Magnetic fields with photon beams: Dose calculation using electron multiple-scattering theory

Strong transverse magnetic fields can produce large dose enhancements and r eductions in localized regions of a patient under irradiation by a photon b eam. We have developed a new equation of motion for the transport of charge d particles in an arbitrary magnetic field, incorporating both energy loss and multiple scattering. Key to modeling the latter process is a new concep t, that of ''typical scattered particles." The formulas which we have arriv ed at are particularly applicable to the transport of, and deposition of en ergy by, Compton electrons and pair-production electrons and positrons gene rated within a medium by a photon beam, and we have shown qualitatively how large dose enhancements and reductions can occur. A companion article exam ines this dose modification effect through systematic Monte Carlo simulatio ns. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(00 )00408-9].