Department of Radiation Oncology, NCCH Rm. CB364, Campus Box 7512, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514, USA.
J Appl Clin Med Phys. 2012 Mar 8;13(2):3402. doi: 10.1120/jacmp.v13i2.3402.
The assumption of cylindrical symmetry in radiotherapy accelerator models can pose a challenge for precise Monte Carlo modeling. This assumption makes it difficult to account for measured asymmetries in clinical dose distributions. We have performed a sensitivity study examining the effect of varying symmetric and asymmetric beam and geometric parameters of a Monte Carlo model for a Siemens PRIMUS accelerator. The accelerator and dose output were simulated using modified versions of BEAMnrc and DOSXYZnrc that allow lateral offsets of accelerator components and lateral and angular offsets for the incident electron beam. Dose distributions were studied for 40 × 40 cm² fields. The resulting dose distributions were analyzed for changes in flatness, symmetry, and off-axis ratio (OAR). The electron beam parameters having the greatest effect on the resulting dose distributions were found to be electron energy and angle of incidence, as high as 5% for a 0.25° deflection. Electron spot size and lateral offset of the electron beam were found to have a smaller impact. Variations in photon target thickness were found to have a small effect. Small lateral offsets of the flattening filter caused significant variation to the OAR. In general, the greatest sensitivity to accelerator parameters could be observed for higher energies and off-axis ratios closer to the central axis. Lateral and angular offsets of beam and accelerator components have strong effects on dose distributions, and should be included in any high-accuracy beam model.
在放射治疗加速器模型中假设圆柱对称性可能会对精确的蒙特卡罗建模造成挑战。这种假设使得难以解释临床剂量分布中的测量不对称性。我们进行了一项敏感性研究，研究了变化蒙特卡罗模型中对称和不对称束以及几何参数对西门子 PRIMUS 加速器的影响。使用允许加速器组件横向偏移以及入射电子束的横向和角度偏移的 BEAMnrc 和 DOSXYZnrc 的修改版本模拟加速器和剂量输出。研究了 40×40cm² 射野的剂量分布。分析了所得剂量分布的平坦度、对称性和离轴比（OAR）的变化。对所得剂量分布影响最大的电子束参数是电子能量和入射角，最大可达 0.25°偏转的 5%。电子光斑尺寸和电子束的横向偏移的影响较小。光子靶厚度的变化对剂量分布的影响较小。准直器的小横向偏移会导致 OAR 发生显著变化。通常，对于更高的能量和更接近中心轴的离轴比，可以观察到对加速器参数的最大敏感性。束和加速器组件的横向和角度偏移对剂量分布有很大影响，应包含在任何高精度束模型中。
