Proton PBS delivery within the treatment volume of an in-beam MR scanner


Proton PBS delivery within the treatment volume of an in-beam MR scanner

Sepúlveda, C.; Gebauer, B.; Schneider, S.; Hoffmann, A. L.; Lühr, A.; Burigo, L.

Introduction: The magnetic fringe field of an in-beam MR scanner integrated with a proton pencil beam scanning (PBS) beamline needs to be taken into account for accurate dose delivery of IMPT plans. This work investigates corrections to proton pencil beams when delivered in the treatment volume of an in-beam MR imager.
Materials and Methods: Monte Carlo (MC) simulations using TOPAS version 3.5 were applied to model the PBS dose delivery to the treatment volume of an in-beam MR imager at the PBS beamline at OncoRay. A 3D map of the full magnetic fringe field of the 0.33 T (vertical field) open MR imager was mapped out and incorporated in the MC simulations. To estimate the distortion of the beam profile in the treatment volume, the delivery of a 10x10 cm2 spot pattern for beam energies of 100, 150 and 200 MeV was simulated in air at the MR isocenter positioned 57 cm downstream of the beam isocenter. The energy-dependent mean lateral deflection was used to correct the beam delivery by a rigid shift of the field.
Results: Lateral deflections of 32.2 mm (100 MeV), 25.6 mm (150 MeV) and 22.2 mm (200 MeV) were observed for all spots. When correcting for these deflections, the mean error in the spot positions were 0.9  0.2 mm (100 MeV), 0.5  0.6 mm (150 MeV) and 0.7  0.1 mm (200 MeV), with maximum differences of 2.0, 2.3 and 0.9 mm, respectively. No distortion of the spot pattern was found.
Conclusions: A submillimeter error in the spot position at the isocenter of the in-beam MR scanner can be achieved for a 10x10 cm2 field when applying energy-dependent corrections in the delivery of the spots. Ongoing research will consider larger fields sizes and corrections needed to account for the beam stopping in water.

Keywords: MRiPT

  • Lecture (Conference) (Online presentation)
    Joint Conference of the ÖGMP, DGMP and SGSMP Dreiländertagung der Medizinischen Physik, 19.-22.09.2021, Wien, Österreich

Permalink: https://www.hzdr.de/publications/Publ-32509
Publ.-Id: 32509