S A Rahimi - PhD Candidate, Department of Medical Physics, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran
B Hashemi - Associate Professor, Department of Medical Physics, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran
S R Mahdavi - Associate Professor, Department of Medical Physics, Faculty of Medicine, Iran University of Medical Sciences, Tehran, Iran

Background: Estimating dosimetric parameters for small fields under non-reference conditions leads to significant errors if done based on conventional protocols used for large fields in reference conditions. Hence, further correction factors have been introduced to take into account the influence of spectral quality changes when various detectors are used in non-reference conditions at different depths and field sizes.Objective: Determining correction factors (KNR and KNCSF) recommended recently for small field dosimetry formalism by American Association of Physicists in Medicine (AAPM) for different detectors at ۶ and ۱۸ MV photon beams.Methods: EGSnrc Monte Carlo code was used to calculate the doses measured with different detectors located in a slab phantom and the recommended KNR and KNCSF correction factors for various circular small field sizes ranging from ۵-۳۰ mm diameters. KNR and KNCSF correction factors were determined for different active detectors (a pinpoint chamber, EDP-۲۰ and EDP-۱۰ diodes) in a homogeneous phantom irradiated to ۶ and ۱۸ MV photon beams of a Varian linac (۲۱۰۰C/D). Results: KNR correction factor estimated for the highest small circular field size of ۳۰ mm diameter for the pinpoint chamber, EDP-۲۰ and EDP-۱۰ diodes were ۰.۹۹۳, ۱.۰۲۰ and ۱.۰۵۴; and ۰.۹۹۲, ۱.۰۵۴ and ۱.۰۰۵ for the ۶ and ۱۸ MV beams, respectively. The KNCSF correction factor estimated for the lowest circular field size of ۵ mm for the pinpoint chamber, EDP-۲۰ and EDP-۱۰ diodes were ۰.۹۹۴, ۱.۰۲۳, and ۱.۰۴۰; and ۱.۰۰۰, ۱.۰۱۴, and ۱.۰۲۲ for the ۶ and ۱۸ MV photon beams, respectively.Conclusion: Comparing the results obtained for the detectors used in this study reveals that the unshielded diodes (EDP-۲۰ and EDP-۱۰) can confidently be recommended for small field dosimetry as their correction factors (KNR and KNCSF) was close to ۱.۰ for all small field sizes investigated and are mainly independent from the electron beam spot size.

Small field radiotherapy, Correction factors, TG۱۵۵, Monte Carlo, Pinpoint chamber, Diode dosimeter