Comparison of the Sensitive Volume Response of the Three Different Cylindrical Ionization Chambers with GEANT4 Simulations Using the Concept of Output Factor

This study aims to contribute to the literature the broadened demonstration of energy-related and field size-related responses of three different cylindrical ionization chambers, commonly used for clinical applications of the therapeutic photon beams, using GEANT4 code. METHODS GEANT4 simulation of Varian TrueBeam STx was compared with the real-time measurements. The variations in the output factor (OF) measurements depending on field sizes ranging from 0.6x0.6 cm2 to 30x30 cm2 and photon energies of 6MV, 6MV-FFF, 10MV, 10MV-FFF, and 15MV were recorded for each chamber and compared with the GEANT4 simulations. Response of the chambers was analyzed depending on those OFs. RESULTS The GEANT4 simulation model was validated. All the chambers were in a good agreement within 1.5% for field sizes between 4x4 cm2 and 30x30 cm2. The farmer chamber presented a major deviation up to 22% for the field sizes smaller than 4x4 cm2. Semiflex and pinpoint chambers have an agreement within 2% and 5.5% for 3x3cm2 and 2x2 cm2, respectively. CONCLUSION The chambers evaluated in this study can be used reliably for all photon energies and field sizes between 4x4 cm2 and 30x30 cm2. To obtain accurate measurements for field sizes smaller than 3x3 cm2, it is recommended to use detectors with a smaller sensitive volume instead of the detectors used in this study.