Frankl Matthias, Macián-Juan Rafael
Department of Nuclear Engineering, Technische Universität München, Garching 85748, Germany
Department of Nuclear Engineering, Technische Universität München, Garching 85748, Germany.
Radiat Prot Dosimetry. 2016 Mar;168(4):537-45. doi: 10.1093/rpd/ncv381. Epub 2015 Aug 25.
The development of intensity-modulated radiotherapy treatments delivering large amounts of monitor units (MUs) recently raised concern about higher risks for secondary malignancies. In this study, optimised combinations of several variance reduction techniques (VRTs) have been implemented in order to achieve a high precision in Monte Carlo (MC) radiation transport simulations and the calculation of in- and out-of-field photon and neutron dose-equivalent distributions in an anthropomorphic phantom using MCNPX, v.2.7. The computer model included a Varian Clinac 2100C treatment head and a high-resolution head phantom. By means of the applied VRTs, a relative uncertainty for the photon dose-equivalent distribution of <1 % in-field and 15 % in average over the rest of the phantom could be obtained. Neutron dose equivalent, caused by photonuclear reactions in the linear accelerator components at photon energies of approximately >8 MeV, has been calculated. Relative uncertainty, calculated for each voxel, could be kept below 5 % in average over all voxels of the phantom. Thus, a very detailed neutron dose distribution could be obtained. The achieved precision now allows a far better estimation of both photon and especially neutron doses out-of-field, where neutrons can become the predominant component of secondary radiation.
近年来，调强放射治疗中大量监测单位（MU）的使用引发了人们对继发恶性肿瘤更高风险的担忧。在本研究中，为了在蒙特卡罗（MC）辐射传输模拟以及使用MCNPX 2.7版在人体模型中计算野内和野外光子及中子剂量当量分布时实现高精度，实施了几种方差减少技术（VRT）的优化组合。计算机模型包括瓦里安Clinac 2100C治疗头和高分辨率头部模型。通过应用VRT，对于光子剂量当量分布，在野内的相对不确定度可小于1%，在模型其余部分平均为15%。计算了在光子能量约>8 MeV时，直线加速器部件中的光核反应产生的中子剂量当量。对模型每个体素计算的相对不确定度，在模型所有体素上平均可保持在5%以下。因此，可以获得非常详细的中子剂量分布。所达到的精度现在使得能够更好地估计野外的光子剂量，尤其是中子剂量，在野外中子可能成为二次辐射的主要成分。
