Interpretation of In-air Output Ratio Of Wedged Fields in Different Measurement Conditions

Abstract

This study aimed to investigate the variation of Sc with different build-up cap materials, wall thickness of build-up caps, Source to chamber Distances (SCDs), ionization chambers and wedge angles in 6MV photon beam.

Methods: In this study, Copper and Perspex build-up caps were fabricated with two different thicknesses for each build-up cap. Measurements were performed on an Elekta Compact medical linear accelerator (6 MV) using RK dosimeter with sensitive volume of 0.120 cm3 and Farmer-type ion chamber with sensitive volume of 0.65 cm3. In all measurements, build-up caps and ionization chambers were positioned in a stand vertical to the beam central axis. It was also investigated the effect of internal wedge with different angles and different SCDs on Sc.

Results: The presence of wedge influences Sc values significantly. Variation of Sc in wedged fields compared to open fields had maximum deviation of 0.9% and 6.8% in 30° and 60° wedge angles, respectively. Different SCDs and type of ion chamber had no significant influence on Sc values. In large field sizes, Sc values in perspex build-up cap is higher than copper one.