ABSTRACT
Methods of calculating the dose distribution in patients undergoing external-beam radio-
therapy have gradually evolved over the years. The emphasis has justifiably been on photon
beams (see Chapter 25 and Chapter 26). The division of a broad beam into a large number of
narrow beams, or pencils, would seem to be a natural line of attack, given that this corresponds
to the way the particles, either photons or electrons, are incident on the phantom or patient.
However, it was not until the early 1980s that pencil-beam methods were first exploited, and
the first models were developed for electrons (Brahme et al. 1981; Hogstrom et al. 1981; Lax
et al. 1983) rather than for photons. This was possibly due to the fact that the inadequacies of
so-called broad-beam approaches (see Section 26.2) in modelling irregular fields and inhomo-
geneities are more dramatic for electrons than for photons (e.g. Figure 27.1).
