Dose distributions in high energy electron beams: production of broad beam distributions from narrow beam data.

In a high energy electron beam the spatial distribution of absorbed energy is dominated by multiple scattering of the electrons. The dose distribution in a broad parallel electron beam entering a uniform phantom can be resolved into a series of narrow beams overlapping one another, in each of which the scattering conditions are identical. It is therefore possible to build up a broad beam dose distribution from a knowledge of the axial and radial dose distribution in a narrow beam. Measurements have been made of such narrow beam distributions using finely collimated electron beams in the energy range 4-10 MeV, and the resulting data added by computer to give broad beam distributions which are compared with measured isodose curves. The method has been extended also to construct isodose distributions in heterogeneous phantoms. The advantages of storing electron beam data in the form of narrow beam distributions are described with particular reference to the problem of irradiating heterogeneous tissue volumes.