Magnetic field-induced proton dose enhancement: Experimental verification and Monte-Carlo simulation

Introduction
Proton therapy (PT) is expected to benefit from integration with magnetic resonance (MR) imaging. However, the magnetic field distorts the dose distribution and induces a local dose enhancement at tissue-air interfaces by the electron return effect (ERE). For MR-integrated photon therapy, a dose enhancement ratio (DER) of 40% compared to no magnetic field has been reported. Here, measurements and calculations of DER for proton beams in transverse magnetic fields are reported.

Methods
Two measurement setups were used: EBT3 films were either attached to the distal face of one or sandwiched between two 10 mm PMMA slabs. Films were irradiated with a 200 MeV proton beam, both with and without transverse magnetic field (0.92 T). High-resolution Monte-Carlo simulations were used to reproduce the experimental findings and to calculate the DER for proton energies between 50−200 MeV and magnetic field strengths between 0.35−3 T within the first 0.05 mm (DERmax) and as function of distance from the air interface.

Results
A DER of (2.2±0.4)% and (0.5±0.6)% was measured at 0.156 and 0.467 mm from the interface, respectively (Fig.1). Measurements and simulations agreed within 0.15%. Simulations using a 200 MeV beam showed a DERmax of 2.6% and 8.2% for 0.35 and 1.5 T, respectively (Fig.2). At 1 T, DERmax increased from 3.2% to 7.6% between 50 and 200 MeV.
Conclusion
For proton beams, the ERE in transverse magnetic fields is measurable. The dose enhancement is well predictable, decreases with distance from the interface, and is negligible after 1 mm. Although small, the impact of the ERE cannot be ignored for dosimetry with air-filled ionization chambers and in porous media (e.g. lung treatment).