title: An experimental extrapolation technique using the Gafchromic EBT3 film for relative output factor measurements in small x-ray fields.
abstract: OBJECTIVE
  An experimental extrapolation technique is presented, which can be used to determine the relative output factors for very small x-ray fields using the Gafchromic EBT3 film.
  
  
  METHODS
  Relative output factors were measured for the Brainlab SRS cones ranging in diameters from 4 to 30 mm(2) on a Novalis Trilogy linear accelerator with 6 MV SRS x-rays. The relative output factor was determined from an experimental reducing circular region of interest (ROI) extrapolation technique developed to remove the effects of volume averaging. This was achieved by scanning the EBT3 film measurements with a high scanning resolution of 1200 dpi. From the high resolution scans, the size of the circular regions of interest was varied to produce a plot of relative output factors versus area of analysis. The plot was then extrapolated to zero to determine the relative output factor corresponding to zero volume.
  
  
  RESULTS
  Results have shown that for a 4 mm field size, the extrapolated relative output factor was measured as a value of 0.651 ± 0.018 as compared to 0.639 ± 0.019 and 0.633 ± 0.021 for 0.5 and 1.0 mm diameter of analysis values, respectively. This showed a change in the relative output factors of 1.8% and 2.8% at these comparative regions of interest sizes. In comparison, the 25 mm cone had negligible differences in the measured output factor between zero extrapolation, 0.5 and 1.0 mm diameter ROIs, respectively.
  
  
  CONCLUSIONS
  This work shows that for very small fields such as 4.0 mm cone sizes, a measureable difference can be seen in the relative output factor based on the circular ROI and the size of the area of analysis using radiochromic film dosimetry. The authors recommend to scan the Gafchromic EBT3 film at a resolution of 1200 dpi for cone sizes less than 7.5 mm and to utilize an extrapolation technique for the output factor measurements of very small field dosimetry.
authors.affiliation: Department of Radiation Oncology, Chris O'Brien Lifehouse, 119-143 Missenden Road, Camperdown, NSW 2050, Australia and School of Chemistry, Physics, and Mechanical Engineering, Queensland University of Technology, Level 4 O Block, Garden's Point, QLD 4001, Australia.
authors.forename: Johnny E
authors.initials: JE
authors.lastname: Morales
authors.affiliation: Department of Radiation Oncology, Chris O'Brien Lifehouse, 119-143 Missenden Road, Camperdown, NSW 2050, Australia and Institute of Medical Physics, University of Sydney, NSW 2006, Australia.
authors.forename: Martin
authors.initials: M
authors.lastname: Butson
authors.affiliation: School of Chemistry, Physics, and Mechanical Engineering, Queensland University of Technology, Level 4 O Block, Garden's Point, QLD 4001, Australia and Cancer Care Services, Royal Brisbane and Women's Hospital, Butterfield Street, Herston, QLD 4029, Australia.
authors.forename: Scott B
authors.initials: SB
authors.lastname: Crowe
authors.affiliation: Department of Radiation Oncology, Chris O'Brien Lifehouse, 119-143 Missenden Road, Camperdown, NSW 2050, Australia and Institute of Medical Physics, University of Sydney, NSW 2006, Australia.
authors.forename: Robin
authors.initials: R
authors.lastname: Hill
authors.affiliation: School of Chemistry, Physics, and Mechanical Engineering, Queensland University of Technology, Level 4 O Block, Garden's Point, QLD 4001, Australia.
authors.forename: J V
authors.initials: JV
authors.lastname: Trapp
mesh_terms: D005373:Film Dosimetry / Q000379:methods*; D014965:X-Rays