Systematic optimization of the double-scatterer system for electron beam field-flattening

The scattering foil widely used for field flattening in electron therapy produces a distortion of the electron spectrum which significantly reduces the slope of the descending part of the depth dose curve. It is already known that this effect can be drastically reduced by replacing the scattering foil by a two-foil scatterer. The second foil, fit with a radial thickness profile, acts as a compensator, attenuating the central part and enhancing the peripheral part of the beam by electron scattering. Calculations of the optimal foil combinations and radial thickness profiles are presented and numerical values of smooth (Gaussian) and three-step profiles for various field sizes are provided. The results are applicable to a variety of electron energies and scattering foil materials.