM for a 0.5-cm-thick slab at the center (along z) of the scan vol-ume. The differences between this value and just the average absorbed dose to the brain were deter-mined as a percent of the average absorbed dose to the brain. Finding that the differences wereextremely small, the average absorbed dose to all three tissues (which was within 1% of the averageabsorbed dose to the brain only) was divided by the simulated CTDIvol,16 to determine the CTDIvol-to-SSDE conversion factors f H16 for each phantom (0, 1, 5, and 21 years old). Dw was calculated for eachphantom in the same manner as was used for the tissue-equivalent head phantoms in section 2.4.1, andf H16 was plotted against Dw.

ResultsWhen comparing the mass-weighted absorbed dose to the radiosensitive tissues of the head (brainparenchyma, shallow, and RBM) to the average absorbed dose to just the brain parenchyma, the dif-ferences were <0.7% across all scanners, phantom sizes, and tube potentials. These negligible differ-ences are a direct consequence of the negligible relative mass of the RBM and shallow marrowcompared to the brain parenchyma. The calculated f H16 values (CV) were 1.14 (4.3%), 0.93 (7.5%),0.82 (9.6%), and 0.69 (12.9%) for the newborn, 1-year-old, 5-year-old, and adult head phantoms,respectively for the VCT scanner, and 1.08 (4.2%), 0.92 (7.1%), 0.83 (9.0%), and 0.71 (12.5%) for theRCT scanner, which are in good agreement with the values reported in Section 3.4.

Figure A2.1. Micro-CT image of the cross-section of the frontal bone of a 56-year-old male. Adapted from thework of Lillie et al.28

size-specific dose estimate (ssde) for head cttion of the water-equivalent diameter, dw, were...

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