The size and shape of the treatment fields applied in radiotherapy account for uncertainties in the daily set-up of the patients during the treatment. We investigated the accuracy of daily patient positioning in the bellyboard pelvic radiotherapy in order to find out the magnitude of the patients movement during the treatment. Translational as well as rotational movements of the patients are explored. Film portal imaging is used in order to find patient positioning error during the treatment of the pelvic region. Patients are treated in the prone position using the bellyboard positioning device. Thirty six patients are included in the study; 15 patients were followed during the whole treatment and 21 during the first 5 consecutive treatment days. The image acquisition was completed in 85% and systematic and random positioning errors in 453 images are analyzed. Translation of the patient during the treatment caused set-up errors that ranged up to 30 mm and rotation of the sacrum ranged up to 14 degrees. We found out that most of the patients had time trend (drift of the position or angle during the time). This is predominant in the first few days while patient accommodate to uncomfortable prone position in the bellyboard. Safety margins that will ensure 90% probability of depositing at least 95% of the prescribed dose in the target are calculated according to translational movement of the patient. No action level, off line, set-up protocol is employed to correct patient position because of the translational movement. To correct for the rotation of the patient anatomy, correction of the custom shielding blocks should be employed.