Abstract. The objective of this study is to analyze the positioning uncertainties and assess the effect of dose distribution to fully optimize their margins. Using a treatment planning system to recalculate dose after the movement of the isocenter in different directions, the change in the dose distribution based on the number of images acquired in the positioning and size fields is analyzed. It is obtained an increase of the maximum dose up to 6%, 8% and 9% respectively on the lateral, longitudinal and vertical axis. The increased dose is of the order of 4% to 10 setup acquired images. This value decreases with decreasing field size. Any change in the positioning of the patient can have very serious consequences. It is therefore proposed that the positioning is controlled in the first three days and weekly with a maximum of 03 images every time. Keywords: Radiotherapy positioning margins, Uncertainties, dose distribution.