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Area of Science:

  • Medical Physics
  • Radiation Oncology
  • Biophysics

Background:

  • Proton therapy (PT) integration with magnetic resonance (MR) imaging offers potential benefits.
  • However, magnetic fields can distort dose distribution via the electron return effect (ERE), enhancing dose at tissue-air interfaces.

Purpose of the Study:

  • To experimentally validate the ERE in proton beams.
  • To systematically characterize the dose enhancement ratio (DER) dependence on magnetic field strength, orientation, proton energy, and voxel size using simulations.

Main Methods:

  • EBT3 films were irradiated with 200 MeV protons under a 0.92 T transverse magnetic field to measure DER at shallow depths.
  • High-resolution Monte Carlo simulations reproduced experiments and calculated DER for energies 50-200 MeV and fields 0.35-3 T, analyzing voxel sizes of 0.05, 0.5, and 1 mm.

Main Results:

  • Measured DERs were 2.2% (±0.4%) at 0.156 mm and 0.5% (±0.6%) at 0.467 mm.
  • Simulations showed DER increases with magnetic field strength (2.6% to 8.2% from 0.35 to 1.5 T for 200 MeV protons) and proton energy (3.2% to 7.6% from 50 to 200 MeV at 1.0 T).
  • DER decreased significantly with increasing voxel size (8.2% in 0.05 mm vs. 1.4% in 1 mm) and depth, becoming negligible beyond 1 mm.

Conclusions:

  • The electron return effect in proton beams within transverse magnetic fields is experimentally verifiable.
  • The local dose enhancement is significant, predictable, and highly dependent on magnetic field strength, proton energy, and voxel size.
  • The impact of ERE on dose calculations in air-filled cavities and porous tissues like the lung must be considered for MR-guided proton therapy.