THE FE-LSPD MODEL FOR ELECTRON-BEAM DOSIMETRY

The FE-lspd model is a two-component electron beam model that distingu ishes between electrons that can be described by small-angle transport theory and electrons that are too widely scattered for small-angle tr ansport theory to be applicable. The two components are called the pri mary beam and the laterally scattered primary distribution (Ispd). The primary beam component incorporates a simple version of the Fermi-Eyg es model and dominates dose calculations at therapeutic depths. The Is pd component corrects errors in the lateral spreading of the primary b eam component, thereby improving the accuracy by which the FE-lspd mod el calculates dose distribution in blocked fields. Comparisons were ma de between dose profiles and central-axis depth dose distributions in small fields calculated by the FE-lspd, Fermi-Eyges and EGS4 Monte Car lo models for a 10 MeV beam in a homogeneous water phantom. The maximu m difference between the dose calculated using the FE-lspd model and E GS4 Monte Carlo is about 6% at a field diameter of about 1 cm, and les s than 2% for field sizes greater than 3 cm diameter. The maximum diff erence between the Fermi-Eyges and Monte Carlo calculations is about 1 8% at a field diameter of about 2.5 cm. A comparison was made with the central-axis depth dose distribution measured in water for a 3 cm dia meter field in a 10 MeV clinical electron beam. The errors in the dose distribution were found to be less than 2% using the FE-lspd model bu t almost 18% using the Fermi-Eyges model. A comparison was also made w ith pencil beam profiles calculated using the second-order Fermi-Eyges transport model.