The effect of beam angulation on central axis per cent depth dose for 4-29 MeV electrons

Electron beams are playing an increasing role in the areas of intraoral and intraoperative radiotherapy. Because of the way the electron beams are used in these two procedures, they are rarely incident normally on the surface of the tissue to be treated, but are inclined at some angle. Previous work has shown that when low-energy electron beams are used in this way, the per cent depth-dose curves, when measured along the oblique axis (the central axis of the beam) show a decrease compared with normal incidence. To quantify this effect in detail the per cent depth dose distributions of electron beams in the energy range 4-29 MeV have been studied as a function of their angle of incidence with respect to the normal (phi) between 0 and 60 degrees. The curves for different phi are compared by plotting the per cent depth dose along the central axis in each case: at low energies, this decreases as phi increases, while at the highest energies, the change is very small. The surface dose also changes, increasing with phi at low energies and decreasing at high energies.