Electron beams have an important role in the treatment of superficial and shallow cancers in modern radiotherapy [1]. Because of the electron interactions with the accelerator head materials including scattering foils, monitoring chambers, photon jaws, and the applicators, they may be thought of as emanating from the virtual source that is not in the real position of the accelerator exit window. The International Commission on Radiation Units and Measurements (ICRU) Report No. 35 [2] , defines an effective extended electron source as the source which when placed in a vacuum at some distance SSD from the phantom surface (Z=0) would produce exactly the same electron fluence at Z=0 as the actual beam. The distance from this source position to the patient's skin surface is called the effective SSD. To obtain an accurate dose calculation of electron beams, correction for measured effective SSD must be applied. In addition, effective SSD may be used to calculate output at extended SSD for corrections of central axis percentage depth dose (PDD) [3] , off-axis dose (OAD) values, output factors, and X-ray contamination [4]. Hence, it is essential that these corrections should be examined for each treatment unit prior to referral to treat at extended SSD using electron beams. The SSD eff is dependent on the electron energy and the field size used.