ESTRO 2021 Abstract Book

S93

ESTRO 2021

Abstract Text Intrafraction motion affects several tumour sites commonly treated with radiotherapy. For example, respiration causes tumour motion of up to several centimetres in the thorax & abdomen, cardiac activity causes high frequency motion in the mediastinum, and gastrointestinal activity causes irregular motion in the abdomen & pelvis. The main promise of hybrid MR-linac platforms is to “see what we treat while we treat” therefore enabling real-time adaptation to mitigate the detrimental effect of intrafraction motion on dose delivery. However, in a 2019 survey of 200 centres worldwide, MR-guided real-time adaptation was performed in only four centres. There was a pronounced interest to implement real-time adaptation more widely on any treatment platform, but high expectations were put on MR-linac systems to fulfil this need. Luckily, new developments in real-time MR-guided radiotherapy are as fast as they are exciting. In this presentation, we will address the three main pillars of real-time treatment adaptation – monitoring, adaptation, dose reconstruction – in the context of MR-guided radiotherapy. On-board MR imaging is a game-changer for the monitoring aspect of real-time adaptation owing to its high soft tissue contrast and flexible acquisition schemes. Cine MR imaging can provide high frequency (~4 Hz) 2D positioning information while 3D information can be obtained with interleaved acquisitions. 3D cine imaging enables slower motion to be surveyed (e.g. prostate drift) with repetition times in the order of 10 s. Higher frequency volumetric imaging (or volumetric image estimation) has also been demonstrated using motion models or machine learning. Real-time treatment adaptation itself is currently limited in clinical practice with only gating (in free- breathing or breath-hold) available on one commercial system. The implementation of more advanced adaptation strategies such as trailing, MLC tracking, or real-time replanning, have nevertheless been demonstrated experimentally or in-silico and are active research & development topics. Finally, the main impact of intrafraction motion is that the delivered dose is always different from the planned dose whether adaptation is used or not. Motion-including dose reconstruction can be performed offline, i.e. after treatment delivery, using treatment log files, for quality assurance and to record the actually delivered dose. Real-time dose calculation for dose-guided radiotherapy and real-time online replanning is a research topic of high interest. Abstract Text Quantitative imaging biomarkers (QIBs) derived from MRI techniques, like diffusion-weighted imaging and dynamic contrast-enhanced MRI, have the potential to be used for personalized treatment of cancer patients. Possible applications of QIBs for radiotherapy are the prediction of outcome based on pretreatment images, the assessment of treatment response with imaging after the completion of radiotherapy or biologically adaptive image-guided radiotherapy. However, large-scale data is missing to validate their added value in clinical practice. Integrated MRI-guided radiotherapy (MRIgRT) systems have the unique advantage that MR images can be acquired during every treatment session. This means that high-frequency imaging of QIBs becomes feasible with reduced patient burden, logistical challenges, and costs compared to extra scan sessions. In this way, a wealth of valuable data will be collected before and during treatment, creating new opportunities to advance QIB research at large. To make optimal use of the unique opportunity that MRIgRT systems offer, we need to ensure that the results of clinical trials on MRIgRT systems are generalizable outside the MRIgRT domain. To this end, the values from qMRI metrics from MRIgRT systems should be comparable to those from traditional diagnostic systems. In addition, to advance QIB development for adaptive radiotherapy, it is important that qMRI values over time are reliable. In this talk, we will present a roadmap towards the clinical use of QIBs on MRIgRT systems. As the integrated MRI scanner differs from traditional MRI scanners, technical validation is an important aspect of this roadmap. We propose to integrate technical validation with clinical trials by the addition of a quality assurance procedure at the start of a trial, the acquisition of in vivo test-retest data to assess the repeatability, as well as a comparison between QIBs from MRIgRT systems and diagnostic MRI systems to assess the reproducibility. These data can be collected with limited extra time for the patient. By including these elements in clinical trials on MRIgRT systems, the results of the trials will also be applicable for measurements on other MRI systems. SP-0149 A roadmap for imaging biomarker studies on MR-linac systems P. van Houdt 1 1 the Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands

Symposium: What's next for breathing motion management in radiotherapy?

SP-0150 Advanced motion management in MR guided radiotherapy D. O’Dwyer Denmark

Abstract not available

SP-0151 Advanced motion management in proton radiotherapy TBC

Abstract not available

SP-0152 Safely regularising breathing motion and prolonging single breath-holds for radiotherapy with a