Purpose: The integration of MR-imaging in a linear accelerator is a great advantage for radiotherapy treatments, as high-contrast, real-time imaging is possible without exposing extra dose to the patient. On the other hand, due to the resulting Lorentz force, the trajectories of the secondary electrons in the radiation field will be strongly affected and the response of ion chambers will change. This response change is taken into account by the magnetic field correction factor kB. In this study two different ionization chambers with sensitive volumes of 0.6 and 0.1 cm3 were investigated in a 6 MV photon field in external magnetic fields up to 1.5 T.
Methods: The Monte Carlo simulations were performed with the EGSnrc code system. Two ion chambers, the SNC 600c and SNC 125c from Sun Nuclear, were modelled according to the information provided by the manufacturer. The correction factor kB was calculated within a 6 MV photon field under TRS-398 reference conditions for different magnetic field directions and strengths. Moreover, the impact of the chambers’ dead volume and the thickness and density of the graphite wall on kB was investigated. The experimental determination of kB was performed at the German national metrology institute, PTB, using an ELEKTA Synergy accelerator and an electromagnet with flux densities up to 1.5 T.
Results: Monte Carlo calculated and measured dose values for the ionization chambers in presence of a homogeneous external magnetic field were in good agreement. The resulting kB values strongly depends on the chamber’s volume, the magnetic field strength and direction.
Conclusion: Using evaluated ionization chamber models, the Monte Carlo code EGSnrc is able to calculate magnetic field corrections kB, which are in good agreement with experimental determinations.
Monte Carlo, Dose, Magnetic Fields
TH- External Beam- Photons: Calibration protocol and primary standards