The purpose of this study was to calculate the magnetic field correction factor $k_B$ for cylindrical ionization chambers and to examine the effect of orientations and beam energies on $k_B$ under different magnetic field strengths using Monte Carlo code TOPAS. Monte Carlo simulations were performed for NE2571, IBA FC65G, IBA FC65P and PTW31010 chambers in a water phantom using data provided by manufacturers and those reported in published literatures. The transverse magnetic field orthogonal to the photon beam was modeled as being parallel or perpendicular to the long axis of the ionization chamber with the magnetic field strength of 0 T, 0.35 T, 0.60 T, 1.0 T, 1.50 T, respectively. The photon beam spectrum of 7 MV Elekta Unity was considered. To quantify the effect of the beam energy on $k_B$, the IBA FC65G chamber was also experimentally irradiated using cobalt-60 beams. To validate the simulation results, the $k_B$ of an IBA FC65G Farmer-type ionization chamber at 1.5 T was compared to data in the literature. The results showed that for the parallel configuration, all magnetic field correction factors $k_B$ differed from unity by less than 2.1%. On the other hand, for the perpendicular configuration, changes up to 5.5% compared to simulations without the magnetic field were observed for 1.0 T. For the PTW31010 chamber, the change was smaller between these two configurations. It was also found that the beam energy had an impact on the $k_B$. When irradiated by the cobalt-60 beams in both configurations, the $k_B$ of IBA FC65G varies similarly within 2% with the case irradiated by 7 MV Elekta Unity beams. The comparison of $k_B$ values from this study with those from the literatures indicate that satisfactory $k_B$ values within 1% and thus can be simulated utilizing TOPAS Monte Carlo simulation methods reported in this study. The magnetic field correction factor $k_B$ at 0.6 T and 1.0 T can be considered as a reference for these two machines at the commissioning stage in the future.

这项研究的目的是计算磁场校正因子 ${ķ}_{乙}$ 用于圆柱电离室，并检查方向和射束能量对 ${ķ}_{乙}$在不同的磁场强度下使用蒙特卡罗代码TOPAS。使用制造商提供的数据和已发表文献中的数据对水模中的NE2571，IBA FC65G，IBA FC65P和PTW31010腔室进行了蒙特卡洛模拟。垂直于光子束的横向磁场被建模为与电离室的长轴平行或垂直，磁场强度分别为0 T，0.35 T，0.60 T，1.0 T和1.50T。考虑了7 MV Elekta Unity的光子束光谱。量化光束能量对${ķ}_{乙}$，IBA FC65G室也使用60钴光束进行了实验辐照。为了验证仿真结果，${ķ}_{乙}$将1.5 T下的IBA FC65G农夫型电离室的最大通气量与文献中的数据进行了比较。结果表明，对于并联配置，所有磁场校正因子${ķ}_{乙}$与统一的差异小于2.1％。另一方面，对于垂直配置，与没有磁场的模拟相比，在1.0 T下的变化高达5.5％。对于PTW31010腔室，这两种配置之间的变化较小。还发现光束能量对${ķ}_{乙}$。在两种配置中，当钴60束辐照时，${ķ}_{乙}$对于7 MV Elekta Unity光束照射的情况，IBA FC65G的光强变化类似地在2％以内。比较${ķ}_{乙}$ 这项研究的值与文献中的值表明，令人满意 ${ķ}_{乙}$值在1％以内，因此可以使用本研究报告的TOPAS蒙特卡罗模拟方法进行模拟。磁场校正系数${ķ}_{乙}$ 将来在调试阶段，这两种机器的0.6 T和1.0 T的参考值可以视为参考。