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• We devised a new dose metric (f(0)CB w) for cone-beam CT dosimetry. • The f(0)CB w method adopted new measurement positions of a cylindrical phantom. • The f(0)CB w method can estimate the average phantom dose within 4.1% error. • The f(0)CB w method is more suitable than the conventional CTDI w method. • The results of this study would be useful for the future of CBCT dosimetry. The conventional weighted computed tomography dose index (CTDI w) may not be suitable for cone-beam computed tomography (CBCT) dosimetry because a cross-sectional dose distribution is angularly inhomogeneous owing to partial angle irradiations. This study was conducted to develop a new dose metric (f(0)CB w) for CBCT dosimetry to determine a more accurate average dose in the central cross-sectional plane of a cylindrical phantom using Monte Carlo simulations. First, cross-sectional dose distributions of cylindrical polymethyl methacrylate phantoms over a wide range of phantom diameters (8–40 cm) were calculated for various CBCT scan protocols. Then, by obtaining linear least-squares fits of the full datasets of the cross-sectional dose distributions, the optimal radial positions, which represented measurement positions for the average phantom dose, were determined. Finally, the f(0)CB w method was developed by averaging point doses at the optimal radial positions of the phantoms. To demonstrate its validity, the relative differences between the average doses and each dose index value were estimated for the devised f(0)CB w , conventional CTDI w , and Haba's CTDI w methods, respectively. The relative differences between the average doses and each dose index value were within 4.1%, 16.7%, and 11.9% for the devised, conventional CTDI w , and Haba's CTDI w methods, respectively. The devised f(0)CB w value was calculated by averaging four "point doses" at 90° intervals and the optimal radial positions of the cylindrical phantom. The devised method can estimate the average dose more accurately than the previously developed CTDI w methods for CBCT dosimetry. [ABSTRACT FROM AUTHOR]

Purpose The weighted computed tomography dose index ( CTDIw) uses measured CTDI values at the center and periphery of a cylindrical phantom. The CTDIw 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 CTDI100 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 CTDIw equation: CTDIw = Wcenter ・ CTDIcenter + Wperiphery ・ CTDIperiphery. In order to demonstrate the validity of the devised new weighting factors, the percentage difference between average dose and CTDIw 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 (Wcenter = 3/8 and Wperiphery = 5/8 where Wcenter and Wperiphery are weighting factors for CTDIcenter and CTDIperiphery) were determined. The maximum percentage differences between average dose and CTDIw 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 (Wcenter = 3/8 and Wperiphery = 5/8) to provide more accurate average dose estimation in PMMA cylindrical phantoms over a wide range of diameter. The CTDIw 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 (Wcenter = 1/3 and Wperiphery = 2/3). [ABSTRACT FROM AUTHOR]