Megavoltage photon beams interactions with any object on their path give rise to secondary photons and electrons. This fact is particularly important in Radiotherapy. These secondary particles produce an unwanted dose contribution both on beam path and outside the geometrical edges of the irradiation field. The goal of this report is to investigate the dose contribution due to secondary electrons. Ionization measurements were made in a polystyrene phantom by means of a Markus ionization chamber, with X-ray beams produced by a "SL5" Philips and by a "Saturne 43" GE CGR linear accelerators. Specific fields for nasopharynx treatment and for supra-diaphragmatic lymphatic chains treatment ("mantle" fields) were investigated. Both depth ionization values on beam path and depth and surface ionization values outside the geometrical edges of the field were obtained. These measurements were made with and without specific electron filters for 6, 15, 25 MV X-ray beams. The results obtained prove that, both on the beam path and outside the geometrical borders of the field, electron contamination reduction by means of specific filters is important only for lower energy X-ray beams. For this sort of beams, the percentage reduction of electron contamination was found. Moreover, the amount of electron contamination outside the field geometrical edge versus field size was evaluated.