Dosimetry with ionization chambers and optically stimulated luminescence detectors in magnetic fields

Abstract

Hybrid magnetic resonance guided radiotherapy (MRgRT) devices allow imaging the patient during treatment by means of non-ionizing radio-frequency. This technology enables real-time image guided radiotherapy, thereby it is expected to improve treatment precision and clinical outcome. In order to use hybrid MRgRT devices clinically, dosimetry needs to be performed in the static magnetic field of the magnetic resonance imaging scanner for commissioning and quality assurance measurements. For this purpose, the applicability of two detector types - ionization chambers and optically stimulated luminescence detectors (OSLDs) - for dosimetry in magnetic fields was investigated in this work by means of experimental measurements and Monte Carlo simulations. The ionization chambers show a response that depends on the magnetic field strength and orientation as well as on the sensitive chamber volume. Likewise, the effective point of measurement is influenced by these parameters. In order to account for these effects, magnetic field correction factors were calculated. Also the response of OSLDs was found to depend on the magnetic field strength and direction. No substantial change in their angular dependence due to the magnetic field could be measured. In conclusion, both detector types can be used for precise dosimetry in high magnetic fields when applying the calculated magnetic field correction factors or when calibrating the detectors directly in the magnetic field.