Small fields dosimetry: Output factors and correction factors determination for an elketa axesse medical linac equipped with circular cones

Abstract

Introduction The peculiarities of small beams (high dose gradient, source occlusion, lack of lateral electronic equilibrium) and the features of the detector (active volume dimension, components with high-Z materials) make the dosimetry very challenging. Purpose The aim of this work is to determine small fields output factors (OF) for several detectors and correction factors for active detectors for comparison with a passive dosimeter. Materials and methods Small fields beams, ranging from 5 mm to 30 mm in diameter, were defined using circular cones. OF measurements were performed with six active detectors (ionizing microchambers air-filled: Exradin A26, Exradin A16; ionizing microchamber isooctane-filled: PTW microLion; plastic scintillator: Exradin W1; diode: Razor IBA) and one passive detector (Gafchromic EBT3 films). Results Exradin W1 and A26 shown excellent agreement with EBT3 films (better than 2%). A significant underestimation was observed for Exradin A16, particularly for the smallest field, up to 12%. The results obtained with the PTW microLion and the IBA RAZOR indicate a dose overestimation for the smaller radiation fields, up to 4% and 7% for the 5 mm-diameter field for microLion and RAZOR respectively. Conclusion The present study points out that it is crucial to apply the appropriate correction factors in order to provide accurate measurements in small beam geometry. The results show that the Exradin W1 and Exradin A26 can be used for small fields dosimetry without correction factors. The correction factors should be employed for the other detectors, in particular for field diameter smaller than 10 mm. Disclosure Nothing to declare.