Determining the Effective Source-Surface Distance for Therapeutic Electron Beams

Abstract

Background: The source point of the irradiated electron beam must be considered to estimate the output factor and dose distribution during electron therapy. Objectives: The aim of this study was to determine the effective source-surface distance (SSDeff) of an electron linear accelerator (Linac), and its dependence on energy and depth. Materials and Methods: A Varian Linac 2100CD with electron energies of 4, 6, 9, 12, and 15 MeV, electron applicator size of 20 × 20 cm2, nominal SSDs of 97 to 113 cm, and air gaps of 2 to 18 cm were studied. Using a Farmer (0.13 cc) ionizing chamber, the percentage depth doses were measured in the water phantom (50 cm3) and then the SSDeff was calculated by applying the inverse square law. Results: For the 100% PDD, the SSDeff values were calculated as 79, 91, 92, 93, and 92 cm for 4, 6, 9, 12, and 15 MeV, respectively. At a depth with a certain PDD, increasing energy also increases SSDeff, and a similar increase is observed at a distinctive energy by increasing the PDD. Conclusions: Using the maximum dose depth from PDD curves and the inverse square law, the required SSDeff to calculate the dose distribution of the electron beam can be calculated.