The use of scattering foil compensators in electron beam therapy.

This study was undertaken to show that scattering foils could be used as electron beam compensators. Two scattering foils were designed to improve the dose homogeneity when a curved surface is irradiated with electrons. One scattering foil, constructed of mylar, was designed for use with a single 6 MeV electron field. The second scattering foil, constructed of lead, was designed to homogenize the dose along the matchline of two abutting electron fields. Measurements with the second compensator were made at energies of 6, 9, and 12 MeV. The compensators were mounted over the topmost opening of the electron cone. A simple method for modeling the effect of the second compensator using a conventional treatment planning system was also evaluated. When the mylar compensator was placed atop the 25 x 25 cm cone and a Rando phantom irradiated with 6 MeV electrons, the dose at the field edges was increased by about 10%. Use of the lead foil compensator for two abutting fields gave a highly uniform dose along the matchline, without perturbing the isodoses at depth or at the other field edges. Measured hot spots in the Rando phantom for 6 and 9 MeV electrons were 104 and 108%, respectively. The effect of the lead foil compensator was successfully modeled on a conventional treatment planning system by summing beams of different field sizes. Both compensators were effective in improving the dose homogeneity within the target volume.