Determining the Effective Source-Surface Distance for Therapeutic Electron Beams

The source point of the irradiated electron beam must be considered to estimate the output factor and dose distribution during electron therapy.
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.
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.
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.
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.