On-line MRI guidance for Radiotherapy

Image-guided radiotherapy has the potential to increase success of treatment by decreasing uncertainties concerning tumour position and shape. MRI is the modality of choice when it comes to imaging for tumour delineation and characterisation, set-up correction, treatment plan adaptation, response monitoring and intra-fraction motion compensation. To boost the possibilities of image guidance by providing images with superior soft-tissue contrast during treatment, integrated diagnostic quality mri functionality with a radiotherapy linear accelerator is pursued. To this end, an integrated MRI-linac was built in cooperation with Elekta AB (Stockholm, Sweden) and Philips (Best, The Netherlands). It features a 6 MV standing wave linear accelerator supported by a circular, continuously rotating gantry around a custom 1.5 T cylindrical MRI scanner with a split gradient coil. Solutions to two technical challenges related to MRI/linac integration are investigated in this thesis. First, the use of MR images in radiotherapy can be hindered by geometrical distortions due to magnetic field inhomogeneity problems. Techniques have been developed to deal with MR image distortions, or to prevent them from occuring at all. Secondly, the MRI scanner and linac must be interfaced seamlessly to allow advanced types of radiation delivery which compensate for target motion based on feedback from MR images. A software architecture was defined and implemented that facilitates the concept of real-time on-line MRI-guidance. Based on this framework a first demonstration of MRI-guided gated and tracked radiotion delivery to a moving target was given. Further research is needed to advance the technical prototype MRI-linac to the status of a clinical prototype.