Advancing size-specific dose estimates in CT examinations: Dose estimates at longitudinal positions of scans

Purpose American Association of Physicists in Medicine (AAPM) (Report 204) introduced the size-specific dose estimate (SSDE) for the average dose to the center of a fixed-mA scan. International standards establish a method that CT manufacturers and radiation dose index monitoring software may use to calculate SSDE(z) at longitudinal positions of scans with fixed mA or tube current modulation, and its scan range average SSDE(z) over bar . We sought to test how accurate SSDE(z) is in tracking the average dose to the transverse slab of an axial image slice (D-slice), evaluated with Monte Carlo calculation, in the chest and abdominopelvic examinations. Methods We retrospectively identified 65 consecutive adult patients undergoing whole-body CT for transcatheter aortic valve implantation planning. Examination parameters (kV, mA, CTDIvol) were extracted from the DICOM headers. Patient water equivalent diameter D-W(z) was calculated at each image slice, excluding the patient table. A previously validated Monte Carlo simulation (Geant4) program was used to evaluate D-slice from the chest and abdominopelvic examinations. Alternatively, SSDE(z) was calculated at each slice. The results of the two methods were compared with descriptive statistical outcomes (R, version 4.0.2). Results In chest and abdominopelvic CT examinations, D-slice largely changed with anatomic location and uniformly fell off toward scan range edges. Scan range averages SSDE(z) over bar and Dslice over bar were consistent within 2.5%-3.1% (median) and 6.3%-10.4% (maximum) in two examinations. On individual image slices, SSDE(z) could be lower or higher than D-slice, with deviation ranging from -18.3% to 85% in two edges (2 x 5 cm) of scan range and from -35.2% to 18.7% in the remaining central region of the scan. Conclusion This study provides critical inputs for quality assurance programs. SSDE(z) over bar is useful to track the average dose of all image slices, but further development may be useful for tracking patient dose at the level of individual image slices, especially near a scan range edge.