Effect of the Air Gap between the Skin Surface and Bolus Material on the Relative Surface Dose in Electron Beam Therapy

Purpose: The existence of an air gap between the bolus material and the skin surface is often encountered in electron beam therapy. This study investigated the perturbation effect of air gaps under bolus material on the surface dose. Materials and Methods: The surface dose was measured for air gaps from a distance of 0.0cm to 2.0cm using 6, 9, 12, 16 and 20 MeV electron beams and various available cone sizes from 6×6, 10×l0, and 20×20 cm^2 on a linear accelerator Results: In the case of 6×6 cm^2 cone size with different thickness of boluses and electron energy, the relative surface dose decreased from 100% to 85.2% as the air gap was increased. In l0×l0 cm^2cone size the relative surface dose decreased from 100% to 93.7%. In 20×20 cm^2 cone size the relative surface dose decreased from 100% to 97.1%. Conclusions: These measurements showed that there was a tendency of relatively decreasing surface dose by lower electron energies, smaller field sizes, larger air gaps, and thicker boluses. It is necessary to correct the problem with individual dosimetry.