THE EFFECTS OF VARIOUS IMMOBILIZATION DEVICES ON BUILD-UP AND SURFACE DOSES

The effect in build-up and surface dose as a result of using immobilization devices for high energy photon beams were investigated. Measurements were made with PTW Markus ion chamber, PTW Unidose electrometer, solid water phantom and Siemens Mevatron MD2 (6 and 15 MV) linear accelerator using 2, 4 and 10 cm thickness of Alpha Cradle, Vac-Form, styrofoam head support foam and thermoplastic mask which stretched as for a normal mask. Open field surface depth doses are 14.3%, 12.5% for 6 and 15 MV, respectively. The 6 and 15 MV surface doses are 51.9%, 62%, 83.4% and 34.0%, 42.3%, 63.8% for 2, 4, and 10 cm Alpha Cradle foam thicknesses, respectively. Similarly, the corresponding 6 and 15 MV surface doses for Vac-Form are 49.2%, 62.1%, 83% and 32%, 42.5%, 63.5% respectively. 2, 4, 10 cm styrofoam head support foam surface doses are 40.7%, 50.3%, 66% and 26.1%, 32.9%, 46% for 6, 15 MV x-rays, respectively. The surface doses of thermoplastic mask were 34.6% and 22.8% for 6, 15 MV x-rays, respectively. The depth of dmax shifts 1 to 3 mm for 2 to 10 cm thick polyurethane cradle foam and approximately 1 mm for stretched thermoplastic mask material. Alpha Cradle, Vac-Form and thermoplastic mask were shown to increase the patient's surface dose. The magnitude of surface dose increase depends on the material's density and thickness.

Keywords: immobilization devices, build-up dose, surface dose