Simulation and experimental verification of magnetic field induced proton dose enhancement effects

Purpose/Objective
Proton therapy (PT) is expected to benefit greatly from integration with magnetic resonance (MR) imaging due to its sensitivity to anatomical variations. Consequently, the concept of MR-guided PT (MRPT) receives increased interest. Previous studies on MR-guided photon therapy (MRXT) have reported local dose enhancement of up to 40% at tissue-air interfaces caused by the electron return effect (ERE) in transverse magnetic fields. For MRPT, however, no consensus on the magnitude and hence the clinical effect of the ERE can be found in the scarcely available literature. The objectives of this study were 1) to confirm the ERE for PT by measurements and 2) to determine its magnitude for clinically relevant proton energies and MR field strengths by simulation.
Material/methods
Measurements were performed with a collimated 200 MeV proton beam traversing a PMMA phantom made of one or two 10 mm vertical slabs. Dose was measured with GafChromic EBT3 films (PMMA equivalent thickness 0.312 mm) using two experimental setups: (A) as reference, one film sandwiched between two slabs and (B) two films attached to the distal end of one slab, resulting in effective measurement depths of 10.156, 0.467, and 0.156 mm from the air interface. Film irradiations were performed under the same conditions without and within a transversal field (0.92 ± 0.02 T) of a permanent magnet. All measurements were repeated 4 to 8 times and the entire experiment was performed twice.
Monte Carlo simulations were performed using Geant4 (V 10.3). The proton beam shaping devices, magnetic field and PMMA slabs were modelled in detail. The EBT3 films were simulated as PMMA slabs and dose was scored in PMMA from 25 to 1000 μm distance to the air interface. Additionally, field strengths were varied between 0.35 and 1.5 T for a 210 MeV proton beam as well as proton energy between 90 and 210 MeV at 1 T. The dose enhancement ratio was defined as dose with divided by dose without magnetic field: DB/D.
Results
Significant dose enhancement was measured at the PMMA-air interface with magnetic field compared to no field (p<0.01) and confirmed by repeated experiments. The dose enhancement decreased with increasing distance from the interface (Fig. 1). Good agreement was achieved between measured and simulated dose both with and without magnetic field.
The dose enhancement ratio was largest in simulations with strong magnetic fields increasing from 2.0% in the presence of a 0.35 T field up to 7.4% for a 1.5 T field near the interface (Fig. 2). A decrease of the proton energy resulted in a decreasing dose enhancement ratio.
Conclusion
For the first time, the ERE for proton beams in a transverse magnetic field was demonstrated experimentally. The significant dose enhancement is predictable and limited to within 1 mm from the air interface for clinically relevant proton energies and magnetic field strengths.
Although smaller than for MRXT, the ERE may affect the clinical treatment of e.g. lung tumors.