AbstractBackgroundQuantifying radiation burden is necessary for optimizing imaging protocols. The normalized dose coefficient (NDC) is determined from the water‐equivalent diameter (WED) and is used to scale the CTDIvol based on body habitus to determine the size specific dose estimate (SSDE). In this study we determine the SSDE prior to the CT scan and how sensitive the SSDE from WED is to the lifetime attributable risk (LAR) from BEIR VII.MethodFor calibration, phantom images are used to relate the mean pixel values along a profile (PPV¯) of the CT localizer to the water‐equivalent area (AW) of the CT axial scan at the same z‐location. Images of the CTDIvol phantoms (32 cm, 16 cm, and ∼1 cm) and ACR phantom (Gammex 464) were acquired on four scanners. The relationship between theAWandPPV¯was used to calculate the WED from the CT localizer for patient scans. A total of 790 CT examinations of the chest and abdominopelvic regions were used in this study. The effective diameter (ED) was calculated from the CT localizer. The LAR was calculated based on the patient chest and abdomen using the National Cancer Institute Dosimetry System for Computed Tomography (NCICT). The radiation sensitivity index (RSI) and risk differentiability index (RDI) were calculated for SSDE and CTDIvol.ResultsThe WED from CT localizers and CT axials scans show good correlation (R2= 0.96) with the maximum percentage difference being 13.45%. The NDC from WED correlates poorly with LAR for lungs (R2= 0.18) and stomach (R2= 0.19), however that is the best correlation.ConclusionThe SSDE can be determined within 20% as recommended by the report of AAPM TG 220. The CTDIvol and SSDE are not good surrogates for radiation risk, however the sensitivity for SSDE improves when using WED instead of ED.