OA025] Response of ionization chambers in the presence of magnetic fields

Abstract

Purpose The next generation of medical electron linear accelerators will integrate Magnetic Resonance Tomography (MRI). Therefore it will be possible to take direct images of the moving tumor during radiotherapy treatment. However serious consideration must be given to the strong magnetic field, which has an impact on the trajectories of the produced secondary electrons, because of the Lorentz force. This force affects both the dose distribution in water and the dose response of used detectors. For an accurate patient dosimetry these effects must be taken into account. Methods Monte Carlo methods correctly describe the radiation transport in different media, even in presence of magnetic fields and are therefore the gold standard for the evaluation of the impact of magnetic fields on clinical dosimetry. In this present study, the beam quality correction factors Kq and the relative response in magnetic fields of different ion chambers (PTW-31013, PTW-31021, EXRADIN-1ASL, NE2571) were investigated with Monte Carlo simulations, using the code EGSnrc. The chambers were modelled in detail according to the information given by the manufacturer and placed in a water phantom (30 × 30 × 30 cm3). The chambers were irradiated under reference conditions, following the recommendations of present dosimetry protocols, such as IAEA TRS-398: i.e. the field size at the phantom surface was 10 × 10cm2; the focus-surface-distance 100 cm and the depth of the chamber’s reference point was 10 cm. The source of the photons were several spectra of clinical medical accelerators applying nominal energies between 4 and 24 MV-X. The magnetic field was applied in different directions relative to the beam axis (z-direction) and the chamber’s symmetry axis and was varied between 0 and 3T. Results All chambers successfully passed the Fano test, the response of all chambers varied up to about 8% depending on the magnetic field strength and the field directions. Conclusions The response of the chambers with the magnetic fields, has a complex dependence on chamber radius, magnetic field strength, the orientation between radiation beams, chamber axis and magnetic field, and lastly the energy spectrum. Those variables consequently played a major role in the increase or decrease of correction factor, Kq.