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With the clinical introduction of MR-guided radiotherapy on hybrid MR-Linacs a new chapter in precision radiotherapy was opened. By integrating a linear accelerator mounted on a rotatable gantry with an MRI scanner, it became possible to leverage the exquisite soft-tissue contrast of MRI for visualisation of difficult-to-see tumours and to adapt the treatment plan to account for daily changes in anatomy (for example due to the different filling of hollow organs). Further, real-time imaging during treatment delivery can provide a basis for further real-time adaptation, if required. During the time in which daily contour adaptation and re-optimization of the treatment plan are carried out, there is a window of opportunity in current MR-Linac treatment workflows to explore different qMRI techniques without the burden of additional or prolonged examinations for the patient.

The translation of quantitative MRI techniques to the MR-Linac system had to overcome several technical challenges. Due to the need for radio-translucency and avoidance of irradiating through radiation-sensitive components, standard receiver arrays have a smaller number of individual coil elements and advanced RF technologies such as parallel transmit or higher order shimming might not be available. Furthermore, imaging gradients coils on the first clinical MR-Linac systems trade performance for spatial accuracy, resulting in lower peak amplitudes. The latter poses challenges in fast imaging and diffusion-weighted MRI in particular.

Despite these challenges a plethora of qMRI techniques were demonstrated feasible on clinical MR-Linac systems, including chemical exchange saturation transfer (CEST) imaging, MR-relaxometry including MR fingerprinting and diffusion-weighted imaging. For some of these techniques a particular aim was to ensure reproducibility of the quantitative measurements acquired on normal clinical scanners. The consensus guidelines for diffusion-weighted imaging on the Unity MR-Linac system are an exemplary consortium effort, where the matching of diffusion time, a parameter that is not on every MRI scanner directly accessible in the user interface, was identified as more crucial than the matching of b-values. Reproducibility of qMRI measurements on dedicated MRI scanners is an important consideration as follow-up examinations of patients after completion of radiotherapy are unlikely to be carried out on MR-Linac systems.

While qMRI faces challenges for integration on MR-Linac systems, it holds potential to gain insights into radiobiology, as measurements can be directly related to the delivered radiation dose. qMRI provides a unique opportunity to study hypofractionated or dose-escalated treatments, which become possible due to the reduced uncertainty resulting from daily adaptation on MR-Linacs. Due to the limited number of different MR-Linac systems clinically available, the fact that the same scanner platform is available to a large number of institutes is a stimulus for multi-centre clinical trials, in particular for the treatment of rarer cancers. Working on the same platform further supports and encourages the use of consensus protocols and analysis tools for qMRI.