Spindeldreier C K, Schrenk O, Bakenecker A, Kawrakow I, Burigo L, Karger C P, Greilich S, Pfaffenberger A
Division of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), 69120 Heidelberg, Germany. Heidelberg Institute for Radiation Oncology (HIRO), National Center for Radiation Research in Oncology (NCRO), 69120 Heidelberg, Germany.
Phys Med Biol. 2017 Aug 1;62(16):6708-6728. doi: 10.1088/1361-6560/aa7ae4.
The aim of this work was to determine magnetic field correction factors that are needed for dosimetry in hybrid devices for MR-guided radiotherapy for Farmer-type ionization chambers for different magnetic field strengths and field orientations. The response of six custom-built Farmer-type chambers irradiated at a 6 MV linac was measured in a water tank positioned in a magnet with magnetic field strengths between 0.0 T and 1.1 T. Chamber axis, beam and magnetic field were perpendicular to each other and both magnetic field directions were investigated. EGSnrc Monte Carlo simulations were compared to the measurements and simulations with different field orientations were performed. For all geometries, magnetic field correction factors, [Formula: see text], and perturbation factors were calculated. A maximum increase of 8.8% in chamber response was measured for the magnetic field perpendicular to chamber and beam axis. The measured chamber response could be reproduced by adjusting the dead volume layer near the chamber stem in the Monte Carlo simulations. For the magnetic field parallel to the chamber axis or parallel to the beam, the simulated response increased by 1.1% at maximum for field strengths up to 1.1 T. A complex dependence of the response was found on chamber radius, magnetic field strength and orientation of beam, chamber axis and magnetic field direction. Especially for magnetic fields perpendicular to beam and chamber axis, the exact sensitive volume has to be considered in the simulations. To minimize magnetic field correction factors and the influence of dead volumes on the response of Farmer chambers, a measurement set-up with the magnetic field parallel to the chamber axis or parallel to the beam is recommended for dosimetry.
这项工作的目的是确定在用于磁共振引导放射治疗的混合设备中，针对不同磁场强度和场方向的Farmer型电离室进行剂量测定所需的磁场校正因子。在置于磁场强度介于0.0 T和1.1 T之间的磁体中的水箱中，测量了在6 MV直线加速器上照射的六个定制Farmer型电离室的响应。电离室轴、射束和磁场相互垂直，且研究了两个磁场方向。将EGSnrc蒙特卡罗模拟与测量结果进行了比较，并进行了不同场方向的模拟。针对所有几何结构，计算了磁场校正因子[公式：见正文]和扰动因子。对于垂直于电离室和射束轴的磁场，测量到电离室响应最大增加了8.8%。通过在蒙特卡罗模拟中调整电离室杆附近的死体积层，可以再现测量到的电离室响应。对于平行于电离室轴或平行于射束的磁场，对于高达1.1 T的场强，模拟响应最大增加1.1%。发现响应与电离室半径、磁场强度以及射束、电离室轴和磁场方向的取向存在复杂的依赖关系。特别是对于垂直于射束和电离室轴的磁场，在模拟中必须考虑精确的灵敏体积。为了最小化磁场校正因子以及死体积对Farmer电离室响应的影响，建议在剂量测定中采用磁场平行于电离室轴或平行于射束的测量设置。
