Can Patient’s Body Weight Represent Body Diameter for Pediatric Size-Specific Dose Estimate in Thoracic and Abdominal Computed Tomography?

Objective: The objective of the study was to determine whether body weight (BW) can be substituted for body diameters to calculate size-specific dose estimate (SSDE) in the children. Materials and Methods: A total of 196 torso computed tomography (CT) studies were retrospectively reviewed. Anteroposterior diameter (DAP) and lateral diameter (Dlat) were measured, and DAP+Dlat, effective diameter, SSDE diameter and SSDEBW were calculated. Correlation coefficients among body diameters, all SSDE types and percentage changes between CT dose index volumes and SSDEs were analyzed by BW and age subgroups. Results: Overall BW was more strongly correlated with body diameter (r = 0.919–0.960, P < 0.001) than was overall age (r = 0.852–0.898, P < 0.001). The relationship between CT dose index volume and each of the SSDE types (r = 0.934–0.953, P < 0.001), between SSDEBW and all SSDE diameters (r = 0.934–0.953, P < 0.001), and among SSDE diameters (r = 0.950–0.989, P < 0.001) overall had strong correlations with statistical significance. The lowest magnitude difference was SSDEBW−SSDEeff. Conclusion: BW can be used instead of body diameter to calculate all SSDE types, with our suggested best accuracy for SSDEeff and the least variation in age < four years and BW < 20 kg. Key Messages: Size-specific dose estimate (SSDE) is a new and accurate dose-estimating parameter for the individual patient which is based on the actual size or body diameter of the patient. BW can be an important alternative for all body diameters to estimate size-specific dose or calculate SSDE in children.


INTRODUCTION
e use of pediatric computed tomography (CT) has grown dramatically in the past decade and the risk of radiation-induced cancers in children is of more concern than in adults. e most commonly used CT parameters for calculating CT radiation dosage are CT dose index volume (CTDIvol) and dose length product (DLP). [1][2][3] However, the CTDIvol is delivered from a specific standard phantom size and does not indicate the actual radiation dose applied to the individual patient, leading to underestimation of the total received radiation dose to children or adults with small body size. [1][2][4][5][6][7][8] Size-specific dose estimate (SSDE) is a new parameter for individual specific patients which was developed by the American Association of Physicists in Medicine (AAPM Report 204). [9] e SSDE is the patient dose estimate with corrections based on the actual size or body diameter of the patient. [4,[9][10] ere have been several reports examining SSDE in children [11][12][13][14][15] and the combination of measurements (sum of body diameters or effective diameters (Deff) is recommended to determine the appropriate SSDE correction. [11] Achieving a patient's body diameters to calculate SSDE is more difficult than obtaining a patient's body weight (BW) in routine work, which would make SSDE calculation more simple and rapid. However, only one report has examined conversion factors for pediatric SSDEBW. [16] e purposes of this study were to determine whether SSDE based on BW could be substituted for other SSDE values and to compare all SSDE values with CTDIvol among pediatric patients who underwent chest and abdominal CT.

Patients and study design
e study was approved by our Human Research Ethics Committee. We retrospectively reviewed the imaging records of pediatric patients (<18 years) who underwent intravenous contrast chest or abdominal CT alone or contiguous chest and abdominal CT examinations from October 2011 to October 2016. Of the 2340 studies, 198 were randomly selected by computer, and two studies were excluded due to incorrect CT dose protocols. [17] Finally, 196 studies were reviewed. e demographic data, age, BW, and gender of the patients were collected from the hospital medical records. e patients were categorized into age and BW subgroups. e age subgroups were 0-<5years (n = 71), 5-<10 years (n = 39), 10-<15 years (n = 31), and 15-<18 years (n = 55). e BW subgroups were classified according to our institutional practice CT protocol: 4-9 kg (n = 19), 10-19 kg (n = 70), 20-29 kg (n = 22), 30-39 kg (n = 15), 40-49 kg (n = 26), and 50-64 kg (n = 26), >64 kg (n = 18). [18] Definitions, dosimetry, and body diameter measurement Anteroposterior diameter (DAP) was defined as the skin-toskin thickness of the body part of the patient at the maximum thickness axial slice image [ Figure 1]. Lateral diameter(Dlat) was defined as the skin-to-skin thickness of the body part of the patient at the maximum thickness axial slice image and/or anterior-posterior dimension localizer image. [19] Anteroposterior plus lateral diameter (DAP+lat) was defined as the diameter calculated as AP diameter plus Dlat. e Deff was calculated as the square root of the AP dimension multiplied by the lateral dimension. [9] e CTDIvol (units: mGy) is the mean radiation absorbed dose to the patient at a given point of scan volume and is defined as weighted CTDIw/pitch. e CTDIvol was calibrated using a pencil-shaped ionization chamber with either a dedicated 16-cm or 32-cm diameter polymethylmethacrylate phantom representing the head or a body region, respectively. e DLP was defined as the CTDIvol x exposed scan length. ese parameters were displayed on the CT scanner consoles and Picture Archiving and Communication System (PACS). In multiphase-scanning, the CTDIvol of the maximum DLP was used. Only CTDIvol based on 32-cm phantom was included in this study. SSDE were calculated as CTDIvol multiplied by the conversion factor in the table and depended on BW, AP, and lateral and Deff according to the AAPM Report 204 and Khawaja et al. study. [9,16] e exact conversion factor for each patient was calculated by the provided equations in the AAPM Report 204. [9] Data collection e two CT scanner models used during the study period were a 64-multislice Philips Brilliance CT scanner and a 160-slice Toshiba Aquilion Prime CT scanner. e images were retrieved from a PACS workstation. e body diameters were independently measured by one 13-year-experience pediatric radiologist and one-third year resident training in diagnostic radiology with consensus. e BW, age, dose indices (CTDIvol and SSDEBW, SSDEAP, SSDElat, SSDEAP+lat, and SSDEeff), and body diameters (AP, lateral, AP+lat, effective) for each patient were recorded into a spreadsheet (Microsoft Office Excel 2010; Microsoft Corporation, Redmond, WA, USA).

Statistical analysis
We presented the quantitative parameters involving BW and body diameters (AP, lateral, AP+lat, effective) using median ± interquartile range (IQR) due to non-normal distribution data. Percentage changes between CTDIvol and each SSDE type and the magnitude differences between the SSDEBW and SSDEdiameters were calculated. Correlations among BW, age, dose indices, and body diameter measurements were established with Spearman Rank correlation coefficients (r) for the following: Correlations between each body diameter and BW and between each body diameter and age; and correlations among dose indices (CTDIvol, SSDEBW, SSDEAP, SSDElat, SSDEAP+lat, and SSDEeff) across BW and age subgroups. e power to determine sample size in BW and age subgroups for calculating correlation among dose indices was >0.8. Estimated relationships between median dose indices (CTDIvol and SSDE) and mean BWs were calculated by quantile regression analysis. Differences among the SSDE values were calculated by Wilcoxon Rank sum test. P = 0.05 or less was considered to indicate a statistically significant difference. Interobserver variations among the two reviewers were calculated using intraclass correlation coefficient (ICC) values.  Table 1. e Dlat was larger than the AP diameter in all BW and age subgroups. All of the body diameters were in ascending order in both BW and age subgroups. Interobserver agreement using ICC between the two reviewers was excellent (ICC = 0.99).

DISCUSSION
Our study found that all body diameters and overall BW were strongly correlated (r = 0.919-0.960, P < 0.001). e Dlat, DAP+lat, and Deff in our study had higher correlations with overall BW than with DAP, which could be explained by understanding the general growth pattern of children, in which the child's body grows in the Dlat more rapidly than in the AP diameter. [2] e correlations for all body diameters and overall age were also strong but not as high as the body diameter-BW relationships (r = 0.852-0.898, P < 0.001). However, the study by Kleinman et al. found that the predicted individual patient size was not correlated with age. [2] All relationships of SSDEBW-all SSDE diameters (r = 0.934-0.953, P < 0.001) and among SSDE body diameters (r = 0.950-0.989, P < 0.001) with overall BW and with overall age showed strong and statistically significant correlations [Tables 3 and 4]. e strongest correlations were found in the 30-39 kg subgroup and the 5-9 years subgroup. Another previous study by Khawaja et al. reached the same conclusion as our study that BW could be substituted to estimate size-specific dose in children. [16] Another study by Parikh et al. also found that BW could be used to estimate SSDE with reasonable accuracy at body width >20 cm. [14] In our study, we could predict the SSDE and CTDIvol using BW from Equations 1-6, while Christner et al. study concluded that only CTDIvol increased linearly with patient size (DAP + Dlat), while SSDE was independent of patient size. [10] Furthermore, almost all SSDE type values (n = 189/196, 96.4%) were higher than CTDIvol, except for large-sized patients and those weighing >100 kg. erefore, emphasizing CTDIvol underestimates the radiation dose in most pediatric or small-sized patients and overestimates the radiation dose in large-sized patients. [2,8,14,20] Although the SSDEBW had a statistically significant difference from SSDElat by Wilcoxon Rank sum test, the magnitude difference between SSDEBW and SSDElat was still in the acceptable range (within 7% of dose index in diagnostic radiology). e lowest magnitude difference was between SSDEBW and SSDEeff, while the highest magnitude difference was between SSDEBW and SSDEAP. ese results could be explained by considering a study from Brady et al., which found that either an individual AP or Dlat measurement alone was less useful than a combination of AP and Dlat measurement for SSDE determination. [11] However, all SSDEBW-SSDEdiameters magnitude differences in our study were still in the acceptable range. e smallest variations of the SSDE differences in all subgroups by age and BW were in the lower BW ranges and younger age groups. In addition, most of the SSDEBW values tended to be lower than the SSDEdiameters. is implies that the SSDEBW can be substituted for SSDEdiameters, especially SSDEeff, but with caution as the SSDEBW tended to be lower than SSDEdiameters.
Our study had a few limitations. First, we could not statistically determine correlations between each body diameter and the BW <20 kg and >64 kg subgroups and between each body diameter and the age >4 year subgroups because the power of the sample size in those subgroups was <0.8. We suggest further research should be conducted with increased sample sizes in each subgroup if the study objective is to determine the correlation between body diameters and age or BW subgroups. Second, we did not calculate the SSDE from the water equivalent diameter (Dw), which is a physical parameter based on patient attenuation. In case of patients having high body attenuation, for example, those suffering from mediastinal or intra-abdominal tumors with low to normal BW, the SSDEDw is more accurate than SSDEdiameter to determine the correct patient dose. [21] We suggest further studies including SSDEDw and clinical indications. Finally, the findings of our study may not be applicable in institutions and hospitals that have automatic software to determine the body measurements and SSDE.

CONCLUSION
Accurate dose-estimating parameters and size-specific dose indices are important for calculating accurate radiation dosage in the pediatric population. Our study found that the body diameter-BW correlation was stronger than the body diameterage relationship. is calculation is simple and rapid to perform, and BW can be an important alternative for all body diameters to estimate size-specific dose or calculate SSDE in children. Our findings indicate this method has the best accuracy for SSDEeff and the least variation in ages less than 4 years and BWs < 20 kg.

Financial support and sponsorship
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Conflicts of interest
ere are no conflicts of interest.