Buildup/surface dose and exit dose measurements for a 6-MV linear accelerator

The central axis dose distribution in the buildup region for the Varian Clinac 6/100 6-MV x-ray beam was measured in a polystyrene phantom using a fixed volume (0.5 cm3) parallel-plate ionization chamber (2.4-mm plate separation). Results for the surface dose measurements ranged from approximately 8% of the maximum dose for a 5 X 5 cm field, up to 36% for a 40 X 40 cm field, 100-cm source-skin distance. The effect of a 0.6-cm-thick polycarbonate blocking tray and metal filters on the surface and buildup dose is also reported. In addition, ionization measurements were made to document the dose perturbations caused by the absence of backscattering material at the exit surface of a polystyrene phantom. Exit dose measurements showed a 15% reduction in dose with essentially no scattering material beyond the measurement point. Near full scatter condition could be restored by placing 5-10 mm (depending on field size) of unit density material directly behind the ion chamber's distal surface.