Determination of square equivalent field for rectangular field in electron therapy.

Equivalent field for electron beams is considered by using pencil beam theory. According to the Fermi-Eyges model the dose distribution of an electron pencil beam has a Gaussian profile. For this function determination of mean square radial displacement scattering of electrons is important. In this study the contribution of back scatter electron has been taken into account by using the multiple scattering theories for calculating mean square radial displacement scattering. The dimension of standard equivalent field depends on depth and shape of treatment field. Here the depth under study is the depth that mean square radial displacement scattering is extremum and the shape of treatment field is rectangular. In this study four energies were used 6, 9,12 and 15 MeV electron beams of 2100C/D Varian Linac. Findings of this study are based on analytical calculations, which are in good agreement with other experimental data. The findings of this study that were resulted from formula, shows, for all circular fields of radius ≥LSE (lateral scattering equilibrium) were considered broad field and equivalent. For validating the findings, Percentage Depth Dose (PDD) and Output factors were measured in 15 MeV electron beams for 7 × 3-cm, 6 × 4-cm and 4 × 2-cm and their equivalent squares and equivalent circular fields and compared.