title: Experimental small field 6 MV output ratio analysis for various diode detector and accelerator combinations.
abstract: OBJECTIVE
  The goal of this work was to measure 6MV small field, detector specific, output ratios (OR(det)) 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.
  
  
  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 OR(det) 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 OR(det) were not consistent across all detectors. The standard percent uncertainty in measured OR(det) 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.
  
  
  CONCLUSIONS
  Experimental small field OR(det) 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%.
authors.affiliation: Department of Medical Physics, Saskatchewan Cancer Agency, Saskatoon, Canada. gavin.cranmer-sargison@saskcancer.ca
authors.forename: Gavin
authors.initials: G
authors.lastname: Cranmer-Sargison
authors.forename: Steve
authors.initials: S
authors.lastname: Weston
authors.forename: Narinder P
authors.initials: NP
authors.lastname: Sidhu
authors.forename: David I
authors.initials: DI
authors.lastname: Thwaites
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D006801:Humans; D009010:Monte Carlo Method; D010315:Particle Accelerators; D011874:Radiometry / Q000295:instrumentation*; D016634:Radiosurgery / Q000295:instrumentation*; D011879:Radiotherapy Dosage; D015203:Reproducibility of Results; D012542:Scattering, Radiation; D012666:Semiconductors