CHARACTERISTICS OF A DEDICATED LINEAR ACCELERATOR-BASED STEREOTAXIC RADIOSURGERY RADIOTHERAPY UNIT

A stereotactic radiosurgery and radiotherapy (SRS/SRT) system on a ded icated Varian Clinac-600SR linear accelerator with Brown-Reberts-Wells and Gill-Thomas-Cosman relocatable Frames along with the Radionics (R SA) planning system is evaluated. The Clinac-600SR has a single 6-MV b eam with the same beam characteristics as that of the mother unit, the Clinac-600C. The primary collimator is a fixed cone projecting to a 1 0-cm diameter at isocenter. The secondary collimator is a heavily shie lded cylindrical collimator attached to the face plate of the primary collimator. The tertiary collimation consists of the actual treatment cones. The cone sizes vary from 12.5 to 40.0 mm diameter. The mechanic al stability of the entire system was verified. The variations in isoc enter position with table, gantry, and collimator rotation were found to be <0.5 mm with a compounded accuracy of less than or equal to 1.0 mm. The radiation leakage under the cones was < 1(a)` o measured at a depth of 5 cm in a phantom. The beam profiles of all cones in the x an d y directions were within +/- 0.5 mm and match with the physical size of the cone. The dosimetric data such as tissue maximum ratio, off-ax is ratio, and cone factor were taken using film, diamond detector, and ion chambers. The mechanical and dosimetric characteristics including dose linearity of this unit are presented and found to be suitable fo r SRS/SRT. The difficult in absolute dose measurement For small cone i s discussed.