ESTRO 2025 - Abstract Book

S21

Invited Speaker

ESTRO 2025

4679

Speaker Abstracts Real-time MR-integrated proton therapy Aswin L. Hoffmann Medical Radiation Physics, OncoRay – National Institute for Radiation Research in Oncology, Dresden, Germany Abstract: It is undisputed that image guidance plays a pivotal role in modern radiotherapy to achieve high local control rates while keeping side effects within tolerable levels. Technological developments in onboard imaging for high-energy X ray therapy (XT) have enabled daily treatment adaptation for anatomical changes that occur in between or during treatment fractions. The rapid adoption of hybrid MR-LINAC systems combining magnetic resonance imaging (MRI) and XT has enabled online adaptive treatment workflows based on high soft-tissue contrast images acquired prior to, during or after dose delivery, allowing dose distributions to be tailored to the individual patient’s changing anatomy with a high targeting precision. Proton therapy (PT) is know to be more sensitive to patient setup uncertainties and anatomical changes than XT due to tissue compositional changes along the beam path and the finite range of the beam. To account for these uncertainties when treating moving tumours, relatively large safety margins around the tumor volume are currently used. This, however, compromises the targeting accuracy of PT for moving tumours. Hence, the dosimetric benefit that PT could offer is not yet fully exploited. The absence of real-time imaging tools offering high-soft tissue contrast to localize and track moving tumours and to synchronize dose delivery with tumour motion is considered a major hindrance to exploiting its full dosimetric potential and outperform image-guided XT with MR-LINAC systems. To break this barrier, following the successful clinical introduction of MR-LINAC systems, the PT community has been triggered to investigate the feasibility of MRI integration into PT. However, the full integration of real-time MRI and PT initially presented a plethora of unresolved technical challenges, beyond those for MR-LINAC, starting from MR scanner integration with the PT beam line, through electromagnetic field interactions between the MR and PT system, the impact of the MR magnetic field on proton beam transport, treatment planning, dosimetry and radiobiological effectiveness, MR-only based treatment planning, online treatment adaptation and quality-assurance, up to online MRI-based proton beam range verification. To overcome these challenges, prototype in-beam MRI systems for MR-integrated proton therapy (MRiPT) have been built, enabling these knowledge gaps to be addressed. After having demonstrated proof-of-principle with the world’s first MRiPT system based on a 0.22T compact in-beam MRI scanner in 2017, the world’s first whole-body MRiPT prototype based on a 0.5T rotatable open MRI scanner has recently been installed. This system offers real-time imaging and enables treating patients in both recumbent and upright position with a horizontal beam. Besides prototype development, substantial progress has been made on proton dosimetry and treatment planning in magnetic fields. The full dosimetric validation and commissioning of a first Monte Carlo based commercial treatment planning system that incorporates the 3D magnetic vector field of an in-beam MR scanner into PT dose calculation is currently ongoing. With no technical showstoppers identified so far, clinical and regulatory preparations for a first-in-human trial have already been initiated. In analogy to the clinical introduction of MR-LINAC treatments, this will initially be for non moving target volumes. The purpose of this trial is to demonstrate the clinical feasibility and safety of online, high precision, MRI guidance of PT using a first clinical prototype MRiPT system. The first patient is expected to be treated inside a compact in-beam MR scanner in the very near future. For MR-guided PT of moving tumours, work is in progress to address the remaining challenges to be solved before a first patient can be treated with the real-time MRiPT prototype system. These include artefact-free imaging during irradiation, implementation and commissioning of MRI-guided beam-gating, 4D dosimetry, workflow development, and comparative treatment planning studies demonstrating the expected clinical benefit of real-time MRiPT over CBCT-guided PT, offline MR-guided PT and MR-LINAC treatments for relevant indications in the thorax, abdomen and pelvis. The concept of real-time MRiPT clearly leverages other current innovations in PT, by uniting online adaptive, upright, arc therapy into one system. Since real-time MRiPT is expected to elevate the accuracy and effectivenes of PT for moving tumours beyond the level of MR-LINAC, it is considered a disruptive innovation that will expand the range of