Challenges for MR integrated Particle Therapy (MRiPT)

The increasing interest for particle therapy (PT) builds on its unique depth-dose characteristics, which are exploited to achieve a significant reduction in normal-tissue dose deposition proximal and distal to the tumor volume. At the same time, this feature makes PT more susceptible to morphological variations (i.e. anatomical changes and organ motion) and patient set-up uncertainties than conventional high-energy X-ray therapy (XT).
The integration of magnetic resonance (MR) imaging and PT (MRiPT) at treatment isocenter would offer an opportunity to fully exploit the dosimetric benefit of PT and realize its true clinical potential, especially for moving tumors in the thorax and abdomen. The unparalleled soft-tissue contrast and real-time imaging capabilities provided by MR imaging allow for online tumor tracking and plan adaptation. Given the steep dose gradients of PT, its targeting accuracy is expected to benefit even more from MR-guidance than XT performed with hybrid MR-linear accelerator systems. Therefore, as a next step in the technological development of image-guided radiation therapy, the concept of integrating real-time MR image guidance with PT has gained significant interest in the scientific community over the past few years.
In this presentation, a number of technological challenges will be discussed that need to be overcome before patient treatment with MRiPT can safely be realized. These challenges include the following aspects: (a) distortion of the proton dose distribution by the magnetic fields of the MR scanner, (b) impact on the MR image quality by the static and dynamic electromagnetic fields of a PT facility, and (c) integration of the MR and PT systems for online adaptive treatment. Furthermore, the current status of a first functional proof-of-concept system for in-beam MR imaging at a PT research beam line installed at OncoRay in Dresden, Germany will be presented.