Radiation dosimetry in magnetic fields with Farmer-type ionization chambers: determination of magnetic field correction factors for different magnetic field strengths and field orientations
Radiation dosimetry in magnetic fields with Farmer-type ionization chambers: determination of magnetic field correction factors for different magnetic field strengths and field orientations
复制标题
DOI：
10.1088/1361-6560/aa7ae4
复制
发表时间：
2017-08-21
影响因子：
3.5
通讯作者：
Pfaffenberger, A.
中科院分区：
文献类型：
--
作者：
Spindeldreier, C. K.;Schrenk, O.;Pfaffenberger, A.
本工作的目的是确定不同磁场强度和磁场方向的Farmer型电离室MR引导放射治疗混合器械中剂量测定所需的磁场校正因子。在一个水箱中测量了6个定制的Farmer型腔室在6 MV直线加速器上辐照的响应，该水箱位于磁场强度在0.0 T和1.1 T之间的磁体中。腔室轴、射束和磁场彼此垂直，并对两个磁场方向进行了研究。EGSnrc蒙特卡罗模拟进行了比较的测量和模拟不同的磁场方向进行。对于所有几何形状，计算磁场校正因子k（B）（Q）和扰动因子。磁场垂直于腔室和射束轴时，腔室响应最大增加8.8%。在蒙特卡罗模拟中，通过调整燃烧室杆附近的死体积层，可以再现所测量的燃烧室响应。对于平行于腔室轴或平行于束流的磁场，模拟的响应增加了1.1%，在最大的场强高达1.1 T。一个复杂的依赖性的响应被发现对腔室半径，磁场强度和定向的束，腔室轴和磁场方向。特别是对于垂直于射束和腔室轴的磁场，在模拟中必须考虑精确的敏感体积。为了最大限度地减少磁场校正因子和死体积对Farmer室响应的影响，建议采用磁场平行于室轴或平行于射束的测量设置进行剂量测定。
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, k(B)(Q), 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.