MR-Guided Radiotherapy

Magnetic resonance-guided radiotherapy (MRgRT) is an emerging modality that integrates high-resolution MRI with linear accelerators to enable superior soft-tissue visualization, intrafractional motion management, and online adaptive radiotherapy. This chapter provides a comprehensive review of the technical foundations, clinical workflows, and implementation strategies associated with MRgRT, focusing on the two commercially available systems: Elekta Unity (1.5 T) and ViewRay MRIdian (0.35 T). Key components of the MRgRT workflow are discussed, including MRI simulation, synthetic CT generation, beam configuration, plan optimization, and adaptive planning. Special emphasis is placed on the management of MR-specific challenges, such as geometric distortion, electron return effects (ERE), and the need for MR-compatible immobilization and QA protocols. The chapter also outlines criteria for patient selection, describes online adaptive planning strategies (ATP and ATS), and highlights site-specific clinical applications. This chapter aims to guide clinicians and physicists through the clinical translation of MRgRT, offering practical insight into treatment planning and adaptive radiotherapy in the MR-Linac environment.