Department of Biomedical Physics, King Faisal Specialist Hospital and Research Center, Riyadh 11211, Kingdom of Saudi Arabia.
Med Phys. 2011 Jan;38(1):188-201. doi: 10.1118/1.3523625.
To commission Monte Carlo beam models for five Varian megavoltage photon beams (4, 6, 10, 15, and 18 MV). The goal is to closely match measured dose distributions in water for a wide range of field sizes (from 2 x 2 to 35 x 35 cm2). The second objective is to reinvestigate the sensitivity of the calculated dose distributions to variations in the primary electron beam parameters.
The GEPTS Monte Carlo code is used for photon beam simulations and dose calculations. The linear accelerator geometric models are based on (i) manufacturer specifications, (ii) corrections made by Chibani and Ma ["On the discrepancies between Monte Carlo dose calculations and measurements for the 18 MV Varian photon beam," Med. Phys. 34, 1206-1216 (2007)], and (iii) more recent drawings. Measurements were performed using pinpoint and Farmer ionization chambers, depending on the field size. Phase space calculations for small fields were performed with and without angle-based photon splitting. In addition to the three commonly used primary electron beam parameters (E(AV) is the mean energy, FWHM is the energy spectrum broadening, and R is the beam radius), the angular divergence (theta) of primary electrons is also considered.
The calculated and measured dose distributions agreed to within 1% local difference at any depth beyond 1 cm for different energies and for field sizes varying from 2 x 2 to 35 x 35 cm2. In the penumbra regions, the distance to agreement is better than 0.5 mm, except for 15 MV (0.4-1 mm). The measured and calculated output factors agreed to within 1.2%. The 6, 10, and 18 MV beam models use theta = 0 degrees, while the 4 and 15 MV beam models require theta = 0.5 degrees and 0.6 degrees, respectively. The parameter sensitivity study shows that varying the beam parameters around the solution can lead to 5% differences with measurements for small (e.g., 2 x 2 cm2) and large (e.g., 35 x 35 cm2) fields, while a perfect agreement is maintained for the 10 x 10 cm2 field. The influence of R on the central-axis depth dose and the strong influence of theta on the lateral dose profiles are demonstrated.
Dose distributions for very small and very large fields were proved to be more sensitive to variations in E(AV), R, and theta in comparison with the 10 x 10 cm2 field. Monte Carlo beam models need to be validated for a wide range of field sizes including small field sizes (e.g., 2 x 2 cm2).
为五款瓦里安兆伏级光子射束（4、6、10、15 和 18 MV）委托制作蒙特卡罗射束模型。目标是在大范围射野尺寸（2×2 至 35×35 cm2）下，使测量的水中剂量分布与计算剂量分布紧密匹配。第二个目标是重新研究计算剂量分布对初级电子射束参数变化的敏感性。
GEPTS 蒙特卡罗代码用于光子射束模拟和剂量计算。直线加速器的几何模型基于：(i) 制造商规格，(ii) Chibani 和 Ma 的修正[《18 MV 瓦里安光子射束的蒙特卡罗剂量计算与测量之间的差异》，《医学物理》34，1206-1216（2007）]，以及 (iii) 最近的图纸。根据射野尺寸，使用 pinpoint 和 Farmer 电离室进行测量。小射野的相空间计算可在是否基于角度的光子分裂。除了三个常用的初级电子射束参数（E(AV) 是平均能量，FWHM 是能谱展宽，R 是射束半径）外，还考虑了初级电子的角发散度（theta）。
在不同能量和从 2×2 至 35×35 cm2 变化的射野尺寸下，在任何深度超过 1 cm 处，计算剂量分布与测量剂量分布的差异在 1%以内。在半影区域，符合程度优于 0.5 mm，除了 15 MV（0.4-1 mm）。测量和计算的输出因子差异在 1.2%以内。6、10 和 18 MV 射束模型使用 theta = 0 度，而 4 和 15 MV 射束模型分别需要 theta = 0.5 度和 0.6 度。参数敏感性研究表明，在解决方案周围改变射束参数会导致小射野（例如 2×2 cm2）和大射野（例如 35×35 cm2）的测量值出现 5%的差异，而 10×10 cm2 射野则保持完美一致。展示了 R 对中央轴深度剂量的影响和 theta 对侧向剂量分布的强烈影响。
与 10×10 cm2 射野相比，非常小和非常大射野的剂量分布被证明对 E(AV)、R 和 theta 的变化更敏感。需要对包括小射野尺寸（例如 2×2 cm2）在内的大范围射野尺寸验证蒙特卡罗射束模型。
