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Influence of Water Equivalent Diameter (Dw) Variability Inside the Scan Area On Size-Specific Dose Estimates (SSDE)

A Abuhaimed1, C Martin2*, (1) King Abdulaziz City for Science and Technology (KACST), Riyadh, Saudi Arabia, (2) University of Glasgow, Glasgow, UK


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Size-specific dose estimates (SSDE) is used to assess doses absorbed by patients of specific sizes from CT scans. SSDE is based on applying a correction factor to volume CT dose index (CTDIvol) that is measured in head and body phantoms of specific sizes. The correction factor is calculated from published data based on size of the scan area for the patient. The size is reported in terms of water equivalent diameter (Dw) of the area. The purpose of this study was to investigate influence of Dw variability inside the scan area on SSDE estimation.

Dw values were assessed for 351 phantoms developed from CT images of pediatric and adult patients. Six scan areas in the trunk were investigated: chest, abdomen, pelvis, chest-abdomen, abdomen-pelvis, and over all the trunk. Dw was assessed for each slide inside these areas using a MATLAB code developed in house, from which minimum and maximum Dw over the area were calculated for each phantom.

Generally, Dw variability for adults was larger than for pediatric patients. The smallest and largest variabilities were for the abdomen and abdomen-pelvis scan areas, respectively, and the variability was found to increase with patient size. For all areas, Dw variability was in the range of 1.3 – 13.5 cm for pediatric and 1.6 – 16.3 cm for adults, with the average values being 6.2 cm and 7.9 cm, respectively. These variabilities can lead to variations in SSDE estimation by up to 58% and 75%, respectively, across the scan areas, and 21% and 29%, on average.

Large variations in Dw values inside the scan areas studied were noticed for some sizes, particularly for large patients. Normally, Dw mean is used for SSDE estimation, but such variations should be taken into consideration to avoid any significant over/underestimation for patient dose.




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