A model for calculating electron beam scattering in treatment planning.

The Fermi-Eyges theory of electron scattering overestimates the scattering of electron beams used in radiation therapy. The reason for this overestimate is the neglect of the loss of electrons which are scattered into highly oblique paths and removed from the beam at relatively shallow depths. A modification of Eyges' solution to Fermi's equation is presented to take this loss of electrons into account. Equations for the calculation of isodose distributions for any medium using pencil beams are developed. Experimental confirmation is presented for electron beams of 13 and 18 MeV in homogeneous water, polystyrene, Lucite, and aluminum phantoms.