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Abstract

Magnetic resonance-guided radiation therapy (MRgRT) enables online adaptive treatment planning and beam delivery based on the visualization of actual location of tumor and normal tissues. This allows for the better tumor coverage and normal tissue sparing. However, magnetic fields affect the dose distribution and the detector response, demanding correction for accurate dose measurements. This thesis investigates the response alteration of six Farmer-type ionization chambers in a carbon ion reference field in presence of magnetic fields of 0.1 to 1 T. A complex dependency on magnetic field strength and chamber volume was obtained with maximum corrections of 0.6 %. Measurements and Monte Carlo simulations revealed a good agreement. Additionally, small field characteristics of photon fields at the 6 MV/0.35 T ViewRay MR-Linac were investigated. Output factors for small fields were measured for two different detectors (PTW PinPoint 3D and microDiamond) for isocentric square and rectangular fields as well as for off-axis fields. Field output factors obtained from Monte Carlo calculations were used to derive output correction factors, which may be used when performing clinical dosimetry of small fields. The derived correction factors reduce the uncertainties of dose measurements in carbon ion reference and small photon fields in presence of magnetic fields.