Comparison of water-equivalent and effective diameter-based size-specific dose estimates in computed tomography pulmonary angiography: implications for radiation dose optimization

This retrospective study investigates dose estimation in computed tomography pulmonary angiography (CTPA) by analyzing 200 examinations from 96 male and 104 female patients. Two size-specific dose estimation (SSDE) methods were evaluated: one based on effective diameter (SSDEDeff) and the other on water-equivalent diameter (SSDEDw). The effective diameter (Deff) was manually measured using the lateral (LAT) and anteroposterior (AP) dimensions, while the water-equivalent diameter (Dw) was calculated from the region of interest (ROI) using mean Hounsfield Unit (HU) values and cross-sectional area. SSDE calculations were compared to the conventional CT dose metric, CTDIvol, to assess its underestimation relative to SSDEDw. The findings revealed a dose underestimation of up to 18% for CTDIvol compared to SSDEDw, particularly for patients with larger body habitus. SSDEDeff consistently overestimated the dose by 7.73% compared to SSDEDw, as Deff relied only on external dimensions while Dw considered tissue attenuation for a more individualized dose assessment. High correlations were observed between lateral diameter (dLAT) and both Deff (R2 = 0.9175) and Dw (R2 = 0.7578). However, the systematic overestimation by SSDEDeff emphasizes the importance of clearly specifying the metric used for SSDE, as differences can influence reported doses by 5–10%, affecting clinical dose monitoring and adherence to diagnostic reference levels. This study highlights the limitations of CTDIvol and SSDEDeff compared to SSDEDw, especially in regions with heterogeneous tissue. The results support the use of individualized radiation dose assessments to improve radiation safety, particularly in CTPA, which has seen increased application during the COVID-19 pandemic.