New weighting factor of weighted CTDI equation for PMMA phantom diameter from 8 to 40 cm: A Monte Carlo study.

Purpose: The weighted computed tomography dose index (CTDI _w ) uses measured CTDI values at the center and periphery of a cylindrical phantom. The CTDI _w value is calculated using conventional, Bakalyar's, and Choi's weighting factors. However, these weighting factors were produced from only 16- and 32-cm-diameter cylindrical phantoms. This study aims to devise new weighting factors to provide more accurate average dose in the central cross-sectional plane of cylindrical phantoms over a wide range of object diameters, by using Monte Carlo simulations.
Methods: Simulations were performed by modeling a Toshiba Aquilion ONE CT scanner, in order to compute the cross-sectional dose profiles of polymethyl methacrylate (PMMA) cylindrical phantoms of each diameter (8-40 cm at 4-cm steps), for various tube voltages and longitudinal beam widths. Two phantom models were simulated, corresponding to the CTDI ₁₀₀ method and the method recommended by American Association of Physicists in Medicine (AAPM) task group 111. The dose-computation PMMA cylinders of 1 mm diameter were located between the phantom surfaces and the centers at intervals of 1 mm, from which cross-sectional dose profiles were calculated. By using linear least-squares fits to the obtained cross-sectional dose profiles data, we determined new weighting factors to estimate more accurate average doses in the PMMA cylindrical phantoms by using the CTDI _w equation: CTDI _w = W _center ・ CTDI _center + W _periphery ・ CTDI _periphery . In order to demonstrate the validity of the devised new weighting factors, the percentage difference between average dose and CTDI _w value was evaluated for the weighting factors (conventional, Bakalyar's, Choi's, and devised new weighting factors) in each calculated cross-sectional dose profile.
Results: With the use of linear least-squares techniques, new weighting factors (W _center = 3/8 and W _periphery = 5/8 where W _center and W _periphery are weighting factors for CTDI _center and CTDI _periphery ) were determined. The maximum percentage differences between average dose and CTDI _w value were 16, -12, -8, and -6% for the conventional, Bakalyar's, Choi's, and devised new weighting factors, respectively.
Conclusions: We devised new weighting factors (W _center = 3/8 and W _periphery = 5/8) to provide more accurate average dose estimation in PMMA cylindrical phantoms over a wide range of diameter. The CTDI _w equation with devised new weighting factors could estimate average dose in PMMA cylindrical phantoms with a maximum difference of -6%. The results of this study can estimate the average dose in PMMA cylindrical phantoms more accurately than the conventional weighting factors (W _center = 1/3 and W _periphery = 2/3).
(© 2017 American Association of Physicists in Medicine.)