Direct electrons are those which do not include electrons scattered from the beam defining system of a treatment head. The difference in surface doses between total and that from direct electrons is indicative of the number and angle of scattered electrons in the beam. The difference between the maximum electron and contaminant photon energy reflects the total thickness of vacuum exit window, scattering foils, beam monitor chamber, mirror and air in the beam. The maximum contaminant photon energy is about the kinetic energy of the incident electron at the exit vacuum window. Table 2 presents the simulation data of clinical electron beams from five medical accelerators.
E is the incident electron kinetic energy at the exit vacuum window of an accelerator.
It is adjusted to match the calculated R to that of the measurement. The data presented in table 2 are for an SGI Indigo with an R4400 cpu. This is due to the combination of the broad angular distributions and the low-energy spectra of electrons from applicators.