Integration of MRI and proton therapy: motivation, challenges and roadmap

• Aswin Hoffmann (UKDD)

Image guidance plays a pivotal role in modern radiation therapy to achieve high local control rates and keep side effects within tolerable levels. Technological developments in onboard imaging for high-energy X-ray beam 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 imaging, 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 beam therapy (PT) is considered a major hindrance to exploiting its full potential to outperform image-guided XT for soft-tissue tumours in general, and those that are subject to motion in particular. The successful clinical introduction of the MR-LINAC systems triggered the PT community to consider the feasibility of MRI guidance for PT. However, the full integration of real-time MRI and PT presents a plethora of technical challenges, starting from scanner integration with the beamline, 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 presentation 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 PT 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 PT inside an in-beam MRI device. These include artefact-free imaging during irradiation, workflow development, and comparative treatment planning studies demonstrating the expected clinical benefit of real-time in-beam MR-integrated PT over CBCT-guided PT, offline MR-guided PT and MR-LINAC treatments for relevant indications in the thorax, abdomen and pelvis.

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