Longitudinal Magnetic Fields on Photon and Electron Radiotherapy Beams

Abstract. Monte Carlo studies have proposed that strong longitudinal magnetic fields can reduce the lateral scattering of primary and secondary electrons to reduce penumbra. In this paper we describe an experimental apparatus using a 3.5 Tesla superconducting solenoidal magnet to investigate the effect of longitudinal magnetic fields on electron and photon radiotherapy beams. The effects were studied using film in air and in phantoms which fit in the magnet bore. The magnetic field focussed and collimated the electron beams. The converging, non-uniform field confined the beam and caused it to converge with increasing depth in the phantom. Due to the field’s collecting and focussing effect, the beam flux density increased leading to increased dose deposition near the magnetic axis, especially near the surface of the phantom. Though the technique introduces some challenges associated with electron focussing, it shows promise for confining and focussing primary and secondary electrons during treatment. The dose delivered by electron and photon radiotherapy beams results from the displacement and transport of electrons. These electrons are either primary particles, in the case of electron beams, or secondary particles set in motion by primary photon beams mostly through Compton recoil interactions. Electrons, once set in motion, scatter an