Electron beam modifications for the treatment of superficial malignancies.

For the treatment of superficial tumors, the surface dose should be high; unfortunately, because of pronounced dose buildup in low energy electron beams, their efficacy for such treatment is reduced. Electron beams can be modified by placing a low atomic number material called a beam spoiler in the beam. In general, the surface dose is a function of electron energy, source to surface distance, field size, thickness of beam spoiler, distance of beam spoiler from surface, atomic number of beam spoiler, and angle of the beam. The effects of these parameters are evaluated with respect to surface dose, bremsstrahlung dose, and field size changes for small fields at standard SSD and electron energies from 6 to 17 MeV. It was found that the use of a beam spoiler can generally increase the surface dose to values exceeding 90% of the maximum buildup value while maintaining a bremsstrahlung dose less than 3%. Changes in field size related to the placement of the beam spoiler were considerable in some cases.