Dosimetry of Ionizing Radiation in Magnetic Fields

Abstract

Magnetic resonance imaging guided radiotherapy (MRIgRT) is a new modality in radiotherapy. The benefits over conventional imaging guided radiotherapy are an increase in soft tissue contrast and the option of real time imaging. Recently, many clinics implemented first devices for this form of therapy (MR-linacs). These new devices offer magnetic resonance imaging but also include a linear accelerator for the treatment of patients. The idea is, that a magnetic resonance image can be acquired immediately before as well as during the irradiation of the patient. As the magnetic field of MR-linacs cannot be shut down easily, measurements for quality assurance are done under the influence of a strong magnetic field. Unfortunately, the magnetic field affects the response of radiation detectors. Because of this, current protocols for reference dosimetry are not applicable for MR-linacs. The aim of this work was to lay the foundations for a new protocol for reference dosimetry for MRIgRT. In this work, several experimental methods were established for the characterization of radiation detectors under the influence of magnetic fields. These included measurements at an experimental setup, that combined a big electromagnet with a conventional linac as well as measurements at MR-linacs. In addition, a simulation method for the quantification of the effects of magnetic fields on the response of ionization chambers has been developed by combining finite element with Monte-Carlo methods. The response of ionization chambers is influenced by magnetic fields. The change of response depends on several conditions, the most important are the geometry of the ionization chamber, the magnetic flux density, and the orientation of the ionization chamber. A dataset of magnetic field correction factors was derived for different types of ionization chambers. These correction factors have been validated by utilizing chemical radiation detectors. The derived correction factors can be used for dosimetry in MRIgRT. It was shown that the way in which Monte Carlo simulation of ionization chambers are done has to be adapted to account for an external magnetic field. A new technique has been developed to overcome the issues found for Monte Carlo simulations. Guidelines have been derived for reference dosimetry under the influence of a magnetic field. The foundation for a new protocol for reference dosimetry for MRIgRT were set in this work and Monte-Carlo methods were refined so that they can be used the questions related to MRIgRT in clinical radiation oncology.