ESTRO 2022 - Abstract Book

S466

Abstract book

ESTRO 2022

SP-0534 Why quantitative MRI in radiotherapy

K.R. Redalen 1

1 Norwegian University of Science and Technology, Department of Physics, Trondheim, Norway

Abstract Text Modern radiotherapy techniques, such as volumetric arc therapy (VMAT), MR-guided radiotherapy with MR-Linacs as well as proton therapy, are very flexible treatment techniques where one can obtain high dose coverage in the tumor volume at the same time as organs at risk (OARs) are spared. This flexibility is also providing the opportunity to escalate the radiation dose to more radioresistant areas of the gross tumor volume without increasing the side effects. In radiation oncology, technology has contributed to an evidence-based scientific discipline determining favorable strategies for radiotherapy delivery with optimal radiation doses at the right time and place to get the optimal outcome. In parallel we have also seen technological developments in medical image acquisition and analysis that increasingly are providing faster and more detailed imaging for contouring of both targets and OARs, treatment planning, response prediction and evaluation, as well as quality assurance. Anatomical MRI has been used for a long time to identify healthy and diseased tissue, tumor, stage and location of the tumor. On the other hand, functional MRI comprises imaging methods that allow us to visualize a range of functional tissue properties using the more advanced protocols such as diffusion-weighted MRI, dynamic contrast-based MRI and susceptibility contrast MRI. These functional sequences can together with post-processing tools provide quantitative MRI (qMRI) measures of radiobiological tissue characteristics, which can be exploited to deliver more tailored radiotherapy to each patient, also having the possibility for adjustments during the course of treatment based on longitudinal changes in qMRI parameters. The use of qMRI may therefore result in a more optimized treatment where the tumor response is increased and normal tissue damage is decreased. The presentation will provide an overview of some of the applications associated with qMRI in radiotherapy: 1) more accurate and automated target and OAR delineation, 2) faster and more accurate qMRI parameter estimation, 3) possibilities for treatment stratification and response monitoring for treatment adaptation, and 4) qMRI maps as input to heterogeneous dose escalation or dose-painting of radioresistant subvolumes. Strategies for how qMRI may be used in these situations will be presented along with examples. The presentation will introduce some challenges and research questions that need to be solved for qMRI to move forward in radiotherapy. Abstract Text MRI offers many potential benefits to radiation therapy treatment planning because of the ability to visualize soft tissues. Additionally, quantitative MRI could be used to distinguish healthy from cancerous tissues. However, quantitative MRI does not always yield the same result across systems and even on the same systems, results can change with time. This has been demonstrated in vivo and in phantoms by several groups (Lee et al, MRM 2019; Stikov et al, MRM 2015; Keenan et al JMRI 2019). One option is to fix the system and software settings across sites, and this yields highly reproducible results (Weiskopf et al, Frontiers in Neuroscience 2013 and Gracien et al NeuroImage 2020). However, requiring certain systems and software versions is not always feasible. Instead, organizations such as RSNA-QIBA and EIBALL are or have developed recommended protocols for quality assurance measurements to enable reproducible quantitative MRI. Recent studies have demonstrated possible quality assurance protocols to assess quantitative MRI measurement repeatability, reproducibility and robustness (Carr et al JACMP 2021). An additional consideration is that some techniques, such as ADC, are highly repeatable across sites (Belli et al JMRI 2015; Winfield et al Radiology 2017), more than single-parameter relaxation measurements (Keenan et al PLOS 2021). In this talk, we'll review the technical challenges, discuss recent studies, and outline how to get started with quality assurance for quantitative MRI. 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom Abstract Text 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. SP-0536 Quantitative MRI on an MR-Linac A. Wetscherek 1 SP-0535 Technical challenges of qMRI K. Keenan 1 1 National Institute of Standards and Technology, Physical Measurement Laboratory, Boulder, USA

The translation of quantitative MRI techniques to the MR-Linac system had to overcome several technical challenges. Due