Session Item

Image guidance plays an undisputed pivotal role in modern radiation therapy to achieve high local control rates and keep side-effects within tolerable levels. Technological developments in on-board imaging for photon beam therapy have enabled daily treatment adaptation for anatomical changes that occur in between or during treatment fractions. The rapid adoption of real-time MR-LINAC systems since its clinical introduction in 2014 has enabled online adaptive treatment workflows based on high soft-tissue contrast magnetic resonance imaging (MRI), allowing dose distributions to be tailored and delivered to the individual patient’s changing anatomy with a high targeting precision.
The lack of fast, high soft-tissue contrast image-guidance in particle therapy is considered a major hindrance for exploiting its full potential to outperform image-guided photon therapy for soft-tissue tumors in general, and those that are subject to motion in particular. The successful clinical introduction of the MR-LINAC systems triggered the particle therapy community to consider the feasibility of MRI guidance for particle therapy. Different scenarios for that include near-room, in-room, and in-beam MRI. An overview of potential clinical indications for these scenarios is presented and discussed. Whereas near-room and in-room MRI may account for interfractional changes and enable offline and online adaptive treatments, respectively, but require post-imaging patient repositioning and do not account for intrafractional changes, in-beam MRI would facilitate imaging in treatment position, both prior to and during dose delivery. The latter real-time approach will improve the targeting precision for moving soft-tissue tumors, which represent a large proportion of clinical cases treated with particle therapy. The added value of MRI-integration in particle therapy is hence expected to be even more favorable than in photon therapy.
However, the full integration of real-time MRI and particle therapy presents a plethora of technical challenges, starting from scanner integration with the beam line, through dosimetry and treatment planning in the presence of the MR magnetic fields, MR-only based treatment planning, online treatment adaptation and quality assurance, up to online MRI-based range verification.
This contribution provides an overview of recent developments and milestones achieved by various groups on the aforementioned topics. A roadmap for the clinical introduction of in-beam MR-integrated proton therapy is presented, showing ongoing research efforts to address the knowledge gaps and remaining challenges to be solved before a first patient can be safely treated with particle therapy inside an in-beam MRI device. These include artifact-free imaging during irradiation, workflow development, and comparative treatment planning studies demonstrating the expected clinical benefit of real-time in-beam MR-integrated particle therapy over CBCT-guided particle therapy, offline MR-guided particle therapy and MR-LINAC treatments for relevant indications in the thorax, abdomen and pelvis.