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MR-integrated proton therapy: challenges and current status

Kolloquium der Abteilung 6

Precise coverage of the tumor volume in proton therapy is even more challenging than in conventional (photon-based) radiotherapy, because protons are more sensitive to anatomical variations (e.g., organ motion and deformation) and patient set-up inaccuracies than photons. This is due to the steep dose fall-off behind the Bragg peak and to the fact that the range of the proton beam strongly depends on the material composition in the beam path. These uncertainties currently translate into relatively large safety margins, thus compromising the dosimetric benefit of proton therapy. This urges the need for image guidance during proton beam delivery. Magnetic resonance imaging (MRI) offers real-time image guidance with unparalleled soft-tissue contrast and absence of radiation dose. Proton therapy is therefore expected to benefit from the integration with real-time MRI. However, a number of hitherto open technological problems have to be solved before MR-integrated proton therapy (MRiPT) can be implemented:

1.       Lorenz-force induced dose distortions need to be quantified and taken into account during treatment planning.

2.       The mutual electromagnetic interaction between the MRI scanner and the proton therapy system needs to be assessed and compensated for during simultaneous operation of these systems.

3.       The magnetic interaction with dosimetry detectors may compromise the results of such measurements. Quality assurance procedures of proton therapy in the presence of magnetic fields have to be established.

4.       For on-line treatment planning, fast and accurate methods that are able to translate MR image information into electronic stopping power need to be developed.

This presentation provides a detailed overview of these challenges and the current status of MRiPT. In particular, first experimental evidence from our group will be shown for (1) the dosimetric effects of proton beams in the presence of magnetic fields, and (2) the mutual electromagnetic interaction between a proton therapy and an MRI system. Initial investigations to solve problems (3) and (4) will be also addressed.