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

Parinaz Mehnati, Ali Jomehzadeh, Farideh Biglari



Background: The collimator scatter factor (Sc) is one of the most important parameters in monitor unit (MU) calculation. There are several factors that impact Sc values, including head structures, backscatter in dose monitoring chambers, and wedges. The objective of this study was to investigate the variation of Sc with different buildup cap materials, wall thickness of buildup caps, source-to-chamber distances (SCDs), ionization chambers, and wedge angles in 6 MV photon beam. Methods: In this study, copper and Perspex buildup caps were made with two different thicknesses for each buildup cap. Measurements were performed on an Elekta Compact medical linear accelerator (6 MV) using RK dosimeter with a sensitive volume of 0.120 cm3 and Farmer-type ion chamber with a sensitive volume of 0.65 cm3. In all measurements, buildup caps and ionization chambers were positioned such as to stand vertically to the beam central axis. It was also investigated the effect of internal wedge with different angles (30° and 60°) different SCDs on Sc. Results: It was found in large field sizes, Sc values in Perspex buildup cap were higher than copper. Different SCDs and type of ion chamber and wall thickness of buildup caps had no significant influence on Sc values. The presence of wedge influenced Sc values significantly. Variation of Sc in wedged fields compared to open fields had a maximum deviation of 0.9% and 6.8% in 30° and 60° wedge angles, respectively. Conclusion: It was found that the presence of wedges had a significant influence on Sc and increases with wedge angles. As such, it should be taken into account in manual MU calculations.


Buildup cap; collimator scatter factor; ionization chamber; wedge


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