Determining the Effective Source-Surface Distance for Therapeutic Electron Beams

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 92cm for 4, 6, 9, 12, and15mev, 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.