Electron dose calculation using multiple-scattering theory: a hybrid electron pencil-beam model.

We have embedded the bipartition model into Fermi-Eyges multiple-scattering theory to produce a more accurate hybrid electron pencil-beam model, by using the fact that away from the edges of a large field, the electron distribution function exactly equals that for an infinitely wide electron beam. The bipartition model calculates various electron transport quantities in a homogeneous or horizontally layered medium with very high accuracy, for an infinitely broad beam. In our hybrid model, we use the bipartition model to calculate the longitudinal part of the pencil-beam distribution function, and the Fermi-Eyges theory to calculate its transverse part. Doing this allows calculation not only of dose distribution, but also of such quantities as electron fluence distribution, energy spectrum, angular distribution, and electron-charge distribution. Using the hybrid electron pencil-beam model, we have calculated the dose distribution for collimated electron beams and compared them with experimental data, for rectangular fields.