Calculations of magnetic field correction factors for ionization chambers in a transverse magnetic field using Monte Carlo code TOPAS

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, 0.6 T, 1.0 T, 1.5 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 Farmer-type ionization chamber was also experimentally irradiated using cobalt-60 beams. To validate the simulation results, the k_B of the IBA FC65G 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.