Longitudinal Magnetic-Field Modulation of Electron Dose Distributions in Homogeneous and Layered Tissue-Equivalent Phantoms

Purpose: To reproduce a 2008 Geant4 investigation of longitudinal magnetic-field effects on electron dose deposition and extend it from 40 MeV to 6 and 20 MeV, representative clinical electron energies. Methods: Monoenergetic parallel electron beams with a 20 cm² circular area irradiated 20 cm cubic bone, muscle, and 3 cm bone–17 cm muscle phantoms. Geant4 11.4.2 with FTFP_BERT_EMZ and step limitation transported 1,000,000 primary electrons for each of 27 combinations of energy (6, 20, and 40 MeV), longitudinal field (0, 2, and 5 T), and target. Dose was scored on a 256 × 256 × 256 grid with 0.78125 mm isotropic voxels. Results: The 5 T minus 0 T change in integral distal R50 was −0.004 to +0.006 cm across the nine energy-target combinations, whereas central-axis R50 changed by −0.029 to +3.251 cm. Lateral FWHM reduction ranged from −5.0% to +3.2%, with positive values denoting narrowing. Median relative statistical uncertainty in slices receiving at least 20% of the maximum dose ranged from 0.09% to 0.10%. Conclusion: Longitudinal fields altered central-axis dose at 20–40 MeV but produced negligible change in integral range and no consistent FWHM narrowing for this broad, uniform source. The deposit includes the preprint, four publication figures, and two numerical tables. Source code remains privately archived and is available for confidential review or from the author on reasonable request.