ABSTRACT
The introduction given in Chapter 5 also serves as an introduction to this chapter. In this chapter, we will give an overview of the work performed in the field of Monte Carlo modeling of clinical electron beams over the last 30 years or so. It is widely believed that to calculate a patient dose correctly in electron beams, Monte Carlo simulations are needed, more so than for radiotherapy photon beams where there exist alternative dose calculation methods that rival Monte Carlo simulations in accuracy and performance. For electron beams, the situation is different, with many papers exposing flaws in analytical dose calculation methods, even in modern treatment planning systems. It is, therefore, not surprising that Monte Carlo simulations are seeing a widespread acceptation in the radiotherapy community.
Electron beam radiotherapy is administered much less frequently than photon beam treatment, most commonly as a boost to a small superficial target. The energies used nowadays mostly range from 4 to 20 MeV. The schematics of an electron beam linac for Monte Carlo modeling are represented in Figure 6.1. The most important differences with a photon beam linac are the absence of the photon target, the presence of a scattering foil to broaden the beam, and a multistage collimator to shape the beam close to the irradiated volume. The latter is needed because electrons scatter much more in air than photons and they therefore require collimation close to the surface of the patient. Monte Carlo modeling of electron beams will be treated more concisely in this chapter than photon beam modeling because of its less frequent clinical use. The BEAM Monte Carlo code is again the most heavily used tool in many studies.
