Experimental small field 6 MV output ratio analysis for various diode detector and accelerator combinations

Background and purpose: The goal of this work was to measure 6 MV small field, detector specific, output ratios (ORdet) using the IBA stereotactic field diode (SFD) and the PTW T60008, T60012, T60016 and T60017 field diodes on both Varian iX and Elekta Synergy accelerators, to establish estimates for the experimental uncertainty and characterize the measurement precision under various conditions. Materials and methods: Data were acquired at depths of 1.5, 5.0 and 10.0 cm for square field sizes of 3.0, 1.0, 0.9, 0.8, 0.7, 0.6 and 0.5 cm. Three isocentric measurements comprised of five readings were made to calculate an experimental output ratio ORdet with respect to a field size of 5.0 cm. The coefficient of variation (CV) was calculated to characterize the precision associated with each detector-linac combination. Another measurement set was made to investigate the influence of jaw position accuracy. Results: As expected for field sizes smaller than 3.0 cm, the measured ORdet were not consistent across all detectors. The standard percent uncertainty in measured ORdet was found to be nearly consistent across all detector-linac combinations: less than +/- 0.25% for the 3.0 cm field size, increasing to approximately +/- 1.25% for the smallest field sizes. As the field size was reduced to 0.5 cm the CV increased to 0.10% and 0.15% on the Varian and Elekta linacs, respectively. Conclusion: Experimental small field ORdet measured with the diode detectors used in this study are reproducible to within +/- 1.25% (standard uncertainty), with the precision of any one set of measurements can be characterized with a CV between 0.10% and 0.15%. (C) 2011 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 100 (2011) 429-435