Schach von Wittenau A E, Cox L J, Bergstrom P M, Chandler W P, Hartmann Siantar C L, Mohan R
Lawrence Livermore National Laboratory, Livermore, California 94550, USA.
Med Phys. 1999 Jul;26(7):1196-211. doi: 10.1118/1.598613.
We present a method for condensing the photon energy and angular distributions obtained from Monte Carlo simulations of medical accelerators. This method represents the output as a series of correlated histograms and as such is well-suited for inclusion as the photon-source package for Monte Carlo codes used to determine the dose distributions in photon teletherapy. The method accounts for the isocenter-plane variations of the photon energy spectral distributions with increasing distance from the beam central axis for radiation produced in the bremsstrahlung target as well as for radiation scattered by the various treatment machine components within the accelerator head. Comparison of the isocenter energy fluence computed by this algorithm with that of the underlying full-physics Monte Carlo photon phase space indicates that energy fluence errors are less than 1% of the maximum energy fluence for a range of open-field sizes. Comparison of jaw-edge penumbrae shows that the angular distributions of the photons are accurately reproduced. The Monte Carlo sampling efficiency (the fraction of generated photons which clear the collimator jaws) of the algorithm is approximately 83% for an open 10x10 field, rising to approximately 96% for an open 40x40 field. Data file sizes for a typical medical accelerator, at a given energy, are approximately 150 kB, compared to the 1 GB size of the underlying full-physics phase space file.
我们提出了一种用于压缩从医用加速器的蒙特卡罗模拟中获得的光子能量和角分布的方法。该方法将输出表示为一系列相关直方图，因此非常适合作为用于确定光子远距离治疗中剂量分布的蒙特卡罗代码的光子源包。该方法考虑了在韧致辐射靶中产生的辐射以及加速器头内各种治疗机器部件散射的辐射，随着离束中心轴距离增加，光子能谱分布在等中心平面的变化。通过该算法计算的等中心能量注量与基础全物理蒙特卡罗光子相空间的等中心能量注量的比较表明，对于一系列开放野尺寸，能量注量误差小于最大能量注量的1%。对射野边缘半值宽度的比较表明，光子的角分布得到了准确再现。对于10×10的开放野，该算法的蒙特卡罗抽样效率（即穿过准直器射野边缘的生成光子的比例）约为83%，对于40×40的开放野则升至约96%。在给定能量下，典型医用加速器的数据文件大小约为150 kB，而基础全物理相空间文件大小为1 GB。
