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

• Published in: Medical physics (Lancaster) 2017-03, Vol.44 (3), p.1149-1156
• Main Authors: Fuchs, Hermann, Moser, Philipp, Gröschl, Martin, Georg, Dietmar
• Format: Article
• Language: English
• Subjects: Algorithms; carbon ion; characterization; Computer Simulation; Feasibility Studies; Heavy Ion Radiotherapy - methods; ion beam therapy; magnetic field; Magnetic Fields; Magnetic Resonance Imaging - methods; Models, Theoretical; Monte Carlo Method; proton; Proton Therapy - methods; Radiotherapy Dosage; Radiotherapy, Image-Guided - methods; Water
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1002/mp.12105

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 γ‐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 γmean (mean value of the γ distribution) of 0.6, with 14.4% of the values above 1 and γ1% (1% of all points have an equal or higher γ 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 μs. The impact of tissue interfaces on the proton dose distributions was found to be less than 2% for a dose voxel size of 1 × 1 × 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 energy‐corrected relativistic Lorentz equation was able to describe the beam path, taking into account the particle energy, magnetic field, and material.