PMMA 体模直径 8 至 40 厘米时加权 CTDI 方程的新加权因子：蒙特卡罗研究。

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

机构信息

Department of Radiology, Fujita Health University Hospital, 1-98 Dengakugakubo, Kutsukake-cho, Toyoake, Aichi, 470-1192, Japan.

Brain and Mind Research Center, Nagoya University, 1-1-20 Daiko-Minami, Higashi-ku, Nagoya, Aichi, 461-8673, Japan.

出版信息

Med Phys. 2017 Dec;44(12):6603-6609. doi: 10.1002/mp.12601. Epub 2017 Oct 23.

DOI:10.1002/mp.12601

PMID:28960375

Abstract

PURPOSE

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

摘要

目的

加权 CT 剂量指数（CTDI）使用圆柱形模体中心和周边的实测 CTDI 值。CTDI 值使用常规、Bakalyar 和 Choi 加权因子计算。然而，这些加权因子仅来自 16 和 32 厘米直径的圆柱形模体。本研究旨在通过使用蒙特卡罗模拟，设计新的加权因子，以在更广泛的物体直径范围内，为圆柱形模体的中央横截面上提供更准确的平均剂量。

方法

通过模拟东芝 Aquilion ONE CT 扫描仪进行模拟，以计算每个直径（8-40 厘米，4 厘米步长）的聚甲基丙烯酸甲酯（PMMA）圆柱形模体的横截面剂量分布，针对各种管电压和纵向射束宽度。模拟了两种模体模型，分别对应于 CTDI 方法和美国医学物理学家协会（AAPM）第 111 任务组推荐的方法。直径为 1 毫米的剂量计算 PMMA 圆柱体位于模体表面和中心之间，间隔 1 毫米，从这些位置计算出横截面剂量分布。通过对获得的横截面剂量分布数据进行线性最小二乘拟合，我们确定了新的加权因子，以便使用 CTDI 方程估计 PMMA 圆柱形模体中更准确的平均剂量：CTDI = W・CTDI + W・CTDI。为了证明所设计的新加权因子的有效性，评估了在每个计算的横截面剂量分布中，平均剂量与 CTDI 值之间的加权因子（常规、Bakalyar、Choi 和新设计的加权因子）的百分比差异。

结果

使用线性最小二乘法技术，确定了新的加权因子（W = 3/8 和 W = 5/8，其中 W 和 W 是 CTDI 和 CTDI 的加权因子）。对于常规、Bakalyar、Choi 和新设计的加权因子，平均剂量与 CTDI 值之间的最大百分比差异分别为 16%、-12%、-8%和-6%。

结论

我们设计了新的加权因子（W = 3/8 和 W = 5/8），以在更广泛的直径范围内提供更准确的 PMMA 圆柱形模体平均剂量估计。使用新设计的加权因子的 CTDI 方程可以估计 PMMA 圆柱形模体的平均剂量，最大差异为-6%。本研究的结果可以比常规加权因子（W = 1/3 和 W = 2/3）更准确地估计 PMMA 圆柱形模体的平均剂量。