Relative Error between Organ Doses and Size-specific Dose Estimates for a Specific Location When the Mean CTDIvol Value Is Used for Calculation

Abstract

Size-specific dose estimates (SSDEs) are dose indices that account for differences in body shape in computed tomography (CT) scans, allowing the evaluation of approximate absorbed doses in any cross section that could not be obtained with the volume CT dose index (CTDIvol). When using automatic exposure control (AEC), CTDIvol is modulated in the body axis direction, but the value displayed after the examination is the mean CTDIvol for the entire scan, and it is expected that the SSDE value will change depending on which value is used in the calculation. In this study, using a human body phantom, we examined the influence of whether the mean CTDIvol or the modulation value for each slice is used to calculate the SSDE on local organ dose evaluation. A program to calculate water equivalent diameter according to the procedure in the American Association of Physicists in Medicine Report No. 220 was developed and compared. As a result, SSDE calculated using the mean CTDIvol (local-SSDEmean) overestimated organ doses in the lung region by 18%-56% compared with those calculated by a web system for evaluating CT exposure doses (WAZA-ARIv2, Japan). In contrast, local-SSDEmodulated, which was calculated using the modulated value of the CTDIvol, was able to estimate the organ dose with a relative error of 10%-13%. The average local-SSDE over the entire body axis direction was not significantly different between the two methods, regardless of which method was used for CTDIvol. If the mean CTDIvol is stored in the Digital Imaging and Communications in Medicine (DICOM) header tag (0018, 9345) of the CT image and the modulated CTDIvol value is not available for each slice, the calculated local SSDE will contain many errors and will not correctly reflect the organ doses at the scan region. In such cases, it is available to use the method of evaluating local organ doses by multiplying the SSDE, which is the average of the SSDE for the entire scan, by a factor for each organ.