title: A methodological approach to reporting corrected small field relative outputs.
abstract: OBJECTIVE
  The goal of this work was to set out a methodology for measuring and reporting small field relative output and to assess the application of published correction factors across a population of linear accelerators.
  
  
  METHODS
  Measurements were made at 6 MV on five Varian iX accelerators using two PTW T60017 unshielded diodes. Relative output readings and profile measurements were made for nominal square field sizes of side 0.5 to 1.0 cm. The actual in-plane (A) and cross-plane (B) field widths were taken to be the FWHM at the 50% isodose level. An effective field size, defined as √FS eff=A · B, was calculated and is presented as a field size metric. FSeff was used to linearly interpolate between published Monte Carlo (MC) calculated [Formula in text] values to correct for the diode over-response in small fields.
  
  
  RESULTS
  The relative output data reported as a function of the nominal field size were different across the accelerator population by up to nearly 10%. However, using the effective field size for reporting showed that the actual output ratios were consistent across the accelerator population to within the experimental uncertainty of ± 1.0%. Correcting the measured relative output using [Formula in text] at both the nominal and effective field sizes produce output factors that were not identical but differ by much less than the reported experimental and/or MC statistical uncertainties.
  
  
  CONCLUSIONS
  In general, the proposed methodology removes much of the ambiguity in reporting and interpreting small field dosimetric quantities and facilitates a clear dosimetric comparison across a population of linacs.
authors.affiliation: Department of Medical Physics, Saskatchewan Cancer Agency, Saskatoon, Canada; Academic Unit of Medical Physics, Faculty of Medicine and Health, University of Leeds, UK. Electronic address: gavin.cranmer-sargison@saskcancer.ca.
authors.forename: Gavin
authors.initials: G
authors.lastname: Cranmer-Sargison
authors.forename: Paul H
authors.initials: PH
authors.lastname: Charles
authors.forename: Jamie V
authors.initials: JV
authors.lastname: Trapp
authors.forename: David I
authors.initials: DI
authors.lastname: Thwaites
mesh_terms: D009010:Monte Carlo Method; D010315:Particle Accelerators; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D035501:Uncertainty
keywords: Small field dosimetry