H. Fuchs, P. Moser, M. Gröschl, D. Georg:

"Magnetic field effects on particle beams and their implications for dose calculation in MR-guided particle therapy";

Medical Physics,44(2017), 3; 1149 - 1156.

Purpose: To investigate and model effects of magnetic fields on proton and carbon ion beams for

dose calculation.

Methods: In a first step, Monte Carlo simulations using Gate 7.1/Geant4.10.0.p03 were performed

for proton and carbon ion beams in magnetic fields ranging from 0 to 3 T. Initial particle energies

ranged from 60 to 250 MeV (protons) and 120 to 400 MeV/u (carbon ions), respectively. The resulting

dose distributions were analyzed focusing on beam deflection, dose deformation, as well as the

impact of material heterogeneities. In a second step, a numerical algorithm was developed to calculate

the lateral beam position. Using the Runge-Kutta method, an iterative solution of the relativistic

Lorentz equation, corrected for the changing particle energy during penetration, was performed. For

comparison, a c-index analysis was utilized, using a criteria of 2%/2 mm of the local maximum.

Results: A tilt in the dose distribution within the Bragg peak area was observed, leading to non-negligible

dose distribution changes. The magnitude was found to depend on the magnetic field strength

as well as on the initial beam energy. Comparison of the 3 T dose distribution with non-B field (nominal)

dose distributions, resulted in a cmean (mean value of the c distribution) of 0.6, with 14.4% of

the values above 1 and c1% (1% of all points have an equal or higher c value) of 1.8. The presented

numerical algorithm calculated the lateral beam offset with maximum errors of less than 2% with calculation

times of less than 5 ls. The impact of tissue interfaces on the proton dose distributions was

found to be less than 2% for a dose voxel size of 1 9 1 9 1 mm3.

Conclusion: Non-negligible dose deformations at the Bragg peak area were identified for high initial

energies and strong magnetic fields. A fast numerical algorithm based on the solution of the energycorrected

relativistic Lorentz equation was able to describe the beam path, taking into account the

particle energy, magnetic field, and material. © 2017 American Association of Physicists in Medicine

[https://doi.org/10.1002/mp.12105]

carbon ion, characterization, ion beam therapy, magnetic field, MR, proton

Created from the Publication Database of the Vienna University of Technology.