## 5 Conclusions  
The SSDE\({}_{\text{Dw}}\) increases with age in pediatric patients. The median (IQR) of the SSDE\({}_{\text{Dw}}\) values for the age groups of 0 to 6 months, 6 months to 3 years, 3 to 6 years, 6 to 12 years, and 12 to 15 years were 15.4 (14.8, 15.8), 20.1 (19.6, 20.6), 25.3 (24.6, 25.7), 28.1 (27.3, 28.8), and 35.1 (34.6, 36) mGy, respectively. Although the CTDI\({}_{\text{vol}}\) remained consistent within each age group, the SSDE\({}_{\text{Dw}}\) varied. The relationship between patient head size, Dw, and SSDE exhibited a strong correlation in the younger age groups, whereas a moderate correlation was observed in the 12-to-15-year age group. The SSDE, particularly when based on the water-equivalent diameter (SSDE\({}_{\text{Dw}}\)), is useful for complementing the DLP and CTDI\({}_{\text{vol}}\), as it closely reflects the patient dose across different pediatric head sizes. The correlation between the conversion factor as a function of the Dw and age indicates a strong negative relationship, with a correlation coefficient of r = \(-\)0.70, which is significant (\(p\) < 0.001). Head size, as a function of the Dw in pediatric patients and the SSDE\({}_{\text{Dw}}\), increased with age. Univariate analysis indicated that patient age and body weight significantly influenced the SSDE\({}_{\text{Dw}}\), with an R-squared value of 0.87 for the SSDE\({}_{\text{Dw}}\) with patient age and an R-squared value of 0.63 for the SSDE\({}_{\text{Dw}}\) relative to weight, both of which were significant, with \(p\) < 0.001. Multivariate linear regression analysis revealed that the SSDE\({}_{\text{Dw}}\) model with age and weight exhibited a high R-squared value of 0.87, which was significant (\(p\) < 0.001) and had a low RSE. Therefore, in a multivariate linear regression model, age and body weight effectively predict the SSDE\({}_{\text{Dw}}\).  
Conceptualization, S.P.; methodology, S.P., K.C. and N.K.; software, S.P.; validation, S.P., K.C. and N.K.; formal analysis, S.P., K.C. and N.K.; investigation, S.P., K.C. and N.K.; resources, S.P., K.C. and N.K.; data curation, S.P.; writing--original draft preparation, S.P., K.C. and N.K.; writing--review and editing, S.P.; visualization, S.P.; supervision, S.P.; project administration, S.P. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
This study was conducted in accordance with the Declaration of Helsinki and was approved by the Institutional Review Board of the Faculty of Medicine, Prince of Songkla University, Thailand, on 14 April 2020 (approval no. REC. 63-066-7-6).  
Patient consent was waived due to the retrospective nature of the study, which utilized patient information and radiation dose data for the size-specific dose estimation (SSDE). The use of pre-existing data minimized potential risks and did not require direct patient interaction. Additionally, the analysis of anonymized data ensured that patient privacy and confidentiality were maintained throughout the research.  
Data Availability Statement: The data supporting the findings of this study are not publicly available due to privacy and ethical restrictions. However, data requests can be directed to the corresponding author.  
The authors wish to thank [PERSON] from the Department of Epidemiology, Faculty of Medicine, Prince of Songkla University, Thailand, for her valuable advice on statistical data analysis.  
The authors declare no conflicts of interest.