ABSTRACT
Stereotactic irradiation, either in the form of stereotactic radiosurgery [SRS] or stereotactic radiotherapy [SRT], of brain lesions requires high precision accuracy in the isocenter, the main determinants were gantry and couch rotations. It is thus necessary to evaluate the isocenter variation due to gantry and couch rotations in the particular setup for SRS/SRT. This paper describes the accuracy of Patients' immobilization and the variation in the isocenter of a Siemens PRIMUS M6/6ST linear accelerator-modified for adapting a couch-mounted radiosurgery system. The accuracy of Patients immobilization was excellent in over twenty patients with 95 setups and 2375 scalp measurements. Ninety two percent of measurements had no displacements from the baseline readings, 7% had 1 mm and one percent had 2 mm displacements. The variations in isocenter position with the table and the gantry rotation were found to be < 0.4 mm with a compounded accuracy of