The MR-Linac Technical Feasibility Protocol

Radiation therapy is predominantly based on a Computed Tomography (CT) scan obtained during the treatment planning phase. During the course of radiotherapy, however, both the tumor and organs at risk (OAR) are variable in position, shape and size between fractions, and during beam delivery within one treatment fraction. Around the clinical target volume (CTV) a safety margin (Planning Target Volume, PTV) is created to account for these geometric uncertainties to ensure that the tumor receives the prescribed dose. Direct integration of imaging at the linear accelerator enables daily monitoring of patient positioning, tumor position, and alterations in patient anatomy. Image guided radiotherapy (IGRT) enables the detection and immediate correction of such deviations and increases the precision of delivery. Adaptive radiotherapy has become an important strategy for serially modifying dose distributions in a manner that can substantially reduce OAR dose and subsequent toxicity, while maintaining adequate target coverage. Current adaptive protocols rely upon a standard CT-based workflow and (cone-beam) CT-based image guidance. Magnetic Resonance Imaging (MRI) has superior soft-tissue contrast over CT and seems a very promising modality to integrate in the radiation treatment process, facilitating better visualization of the tumor and OAR during treatment.