Magnetic fields with photon beams: Monte Carlo calculations for a model magnetic field

Strong transverse magnetic fields can produce very large dose enhancements and reductions in localized regions of a patient under irradiation by a photon beam. We have suggested a model magnetic field which can be expected to produce such large dose enhancements and reductions, and we have carried out EGS4 Monte Carlo calculations to examine this effect for a 6x6 cm2 photon beam of energy 15, 30, or 45 MV penetrating a water phantom. Our model magnetic field has a nominal (center) strength B0 ranging between 1 and 5 T, and a maximum strength within the geometric beam which is 2.2xB0. For all three beam energies, there is significant dose enhancement for B0 = 2 T which increases greatly for B0 = 3 T, but stronger magnetic fields increase the enhancement further only for the 45-MV beam. Correspondingly, there is major reduction in the dose just distal to this region of large dose enhancement, resulting from secondary electrons and positrons originating upstream which are depositing energy in the dose-enhancement region rather than continuing further into the patient. The dose peak region is fairly narrow (in depth), but the magnetic field can be shifted along the longitudinal axis to produce a flat peak region of medium width (approximately 2 cm) or of large width (approximately 4 cm), with rapid dose dropoffs on either side. For the 30-MV beam with B0 = 3 T, we found a dose enhancement of 55% for the narrow-width configuration, 32% for the medium-width configuration, and 23% for the large-width configuration; for the 45-MV beam with B0 = 3 T, the enhancements were quite similar, but for the 15-MV beam they were considerably less. For all of these 30-MV configurations, the dose reductions were approximately 30%, and they were approximately 40% for the 45-MV configurations.