German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany.
Faculty of Medicine, University of Heidelberg, Heidelberg, Germany.
Med Phys. 2023 Jul;50(7):4590-4599. doi: 10.1002/mp.16368. Epub 2023 Mar 29.
Magnetic resonance-guided proton therapy is promising, as it combines high-contrast imaging of soft tissue with highly conformal dose delivery. However, proton dosimetry in magnetic fields using ionization chambers is challenging since the dose distribution as well as the detector response are perturbed.
This work investigates the effect of the magnetic field on the ionization chamber response, and on the polarity and ion recombination correction factors, which are essential for the implementation of a proton beam dosimetry protocol in the presence of magnetic fields.
Three Farmer-type cylindrical ionization chambers, the 30013 with 3 mm inner radius (PTW, Freiburg, Germany) and two custom built chambers "R1" and "R6" with 1 and 6 mm inner radii respectively were placed at the center of an experimental electromagnet (Schwarzbeck Mess - Elektronik, Germany) 2 cm depth of an in-house developed 3D printed water phantom. The detector response was measured for a 3 × 10 cm field of mono-energetic protons 221.05 MeV/u for the three chambers, and with an additional proton beam of 157.43 MeV/u for the chamber PTW 30013. The magnetic flux density was varied between 0.1 and 1.0 Tesla in steps of 0.1 Tesla.
At both energies, the ionization chamber PTW 30013 showed a non-linear response as a function of the magnetic field strength, with a decrease of the ionization chamber response of up to 0.27% ± 0.06% (1 SD) at 0.2 Tesla, followed by a smaller effect at higher magnetic field strength. For the chamber R1, the response decreased slightly with the magnetic field strength up to 0.45% ± 0.12% at 1 Tesla, and for the chamber R6, the response decreased up to 0.54% ± 0.13% at 0.1 Tesla, followed by a plateau up to 0.3 Tesla, and a weaker effect at higher magnetic field strength. The dependence of the polarity and recombination correction factor on the magnetic field was ⩽0.1% for the chamber PTW 30013.
The magnetic field has a small but significant effect on the chamber response in the low magnetic field region for the chamber PTW 30013 and for R6, and in the high magnetic field region for the chamber R1. Corrections may be necessary for ionization chamber measurements, depending on both the chamber volume and the magnetic flux density. No significant effect of the magnetic field on the polarity and recombination correction factor was detected in this work for the ionization chamber PTW 30013.
磁共振引导质子治疗很有前途，因为它将软组织的高对比度成像与高度适形的剂量输送相结合。然而，在磁场中使用电离室进行质子剂量测定具有挑战性，因为剂量分布以及探测器响应都受到干扰。
本研究旨在探讨磁场对电离室响应的影响，以及对极性和离子复合校正因子的影响，这些因子对于在磁场存在下实施质子束剂量测定协议至关重要。
将三个 Farmer 型圆柱形电离室（PTW，德国弗莱堡），内半径为 3mm 的 30013 型和两个分别具有 1mm 和 6mm 内半径的定制的“R1”和“R6”型，放置在一个实验性电磁体（德国 Schwarzbeck Mess-Elektronik）的中心，深度为 2cm，在一个内部开发的 3D 打印水模体中。对于三个电离室，测量了三个能量为 221.05MeV/u 的单能质子 3×10cm 场的探测器响应，以及能量为 157.43MeV/u 的质子束的 PTW 30013 电离室的响应。磁场强度在 0.1 至 1.0 特斯拉之间以 0.1 特斯拉的步长变化。
在这两种能量下，电离室 PTW 30013 的响应均表现出与磁场强度的非线性关系，在 0.2 特斯拉时，电离室响应降低了高达 0.27%±0.06%（1 SD），随后在更高的磁场强度下，影响较小。对于 R1 室，响应随着磁场强度的增加略有降低，在 1 特斯拉时降低了高达 0.45%±0.12%，对于 R6 室，响应在 0.1 特斯拉时降低了高达 0.54%±0.13%，随后在 0.3 特斯拉时达到一个平台，在更高的磁场强度下影响较弱。对于 PTW 30013 室，极性和复合校正因子对磁场的依赖性小于 0.1%。
在低磁场区域，PTW 30013 室和 R6 室的磁场对室响应有较小但显著的影响，在高磁场区域，R1 室的磁场对室响应有较小但显著的影响。取决于电离室体积和磁通密度，可能需要对电离室测量进行校正。在这项工作中，对于电离室 PTW 30013，没有检测到磁场对极性和复合校正因子有显著影响。
