Experimental setup to measure magnetic field effects of proton dose distributions: simulation study


Experimental setup to measure magnetic field effects of proton dose distributions: simulation study

Schellhammer, S.; Oborn, B.; Lühr, A.; Gantz, S.; Wohlfahrt, P.; Bussmann, M.; Hoffmann, A.

Purpose/Objective:

As a first step towards proof-of-concept for MR-integrated proton therapy, the dose deposited by a slowing down proton pencil beam in tissue-equivalent material is assessed within a realistic magnet assembly. Furthermore, radiation-induced activation and demagnetization effects of the magnet are studied.

Material/Methods:

The dose distributions of proton pencil beams (energy range 70-180 MeV) passing through a transverse magnetic field of a permanent C-shaped NdFeB dipole magnet (maximum magnetic flux density Bmax = 0.95 T) while being stopped inside a tissue-equivalent slab phantom of PMMA were simulated (Figure 1). The beam was collimated to a diameter of 10 mm. A radiochromic EBT3 film dosimeter was placed centrally between the two phantom slabs parallel to the beam’s central axis. 3D magnetic field data was calculated using finite-element modelling (COMSOL Multiphysics) and experimentally validated using Hall-probe based magnetometry. A Monte Carlo model was designed using the simulation toolkit Geant4.10.2.p02 and validated by reference measurements of depth-dose distributions and beam profiles obtained with Giraffe and Lynx detectors (IBA Dosimetry), respectively. The beam trajectory and lateral deflection were extracted from the film’s planar dose distribution. Demagnetization was assessed by calculating the dose deposited in the magnet elements, and by relating this to radiation hardness data from literature. A worst-case estimate of the radioactivation of the magnet was obtained by taking into account the most common produced mother nuclides and their corresponding daughter nuclides.

Results:

The Monte Carlo model showed excellent agreement with the reference measurements (mean absolute range difference: 0.2 mm). The predicted planar dose distribution clearly showed the magnetic field induced beam deflection (Figure 2). The estimated in-plane deflection of the Bragg peak ranged from 0 cm for 70 MeV to 1 cm for 180 MeV in comparison to no magnetic field. No out-of-plane beam deflection was observed. Exposing the film to 2 Gy at the Bragg peak was estimated to cause a mean dose to the magnets of 20 μGy, which is expected to produce negligible magnetic flux loss. The initial activation was estimated to be below 25 kBq.

Conclusion:

A first experimental setup capable of measuring the trajectory of a proton pencil beam slowing down in a tissue-equivalent material within a realistic magnetic field has been designed and built. Monte Carlo simulations of the design show that magnetic field induced lateral beam deflections are measurable at the energies studied and radiation-induced magnet damage is expected to be manageable. These results have been validated by irradiation experiments, as reported by Lühr et al. in a separate abstract.

  • Lecture (Conference)
    ESTRO 2017, 05.-09.05.2017, Wien, Österreich

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Publ.-Id: 24214