Room: AAPM ePoster Library
The concept of size-specific dose estimates (SSDE) has been recommended for estimating doses delivered to patients of specific sizes from CT examinations. SSDE converts volume CT dose index (CTDIvol) measured in phantoms of reference sizes to dose for a specific size using a conversion factor, which is determined by water equivalent diameter (Dw) for the region of interest in the patient. The aim of this study was to investigate the correlation between body mass index (BMI) of the patient and Dw of the region.
A total of 158 pediatric phantoms ranging from 1 to 15 years and 193 adults phantoms reconstructed from images of patients who underwent CT examinations were involved. The BMIs were in the ranges of (10 – 115) kg/m² and (40 – 125) kg/m² for pediatric and adults, respectively. Dw was assessed for six scan regions for each phantom: chest, abdomen, pelvis, chest-abdomen, abdomen-pelvis, and over all the regions, i.e. the trunk.
Good correlations were found between BMI and Dw for the regions studied. Values of Dw increased for each region with BMI. The correlations were analyzed by linear regression for each region. R² values were in the ranges of (0.84 – 0.89) for pediatric and (0.89 – 0.94) for adults, and root mean square error (RMSE) of the correlations were slightly larger for pediatric (1.86 – 2.33) cm than those for adults (1.02 – 1.46) cm.
The correlations found between BMI and Dw may be considered as a simple and quick approach to determine Dw of a patient, and hence the conversion factor and SSDE. This approach only gives a good estimation for Dw prior to a CT exam, but the accurate assessment should be made with one of the methods described in the AAPM task group report 220.