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İstanbul Üniversitesi Cerrahpaşa Tıp Fakültesi Radyasyon Onkolojisi AD, İstanbul Due to the anatomical obstructions electron beam treatment is sometimes performed at extended source to skin distances. To eliminate the calculation errors in output which arise from the inverse square law the effective source to skin distances were determined and the variation with field sizes and electron energies were examined. Measurements were made on Saturne 42 linear accelerator using the electron energies 6, 9, 12 and 15 MeV on 4x4, 6x6, 8x8, 10x10, 15x15, 20x20, 25x25 cm2 field sizes. A 0.6 cm3 PTW ionization chamber was placed in a solid water phantom and readings were recorded from a Victoreen 525 model electrometer. I0 is the reading for depth d at nominal source to skin distance=100 cm, I is the reading for a gap g (distance between the measurement nominal source to skin distance and 100 cm). When plotted the square root of the ionization ratio (Io/I)1/2 aganist the air gap g the slope of the straight line gives us the effective source to skin distance positions for each energy and field size. The variation of effective source to skin distance with electron energies and field sizes was studied. Effective source to skin distance varies with electron energies and field sizes. Due to the lack of lateral scatter equilibrium we observed small effective source to skin distances for small fields, whereas for larger field sizes the lateral scatter equilibrium exists, therefore we observed larger effective source to skin distances. The results showed that the output of the patients who undergo at extended source to skin distances have to be calculated by using the inverse square law from the effective source to skin distances especially for small field sizes and low energies. Keywords : source to skin distance, electron, linear accelerator