Measurement and comparison of head scatter factor for 9MV photon beam using the build-up cap and a columnar mini-phantom

Introduction: The Sc is an important parameter to monitor unit (MU) calculation in radiation therapy procedure. The aim of this study is to measurement and comparison of head scatter factor (Sc) for 9MV photon beam using the build-up cap and a columnar mini-phantom.
 
Materials and Methods: The measurements performed using a calibrated ion chamber (Scanditronix / Wellhofer Farmer Type Chamber FC65-G), and electrometer (Scanditronix / Wellhofer, Dose 1 Reference class electrometer). Sc’s values were measured on Neptun linear accelerator. The Sc measured in the source to surface distance (SSD) of 100cm in different square field sizes (5×5 to 38×38 cm) using two available devices: the PMMA build- up cap and dmax dose water equivalent columnar mini-phantom.
Results: Sc’s values for 5×5, 10×10, 15×15, 20×20, 25×25, 30×30, 35×35 and 38×38 cm2 field
sized with columnar mini-phantom were 0.97, 1, 1.02, 1.04, 1.06, 1.07, 1.08, and 1.09, respectively.
Sc’s values for 5×5, 10×10, 15×15, 20×20, 25×25, 30×30, 35×35 and 38×38 cm2 field sized
with build-up cap were 0.96, 1, 1.03, 1.06, 1.08, 1.09, 1.10, and 1.11, respectively. Conclusion: Though measured Sc value using build-up cap in 5×5 cm2 field size was smaller than of those Sc measured using columnar mini-phantom, higher values result in larger field sizes than 10×10 cm2.