Islamic Azad University, Fars, Persepolis, Iran.
Med Phys. 2012 Apr;39(4):1971-9. doi: 10.1118/1.3693046.
The goal of this study is to determine a method for Monte Carlo (MC) characterization of the miniature electronic brachytherapy x-ray sources (MEBXS) and to set dosimetric parameters according to TG-43U1 formalism. TG-43U1 parameters were used to get optimal designs of MEBXS. Parameters that affect the dose distribution such as anode shapes, target thickness, target angles, and electron beam source characteristics were evaluated. Optimized MEBXS designs were obtained and used to determine radial dose functions and 2D anisotropy functions in the electron energy range of 25-80 keV.
Tungsten anode material was considered in two different geometries, hemispherical and conical-hemisphere. These configurations were analyzed by the 4C MC code with several different optimization techniques. The first optimization compared target thickness layers versus electron energy. These optimized thicknesses were compared with published results by Ihsan et al. [Nucl. Instrum. Methods Phys. Res. B 264, 371-377 (2007)]. The second optimization evaluated electron source characteristics by changing the cathode shapes and electron energies. Electron sources studied included; (1) point sources, (2) uniform cylinders, and (3) nonuniform cylindrical shell geometries. The third optimization was used to assess the apex angle of the conical-hemisphere target. The goal of these optimizations was to produce 2D-dose anisotropy functions closer to unity. An overall optimized MEBXS was developed from this analysis. The results obtained from this model were compared to known characteristics of HDR (125)I, LDR (103)Pd, and Xoft Axxent™ electronic brachytherapy source (XAEBS) [Med. Phys. 33, 4020-4032 (2006)].
The optimized anode thicknesses as a function of electron energy is fitted by the linear equation Y (μm) = 0.0459X (keV)-0.7342. The optimized electron source geometry is obtained for a disk-shaped parallel beam (uniform cylinder) with 0.9 mm radius. The TG-43 distribution is less sensitive to the shape of the conical-hemisphere anode than the hemispherical anode. However, the optimized apex angle of conical-hemisphere anode was determined to be 60°. For the hemispherical targets, calculated radial dose function values at a distance of 5 cm were 0.137, 0.191, 0.247, and 0.331 for 40, 50, 60, and 80 keV electrons, respectively. These values for the conical-hemisphere targets are 0.165, 0.239, 0.305, and 0.412, respectively. Calculated 2D anisotropy functions values for the hemispherical target shape were F(1 cm, 0°) = 1.438 and F(1 cm, 0°) = 1.465 for 30 and 80 keV electrons, respectively. The corresponding values for conical-hemisphere targets are 1.091 and 1.241, respectively.
A method for the characterizations of MEBXS using TG-43U1 dosimetric data using the MC MCNP4C has been presented. The effects of target geometry, thicknesses, and electron source geometry have been investigated. The final choices of MEBXS design are conical-hemisphere target shapes having an apex angle of 60°. Tungsten material having an optimized thickness versus electron energy and a 0.9 mm radius of uniform cylinder as a cathode produces optimal electron source characteristics.
本研究旨在确定一种蒙特卡罗（MC）方法来对微型电子近距离治疗 X 射线源（MEBXS）进行特性描述，并根据 TG-43U1 形式设置剂量学参数。TG-43U1 参数用于获得 MEBXS 的最佳设计。评估了影响剂量分布的参数，如阳极形状、靶厚度、靶角和电子束源特性。获得了优化的 MEBXS 设计，并用于在 25-80keV 的电子能量范围内确定径向剂量函数和二维各向异性函数。
考虑了两种不同几何形状的钨阳极材料，即半球形和圆锥形-半球形。这些配置由 4C MC 代码进行了分析，并采用了几种不同的优化技术。第一次优化比较了靶厚度层与电子能量的关系。将这些优化后的厚度与 Ihsan 等人发表的结果进行了比较。[Nucl. Instrum. Methods Phys. Res. B 264, 371-377 (2007)]。第二次优化通过改变阴极形状和电子能量来评估电子源特性。研究的电子源包括：（1）点源，（2）均匀圆柱，（3）非均匀圆柱壳几何形状。第三次优化用于评估圆锥形-半球形靶的顶点角度。这些优化的目的是生成更接近 1 的二维剂量各向异性函数。从这个分析中开发了一个整体优化的 MEBXS。从这个模型中获得的结果与已知的 HDR(125)I、LDR(103)Pd 和 Xoft AxxentTM 电子近距离治疗源（XAEBS）[Med. Phys. 33, 4020-4032 (2006)]的特性进行了比较。
优化后的阳极厚度与电子能量的关系可以用线性方程 Y(μm)=0.0459X(keV)-0.7342 来拟合。优化后的电子源几何形状是一个直径为 0.9mm 的圆盘形平行束（均匀圆柱）。TG-43 分布对圆锥形-半球形阳极的形状比半球形阳极更不敏感。然而，确定了圆锥形-半球形阳极的最佳顶点角度为 60°。对于半球形靶，在距离 5cm 处计算得到的径向剂量函数值分别为 40、50、60 和 80keV 电子时为 0.137、0.191、0.247 和 0.331。对于圆锥形-半球形靶，这些值分别为 0.165、0.239、0.305 和 0.412。半球形靶形状的计算二维各向异性函数值分别为 F(1cm，0°)=1.438 和 F(1cm，0°)=1.465，分别对应于 30 和 80keV 电子。圆锥形-半球形靶的相应值分别为 1.091 和 1.241。
提出了一种使用 MC MCNP4C 基于 TG-43U1 剂量学数据对 MEBXS 进行特性描述的方法。研究了靶几何形状、厚度和电子源几何形状的影响。MEBXS 设计的最终选择是具有 60°顶点角的圆锥形-半球形靶。优化后的厚度与电子能量和 0.9mm 半径的均匀圆柱作为阴极的钨材料产生了最佳的电子源特性。
