A magnetic resonance guided radiotherapy system comprising - a magnetic resonance examination system configured for acquiring magnetic resonance signals for monitoring of a structure motion of a subject within an examination zone; - a radiotherapy system configured for performing radiotherapy on the subject based on the structure motion and a radiotherapy plan, wherein the radiotherapy plan is optimized based on structure motion information determined during an initial quality assurance phase and wherein structure motion information is the structure motion itself or a statistical measure determined by the structure motion; wherein the radiotherapy system is configured to move a radiation source about the subject within the imaging zone; - the magnetic resonance examination system is also configured for monitoring during a quality assurance phase preceding the radiotherapy but following the initial quality assurance phase, wherein the quality assurance phase is a phase for determining if the radiotherapy plan can be safely delivered to the subject given the structure motion - wherein the magnetic resonance guided radiotherapy system further comprises - structure tracking software configured for structure tracking based on the magnetic resonance signals acquired for monitoring during the quality assurance phase, wherein the structure tracking software is configured to provide structure motion information to motion analysis; characterised by: - motion analysis software configured for calculating a quality factor for the radiotherapy, based on a comparison between structure motion information retrieved during the initial quality assurance phase with structure motion information retrieved during the quality assurance phase, wherein an outcome of the comparison is compared with at least one pre-set limit for the outcome. - a magnetic resonance examination system configured for acquiring magnetic resonance signals for monitoring of a structure motion of a subject within an examination zone; - a radiotherapy system configured for performing radiotherapy on the subject based on the structure motion and a radiotherapy plan, wherein the radiotherapy plan is optimized based on structure motion information determined during an initial quality assurance phase and wherein structure motion information is the structure motion itself or a statistical measure determined by the structure motion; - the magnetic resonance examination system is also configured for monitoring during a quality assurance phase preceding the radiotherapy but following the initial quality assurance phase, wherein the quality assurance phase is a phase for determining if the radiotherapy plan can be safely delivered to the subject given the structure motion - wherein the magnetic resonance guided radiotherapy system further comprises - structure tracking software configured for structure tracking based on the magnetic resonance signals acquired for monitoring during the quality assurance phase, - motion analysis software configured for calculating a quality factor for the radiotherapy, based on a comparison between structure motion information retrieved during the initial quality assurance phase with structure motion information retrieved during the quality assurance phase, wherein an outcome of the comparison is compared with at least one pre-set limit for the outcome.