German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany.
Faculty of Physics and Astronomy, University of Heidelberg, Heidelberg, Germany.
Phys Med Biol. 2024 Oct 15;69(20). doi: 10.1088/1361-6560/ad67a5.
To investigate magnetic field effects on the dose distribution and ionization chambers response in carbon ion reference fields and determine magnetic field correction factors for chambers of different volumes.The response of six Farmer-type chambers with varying radii (1-6 mm, termed as R1-R6) was measured in magnetic fields up to 1 T in 0.1 T increments using an experimental electromagnet and compared with Monte Carlo simulations. Chamber readings were measured in the entrance region of a monoenergetic carbon ion beam of 390.75 MeV u. A lower energy of 200.28 MeV uwas applied to chamber R3 for comparison. Polarity and recombination corrections were investigated for the R3 chamber. The local dose change induced by the magnetic field was calculated by Monte Carlo, which together with change of the chamber's response, was used to calculate the final magnetic field correction factors.The dependence of the chamber response on the magnetic field was non-linear and volume-dependent. Maximum changes ranged from 0.30% (R4) to 0.62% (R5) at 0.2 T. For R3, the response for the lower energy was systematically decreased by 0.2% in the range of 0.2 T to 0.7 T. No significant effect of the magnetic field on polarity and ion recombination correction was found. The maximum variation of the local dose was found to be (0.03 ± 0.08) % at 0.2 T for beam energy of 390.75 MeV u. Magnetic field correction factors for the different chambers ranged from 0.28% (R4) to 0.60% (R5).This study provides the first detailed analysis of chambers' response to magnetic flux densities of up to 1 T using chambers of different radii and comparison with simulations. By combining the chamber response alterations with local dose changes magnetic field correction factors were calculated for all six chambers, including the commercial Farmer-type chamber.
为了研究磁场对碳离子参考场中剂量分布和电离室响应的影响，并确定不同体积电离室的磁场校正因子，我们使用实验电磁铁在 0.1 T 的增量下测量了六个半径（1-6 毫米，分别称为 R1-R6）不同的 Farmer 型电离室在高达 1 T 的磁场中的响应，并与蒙特卡罗模拟进行了比较。在 390.75 MeV u 的单能碳离子束的入口区域测量了腔室读数。为了进行比较，将 200.28 MeV u 的较低能量应用于腔室 R3。对 R3 腔室进行了极性和复合修正的研究。通过蒙特卡罗计算了磁场引起的局部剂量变化，将该变化与腔室响应的变化结合起来，计算了最终的磁场校正因子。腔室响应对磁场的依赖性是非线性和体积依赖性的。在 0.2 T 时，最大变化范围从 0.30%（R4）到 0.62%（R5）。对于 R3，在 0.2 T 到 0.7 T 的范围内，较低能量的响应系统地降低了 0.2%。没有发现磁场对极性和离子复合修正有显著影响。在 390.75 MeV u 的束能下，在 0.2 T 时，局部剂量的最大变化为（0.03 ± 0.08）%。不同腔室的磁场校正因子范围从 0.28%（R4）到 0.60%（R5）。本研究首次使用不同半径的腔室进行了高达 1 T 的磁场通量密度的详细分析，并与模拟进行了比较。通过结合腔室响应变化和局部剂量变化，计算了所有六个腔室（包括商业 Farmer 型腔室）的磁场校正因子。
