الفهرس 
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Abstract
Patient immobilization and position are important contributors to the reproducibility and accuracy of radiation therapy. In addition the choice of position can alter the external contour of the treated area and has the potential to alter the spatial relationship between internal organs. The imaging and target volume definition have demonstrated changes in the calculated therapeutic ratio comparing the different groups for masks used. The planning target volume (PTV) would impact on dose–volume histograms to organs at risk (OAR). We also determined if any such improvement might permit dose escalation. The purpose of this study was to evaluate the repositioning accuracy of different mask time used in order to determine their applicability for potential use in conformal radiation therapy for patients. Fifty patients with different tumors in Head and Neck were included. The Patients were divided into 5 groups determined according to the time for Masks used. group 1 was positioned using a thermoplastic mask (only one used). group 2 to 5 were positioned using different types of mask (used one to four times before). The time of presentation determined which position device was used. Port films of the fifty patients were reviewed retrospectively, and the repositioning precision was recorded by measurements in three orthogonal planes. group 1 significantly better repositioning accuracy (P < 0.05) compared to group 2 to 5. The position variation was significantly different (P < 0.04) between group 3, 4 and 5 in the lateral and longitudinal direction highly significantly (P<0.0013).