Prototype system for first-in-human MR-integrated proton therapy combining a 0.32T open MR scanner with a horizontal proton pencil beam scanning beamline

Target audience: Audience interested in image-guided radiation therapy, innovative use-cases for MRI and experimental hardware developments.

Purpose: The physical integration of magnetic resonance imaging (MRI) with proton therapy (PT) into an MR-integrated PT (MRiPT) system is expected to improve the targeting accuracy of PT [1]. However, a successful integration has so far only been achieved in a proof-of-concept study [2], which demonstrated that in-beam MR imaging during proton beam irradiation at a fixed PT beamline is feasible. The purpose of this study was to develop a prototype system combining a low-field MRI scanner with a proton pencil beam scanning (PBS) beamline to enable a first in-human MRiPT treatment. This contribution presents first results of the installation and commissioning of the MRiPT system where the positioning reproducibility, magnet shimming performance and image quality were analyzed.

Methods: The setup consists of an open C-shaped 0.32 T MRI scanner (MRJ3300, ASG Superconductors SpA, Genoa, Italy) positioned in close proximity of the nozzle of a horizontal proton PBS beamline (Figure 1). The MRI scanner, which utilizes a permanent magnet, was encased in a custom-designed compact aluminum Faraday cage. At the location of the beam exit window of the nozzle, a beam entrance opening was incorporated in the wall of the RF cage, which was sealed by a thin (40 µm) aluminum foil to combine high RF attenuation and small lateral spreading of the traversing proton beam. The scanner and RF cage were mounted on top of an air-cushion-based transport platform, allowing the assembly to be accurately positioned in the beam path exiting the nozzle. The maneuvering of the assembly into treatment position was thereby visually guided based on room lasers that intersect at the natural beam isocenter and project onto the outer wall of the cage. The magnet was shimmed in treatment position close to ferromagnetic components of the nozzle where field homogeneity was measured using a magnetic field camera (MFC3045, Metrolab Technology SA, Geneva, Switzerland). During commissioning the MR image quality was assessed both quantitatively and quantitatively using the ACR Small MRI Phantom [3] and images of a healthy volunteer’s extremities acquired in coronal and transversal directions using various scan protocols (T1w SE, T2w TSE, T2w STIR, T1w 3D GE), respectively.

Results: The positioning accuracy and precision of the mobile in-beam MRI system was below 1 mm. A peak-to-peak B0 field homogeneity of 43 ppm over a 25 cm diameter spherical volume (DSV) around the MR magnetic isocenter was achieved during magnet shimming. Phantom imaging revealed a signal-to-noise ratio (SNR) of >80 and a geometric distortion of <1 mm over a 10 cm DSV around the magnetic isocenter. The quality of the MR images was deemed clinically useful for structural imaging and promising for the localization and monitoring of extremity soft-tissue tumors.

Discussion: Tumor visualization capabilities should be further investigated in patients with malignant soft-tissue tumors. A full workflow for patient positioning and irradiation is currently under development, including the development of accurate methods for PT treatment planning and dose verification that take into account the presence of the MR magnetic fields during dose delivery.

Conclusion: A 0.32 T in-beam MRI scanner was successfully installed and commissioned in front of a horizontal proton PBS beamline in preparation for the development of a first clinical prototype MRiPT system. Further developments in patient positioning, dosimetry and treatment planning are indispensable before a first in-human irradiation can be safely performed.

References:

[1] A. Hoffmann et al. 2020 Radiat. Oncol.
[2] S. Schellhammer et al. 2018 Phys. Med. Biol.
[3] Small Phantom Guide 4/17/18. American College of Radiology