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

Document Type: Conference Proceedings

Authors

1 Department of Medical Physics, Tabriz University of Medical Sciences, Tabriz, Iran.

2 Medical Radiation Sciences Research Team, Department of Medical Physics, Tabriz University of Medical Sciences, Tabriz, Iran.

3 Department of Medical Physics, Kerman University of Medical Sciences, Kerman, Iran.

Abstract

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.

Keywords