Detector comparison for small field output factor measurements in flattening filter free photon beams

Abstract

PurposeThe applicability of various detectors for small field dosimetry and whether there are differences in the detector response when irradiated with FF- and FFF-beams was investigated. Materials and methodsOutput factors of 6 and 10MV FF- and FFF-beams were measured with 14 different online detectors using field sizes between 10×10 and 0.6×0.6cm2 at a depth of 5cm of water in isocentric conditions. Alanine pellets with a diameter of 5 and 2.5mm were used as reference dosimeters for field sizes down to 1.2×1.2 and 0.6×0.6cm2, respectively. The ratio of the relative output measured with the online detectors to the relative output measured with alanine was evaluated (referred to as dose response ratio). ResultsThe dose response ratios of two different shielded diodes measured with 10MV FF-beams deviated substantially by 2–3% compared to FFF-beams at a field size of 0.6×0.6cm2. This difference was less pronounced for 6MV FF- and FFF-beams. For all other detectors the dose response ratios of FF- and FFF-beams showed no significant difference. ConclusionThe dose response ratios of the majority of the detectors agreed within the measurement uncertainty when irradiated with FF- and FFF-beams. Of all investigated detectors, the microDiamond and the unshielded diodes would require only small corrections which make them suitable candidates for small field dosimetry in FF- and FFF-beams.