RESUMEN
In this work the effect of patient immobilization was studied in cases of head and neck malignancies, which treated in the supine position with a cobalt-60 machine at Clinical Oncology Department in Tanta University Hospitals. Four patients were immobilized using orfit and planned on the x-ray diagnosis machine with and without the orfit. Different headrest types were used to detect which of them help in the reproducibility from day to day. Then portal images were obtained to perform weekly "on-line" setup errors. Comparison between manually dose distribution and those calculated using C.T. planning images and calculated by planning system were investigated are done. By measurements and calculation the HORTON's equation of shadow tray factor must be development