## Abstract

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).

Original language | English |
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Pages (from-to) | 6603-6609 |

Number of pages | 7 |

Journal | Medical Physics |

Volume | 44 |

Issue number | 12 |

DOIs | |

Publication status | Published - 12-2017 |

## All Science Journal Classification (ASJC) codes

- Biophysics
- Radiology Nuclear Medicine and imaging