Sikora M, Dohm O, Alber M
Section for Biomedical Physics, University Hospital for Radiation Oncology, Hoppe-Seyler-Str. 3, 72076 Tübingen, Germany.
Phys Med Biol. 2007 Aug 7;52(15):4449-63. doi: 10.1088/0031-9155/52/15/006. Epub 2007 Jun 26.
A dedicated, efficient Monte Carlo (MC) accelerator head model for intensity modulated stereotactic radiosurgery treatment planning is needed to afford a highly accurate simulation of tiny IMRT fields. A virtual source model (VSM) of a mini multi-leaf collimator (MLC) (the Elekta Beam Modulator (EBM)) is presented, allowing efficient generation of particles even for small fields. The VSM of the EBM is based on a previously published virtual photon energy fluence model (VEF) (Fippel et al 2003 Med. Phys. 30 301) commissioned with large field measurements in air and in water. The original commissioning procedure of the VEF, based on large field measurements only, leads to inaccuracies for small fields. In order to improve the VSM, it was necessary to change the VEF model by developing (1) a method to determine the primary photon source diameter, relevant for output factor calculations, (2) a model of the influence of the flattening filter on the secondary photon spectrum and (3) a more realistic primary photon spectrum. The VSM model is used to generate the source phase space data above the mini-MLC. Later the particles are transmitted through the mini-MLC by a passive filter function which significantly speeds up the time of generation of the phase space data after the mini-MLC, used for calculation of the dose distribution in the patient. The improved VSM model was commissioned for 6 and 15 MV beams. The results of MC simulation are in very good agreement with measurements. Less than 2% of local difference between the MC simulation and the diamond detector measurement of the output factors in water was achieved. The X, Y and Z profiles measured in water with an ion chamber (V = 0.125 cm(3)) and a diamond detector were used to validate the models. An overall agreement of 2%/2 mm for high dose regions and 3%/2 mm in low dose regions between measurement and MC simulation for field sizes from 0.8 x 0.8 cm(2) to 16 x 21 cm(2) was achieved. An IMRT plan film verification was performed for two cases: 6 MV head&neck and 15 MV prostate. The simulation is in agreement with film measurements within 2%/2 mm in the high dose regions (> or = 0.1 Gy = 5% D(max)) and 5%/2 mm in low dose regions (<0.1 Gy).
为了对微小的调强放疗（IMRT）射野进行高度精确的模拟，需要一个专门用于调强立体定向放射外科治疗计划的高效蒙特卡罗（MC）加速器头部模型。本文提出了一种迷你多叶准直器（MLC）（医科达束流调制器（EBM））的虚拟源模型（VSM），即使对于小射野也能高效生成粒子。EBM的VSM基于先前发表的虚拟光子能量注量模型（VEF）（Fippel等人，2003年，《医学物理》，30卷，301页），该模型通过在空气和水中的大射野测量进行了调试。仅基于大射野测量的VEF原始调试程序对于小射野会导致不准确。为了改进VSM，有必要通过开发以下内容来改变VEF模型：（1）一种确定与输出因子计算相关的初级光子源直径的方法；（2）一种描述均整器对次级光子能谱影响的模型；（3）一种更符合实际的初级光子能谱。VSM模型用于生成迷你MLC上方的源相空间数据。随后，粒子通过一个被动滤波函数穿过迷你MLC，这显著加快了生成迷你MLC之后用于计算患者剂量分布的相空间数据的时间。改进后的VSM模型针对6和15 MV射束进行了调试。MC模拟结果与测量结果非常吻合。在水中，MC模拟与钻石探测器对输出因子的测量之间的局部差异小于2%。使用电离室（V = 0.125 cm³）和钻石探测器在水中测量的X、Y和Z方向的剂量分布曲线用于验证模型。对于从0.8×0.8 cm²到16×21 cm²的射野尺寸，在高剂量区域测量值与MC模拟值之间的总体一致性为2%/2 mm，在低剂量区域为3%/2 mm。对两个病例进行了调强放疗计划胶片验证：6 MV的头颈部病例和15 MV的前列腺病例。在高剂量区域（≥0.1 Gy = 5% D(max)）模拟结果与胶片测量结果的一致性在2%/2 mm以内，在低剂量区域（<0.1 Gy）为5%/2 mm。
