Effects of immobilization beds on the dose in the entrance and exit dose region for Co-60, 4, 6 and 15 MV photons

Abstract

Purpose: The aim of this study was to determine the effects of Styrofoam beds used for immobilization on build-up and exit dose regions for high energy photon beams. Materials and methods: Build-up dose and exit dose measurements in central axis of Co-60 and 4, 6 and 15 MV photons at various field sizes and source to phantom distances were made in a water equivalent solid phantom with 2, 5 and 10 cm thick uniform Styrofoam beds at the surface. A Markus type plane-parallel ion chamber with fixed separation between collecting electrodes was used to measure the percent depth doses. Results: The surface dose increased almost linearly with field size for Co-60, 4, 6 and 15 MV X-ray beams. The effect of immobilization (Styrofoam beds) on the surface dose increased with the thickness and this effect was lower with higher energies. When a 2 cm thick Styrofoam bed was used for immobilization, the surface dose in a 10 x 10 cm field was higher (43.9, 36.8, 28.8 and 14.9% for Co-60, 4, 6 and 15 MV, respectively). Conclusion: As the Styrofoam bed was thicker, the maximum dose point moved closer to the surface of the phantom for all energies. The exit surface dose was also enhanced with the presence of Styrofoam beds and similar to the effects on the surface dose. This enhancement was the maximum 5% for high energy photon beams and 6% for Co-60 beam. The introduction of Syrofoam beds in the radiation beam for the immobilization of the patient increases surface and exit doses to a considerable extent.