目的:磁场校正因子kB,已通过实验研究了紧凑型空气填充电离室的Q,并使用高达1.5T的蒙特卡罗模拟。阐明了空气腔内非敏感区域的作用以及腔室结构对其剂量响应的影响。
方法:研究了PTWSemiflex3D31021,PinPoint3D31022和Sun核合作SNC125c腔室。KB,在德国国家计量研究所(PTB)的实验设施中,使用6MV光子束测量了高达1.4T的Q因子。将腔室定位成使得腔室轴线垂直于射束轴线(径向);并且平行于射束轴线(轴向)。在这两种情况下,磁场垂直于束轴和腔轴。此外,这些腔室的敏感体积已使用聚焦质子微束和有限元方法通过实验确定。除了模拟kB,Q因素,详细的蒙特卡罗技术已被用来分析空气腔内的二次电子通量,也就是说,二次电子的数量和作为磁场强度的函数的平均路径长度。
结果:根据微束测量和有限元计算,已经确定了PTW腔室的腔室杆附近的空气腔内的非敏感体积。在这项工作中研究的径向和轴向方向的磁场范围内,三个研究的电离室的剂量响应与无磁场情况的偏差不超过4%。模拟的kB,如果在模拟期间未正确考虑其敏感体积,则完全防护的PTW腔室的Q偏差高达6%。在实现从微束测量得出的敏感体积之后,实验和蒙特卡罗kB之间的一致性优于1%,可以实现所有三个腔室的Q因子。详细的分析表明,PTW腔室的茎可能会产生屏蔽效应,从而减少在存在磁场的情况下进入气腔的二次电子的数量。然而,它们在空气腔内的路径长度的磁场依赖性显示为比SNC125c腔室弱,其中气腔的长度大于其直径。对于该室,表明腔中电子的数量及其路径长度对磁场的依赖性更强。
结论:对于高达1.5T的临床测量,所需的kB,在所研究的两个腔室方向上,三个腔室的Q校正都可以保持在3%以内。结果重申,必须考虑到充分守卫的房间的敏感体积,即使是被调查的紧凑型房间,在磁场中燃烧室响应的蒙特卡罗模拟中。已证明所产生的磁场依赖性剂量反应取决于腔室的结构。例如空气腔的长度和直径之间的比率以及室杆的设计。
OBJECTIVE: The magnetic-field correction factors k B , Q of compact air-filled ionization chambers have been investigated experimentally and using Monte Carlo simulations up to 1.5 T. The role of the nonsensitive region within the air cavity and influence of the chamber construction on its dose response have been elucidated.
METHODS: The PTW Semiflex 3D 31021, PinPoint 3D 31022, and Sun Nuclear Cooperation SNC125c chambers were studied. The k B , Q factors were measured at the experimental facility of the German National Metrology Institute (PTB) up to 1.4 T using a 6 MV photon beam. The chambers were positioned with the chamber axis perpendicular to the beam axis (radial); and parallel to the beam axis (axial). In both cases, the magnetic field was directed perpendicular to both the beam axis and chamber axis. Additionally, the sensitive volumes of these chambers have been experimentally determined using a focused proton microbeam and finite element method. Beside the simulations of k B , Q factors, detailed Monte Carlo technique has been applied to analyse the secondary electron fluence within the air cavity, that is, the number of secondary electrons and the average path length as a function of the magnetic field strength.
RESULTS: A nonsensitive volume within the air cavity adjacent to the chamber stem for the PTW chambers has been identified from the microbeam measurements and FEM calculations. The dose response of the three investigated ionization chambers does not deviate by more than 4% from the field-free case within the range of magnetic fields studied in this work for both the radial and axial orientations. The simulated k B , Q for the fully guarded PTW chambers deviate by up to 6% if their sensitive volumes are not correctly considered during the simulations. After the implementation of the sensitive volume derived from the microbeam measurements, an agreement of better than 1% between the experimental and Monte Carlo k B , Q factors for all three chambers can be achieved. Detailed analysis reveals that the stem of the PTW chambers could give rise to a shielding effect reducing the number of secondary electrons entering the air cavity in the presence of magnetic field. However, the magnetic field dependence of their path length within the air cavity is shown to be weaker than for the SNC125c chamber, where the length of the air cavity is larger than its diameter. For this chamber it is shown that the number of electrons and their path lengths in the cavity depend stronger on the magnetic field.
CONCLUSIONS: For clinical measurements up to 1.5 T, the required k B , Q corrections of the three chambers could be kept within 3% in both the investigated chamber orientations. The results reiterate the importance of considering the sensitive volume of fully guarded chambers, even for the investigated compact chambers, in the Monte Carlo simulations of chamber response in magnetic field. The resulting magnetic field-dependent dose response has been demonstrated to depend on the chamber construction, such as the ratio between length and the diameter of the air cavity as well as the design of the chamber stem.