Magnetic resonance imaging and radiotherapy: Precision begins with imaging

Magnetic resonance imaging has played an increasingly important role in radiotherapy, both before and during treatment. Before irradiation, its excellent soft-tissue contrast has provided a major advantage over simulation computed tomography by improving the visualization of tumour volumes and organs at risk, and by reducing delineation uncertainties, particularly in high-precision treatments such as stereotactic radiotherapy. Beyond morphological information, magnetic resonance imaging has also provided access to functional and quantitative data, such as diffusion-weighted imaging, apparent diffusion coefficient maps, perfusion and T1/T2 mapping, which may contribute to the biological characterization of tissues and to the early assessment of treatment response. During irradiation, onboard magnetic resonance imaging has helped overcome some of the limitations of cone beam computed tomography, owing to improved soft-tissue visualization, motion monitoring, and adaptation of the treatment plan to inter- or intrafraction anatomical variations. The development of magnetic resonance-only workflows, based on synthetic computed tomography generation, could also reduce dependence on planning computed tomography. However, these benefits have imposed specific requirements: acquisition under treatment-position conditions, geometric distortion control, reproducibility of quantitative acquisitions, accurate computed tomography-magnetic resonance imaging registration, and rigorous validation. Their implementation requires harmonization of practices, strengthened coordination between imaging and radiotherapy, and official recommendations, or even a dedicated regulatory framework.