Bush Karl, Zavgorodni Sergei, Beckham Wayne
Department of Physics and Astronomy, University of Victoria, P. O. Box 3055 STN CSC, Victoria, British Columbia V8W 3P6, Canada.
Med Phys. 2009 Jun;36(6):2309-19. doi: 10.1118/1.3130102.
The purpose of this study was to develop an efficient method to determine the optimal intensity distribution of the pretarget electron beam in a Monte Carlo (MC) accelerator model able to most accurately reproduce a set of measured photon field profiles for a given accelerator geometry and nominal photon beam energy. The method has the ability to reduce the number of simulations required to commission a MC accelerator model and has achieved better agreement with measurement than other methods described in literature. The method begins from a cylindrically symmetric pretarget electron beam (radius of 0.5 cm) of uniform intensity. This beam is subdivided into annular regions of fluence for which each region is individually transported through the accelerator head and into a water phantom. A simulated annealing search is then performed to determine the optimal combination of weights of the annular fluences that provide a best match between the measured dose distributions and the weighted sum of annular dose distributions for particular pretarget electron energy. When restricted to Gaussian intensity distributions, the optimization determined an optimal FWHM=1.34 mm for 18.0 MeV electrons, with a RMSE=0.49% on 40 x 40 cm2 lateral profiles. When allowed to deviate from Gaussian intensities a further reduction in RMSE was achieved. For our Clinac 21 EX accelerator MC model (based on the 1996 Varian Oncology Systems, Monte Carlo Project package), the optimal unrestricted intensity distribution was found to be a Gaussian-like solution (18.0 MeV, FWHM= 1.10 mm, 40 x 40 cm2 profile, and RMSE=0.15%) with the presence of an extra focal halo contribution on the order of 10% of the maximum Gaussian intensity. Using the optimally derived intensity, 10 x 10 and 4 x 4 cm2 profiles were found to be in agreement with measurement with a maximum RMSE=0.49%. The optimized Gaussian and unrestricted values of the electron beam FWHM were both within the range of those inferred by focal spot image measurements performed by Jaffray et al. ["X-ray sources of medical linear accelerators: Focal and extra-focal radiation," Med. Phys. 20, 1417-1427 (1993)]. The inference of an extra focal pretarget electron component may be an indicator of a deficiency in the MC model and needs further investigation.
本研究的目的是开发一种高效方法，以确定蒙特卡罗（MC）加速器模型中预靶电子束的最佳强度分布，该模型能够在给定加速器几何结构和标称光子束能量的情况下，最准确地重现一组测量的光子场轮廓。该方法能够减少调试MC加速器模型所需的模拟次数，并且与文献中描述的其他方法相比，与测量结果的一致性更好。该方法从强度均匀的圆柱形对称预靶电子束（半径0.5 cm）开始。该束被细分为注量的环形区域，每个区域分别通过加速器头部传输到水体模中。然后进行模拟退火搜索，以确定环形注量权重的最佳组合，该组合能在特定预靶电子能量下，使测量剂量分布与环形剂量分布的加权和之间达到最佳匹配。当限于高斯强度分布时，优化确定18.0 MeV电子的最佳半高宽（FWHM）=1.34 mm，在40×40 cm2横向轮廓上的均方根误差（RMSE）=0.49%。当允许偏离高斯强度时，RMSE进一步降低。对于我们的Clinac 21 EX加速器MC模型（基于1996年瓦里安肿瘤系统蒙特卡罗项目包），发现最佳无限制强度分布是一种类似高斯的解（18.0 MeV，FWHM = 1.10 mm，40×40 cm2轮廓，RMSE = 0.15%），存在一个额外的焦点晕贡献，约为最大高斯强度的10%。使用最优推导的强度，发现10×10和4×4 cm2轮廓与测量结果一致，最大RMSE = 0.49%。电子束FWHM的优化高斯值和无限制值均在Jaffray等人[《医用直线加速器的X射线源：焦点和焦点外辐射》，《医学物理》20，1417 - 1427（1993）]通过焦点斑图像测量推断的范围内。焦点外预靶电子成分的推断可能表明MC模型存在缺陷，需要进一步研究。
