Borzov Egor, Nevelsky Alexander, Bar-Deroma Raquel, Orion Itzhak
Department of Radiotherapy, Division of Oncology, Rambam Health Care Campus, Haifa 32000, Israel.
Department of Nuclear Engineering, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel.
Phys Imaging Radiat Oncol. 2019 Dec 9;12:67-73. doi: 10.1016/j.phro.2019.11.010. eCollection 2019 Oct.
The electron source intensity distribution of a clinical linear accelerator has a great influence on the calculation of output factors for small radiation fields where source occlusion by the collimating devices takes place. The purpose of this study was to present a new method for the electron source reconstruction problem.
The measurements were performed in-air using diode and 6 MV 1 × 1 cm photon field in flattening filter-free mode. In Monte Carlo simulation, an electron target area was divided into a number of square subsources. Then, the in-air doses in 2D silicon chip array were calculated individually from each subsource. A genetic algorithm search was applied in order to determine the optimal weight factors for all subsources that provide the best agreement between simulated and measured doses.
It was found that the reconstructed electron source intensity from a clinical linear accelerator has the two-dimensional elliptical double Gaussian distribution. The source intensity distribution consisted of two intensity components along the in-plane (x) and cross-plane (y) directions characterized by full width half-maximum (FWHM): FWHM = 0.27 cm, FWHM = 0.08 cm, FWHM = 0.24 cm, FWHM = 0.06 cm, where broader components are 81% and 53% of the total intensity along × and y axis respectively.
The obtained results demonstrated an elliptical double Gaussian intensity distribution of the incident electron source. We anticipate that the proposed method has universal applications independent of the type of linear accelerator, modality or energy.
临床直线加速器的电子源强度分布对小射野输出因子的计算有很大影响，在小射野中准直器会遮挡电子源。本研究的目的是提出一种解决电子源重建问题的新方法。
测量在空气中使用二极管及6兆伏1×1厘米无均整器模式的光子野进行。在蒙特卡罗模拟中，将电子靶区分成若干方形子源。然后，从每个子源单独计算二维硅芯片阵列中的空气比释动能率。应用遗传算法搜索以确定所有子源的最佳权重因子，使模拟和测量的剂量达到最佳吻合。
发现从临床直线加速器重建的电子源强度具有二维椭圆形双高斯分布。源强度分布由沿平面内（x）和平面外（y）方向的两个强度分量组成，其半高宽（FWHM）分别为：FWHM = 0.27厘米，FWHM = 0.08厘米，FWHM = 0.24厘米，FWHM = 0.06厘米，其中较宽的分量分别占x轴和y轴总强度的81%和53%。
所得结果表明入射电子源具有椭圆形双高斯强度分布。我们预计所提出的方法具有普遍适用性，与直线加速器的类型、模式或能量无关。
