MAGNETIC FIELD INFLUENCE ON THE RESPONSE OF THE EXRADIN A1SLMR IONIZATION CHAMBER

Recently, dedicated MR-compatible ionization chambers have become commercially available for dosimetry in hybrid devices for MR-guided radiotherapy (MRgRT). The response of ionization chambers is known to depend on the chamber type, the magnetic flux density and the orientation between chamber axis, beam and magnetic field [1-3]. The aim of this study was to investigate the response of the MR-compatible Exradin A1SLMR slimline miniature ionization chamber in magnetic fields of different magnetic flux densities and field orientations. The chamber response to a 6 MV photon beam was measured in a water phantom positioned in an experimental electromagnet (magnetic flux densities between 0 - 1.1 T). The magnetic field was aligned perpendicular to the chamber and beam axis. The response was simulated using the Monte Carlo code EGSnrc and compared to the measurement. Moreover, simulations for magnetic fields parallel to the chamber and parallel to the beam were performed. For a magnetic field perpendicular to the chamber and the beam, the response increased by maximally 3.7%. Measured and simulated results showed a good agreement. For a magnetic field parallel to the chamber and parallel to the beam, only small changes in the simulated response were observed.