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

Introduction: The head scatter factor (Sc) is one of the important parameters for monitor unit (MU) calculation. There are multiple factors that impact the Sc values, such as, head structures, back scattering in to dose monitoring chambers, wedges and so on. This study aimed to investigate the variations of SC with different build-up cap materials, wall thickness, Source to Skin Distance (SSDs), ionization chambers and variation of the SC  with wedge angles in 6MV photon beam. Materials and Methods: In this study, Copper and Perspex build-up caps were designed and fabricated with two different thicknesses for each build-up cap. Measurements were performed on an Elekta Compact medical linear accelerator (6 MV) using RK with sensitive volume of 0.12 cc and Farmer-type ion chamber with sensitive volume of 0.65 cc. In all measurements, build-up caps and ionization chambers were positioned in a stand vertical to the beam central axis. It is also investigated the effect of different SSDs on SC. Results: The presence of wedge influences the SC values significantly. Variation of SC in open and wedged fields had maximum deviation of 0.9% and 6.8% and minimum of 0.4% and 2.7% for 30° and 60° angles of wedge, respectively. different SSD and type of ion chamber had no significant influence on the SC. In small field sizes, the SC values in copper build-up cap is higher than Perspex and in large field sizes, measured SC in perspex build-up cap is higher than copper. Conclusion: It was found that the presence of internal wedge has significant contribution to head scatter factor and it increases with increasing wedge angle. Therefore, it should be takes into account in manually MU calculations.