Source of Support: None, Conflict of Interest: None
Electron beams from an accelerator appear to come from a point in vacuum designated as virtual source that lies at a distance not coinciding with the scattering foil or the beam exit window. This is due to the influence of the various scattering processes taking place in the foils, collimator and the electron applicator apart from the fringe magnetic fields in the blending magnet. Virtual source to isocentric distances are experimentally determined by measurements made in a polystyrene phantom using an ionization chamber and applying inverse square law. The results for the narrow and broad beam geometry are presented for the electron beams available with Clinac-1800.