Comparison between Deff and Dw approaches for estimation of size-specific dose in paediatric CT imaging

Individual patient size is an important determinant for size-specific dose estimate (SSDE) since the size correction factors are highly dependent on this factor. This study aims to compare two approaches for patient size measurement, the effective diameter (Deff) and water-equivalent diameter (Dw) for estimation of paediatric SSDE. The individual size of 318 paediatric patients (160 head and 158 abdomen CT) aged 0-12 years were measured retrospectively. The patient size was estimated using attenuation-based; Dw approach and geometry-based; Deff approach using anteroposterior (AP) and lateral dimensions on CT axial images. A comparative study was performed to determine the difference and association between these two approaches and their effects on SSDE. The measured patient size using Dw was higher than Deff by an average difference of about 12.8% and 9.3% for head and abdomen, respectively. There were high positive correlations between both approaches, head (R2 = 0.9176) and abdomen (R2 = 0.9491). The difference between measured Dw and Deff were observed by the Bland-Altman's analysis that showed good agreement between Dw and Deff. The overall disparity in SSDE calculated using Deff and Dw was less than 10%. The disparity was higher in head CT SSDE as compared to abdomen CT SSDE, due to lesser correlation between Dw and Deff in head CT scan. Our findings show Deff and Dw to be comparable size metrics for size-specific dose estimate in paediatric CT. The discrepancy between both dose metrics led to slight difference in SSDE calculated using Deff and Dw.