title: Small field in-air output factors: the role of miniphantom design and dosimeter type.
abstract: OBJECTIVE
  The commissioning of treatment planning systems and beam modeling requires measured input parameters. The measurement of relative output in-air, Sc is particularly difficult for small fields. The purpose of this study was to investigate the influence of miniphantom design and detector selection on measured Sc values for small fields and to validate the measurements against Monte Carlo simulations.
  
  
  METHODS
  Measurements were performed using brass caps (with sidewalls) or tops (no sidewalls) of varying heights and widths. The performance of two unshielded diodes (60012 and SFD), EBT2 radiochromic film, and a fiber optic dosimeter (FOD) were compared for fields defined by MLCs (5-100 mm) and SRS cones (4-30 mm) on a Varian Novalis linear accelerator. Monte Carlo simulations were performed to theoretically predict Sc as measured by the FOD.
  
  
  RESULTS
  For all detectors, Sc agreed to within 1% for fields larger than 10 mm and to within 2.3% for smaller fields. Monte Carlo simulation matched the FOD measurements for all size of cone defined fields to within 0.5%.
  
  
  CONCLUSIONS
  Miniphantom design is the most important variable for reproducible and accurate measurements of the in-air output ratio, S(c), in small photon fields (less than 30 mm). Sidewalls are not required for fields ≤ 30 mm and tops are therefore preferred over the larger caps. Unlike output measurements in water, S(cp), the selection of detector type for Sc is not critical, provided the active dosimeter volume is small relative to the field size.
authors: Illawarra Cancer Care Centre, Wollongong Hospital, Wollongong, New South Wales 2521, Australia and Centre for Medical Radiation Physics, University of Wollongong, Wollongong, New South Wales 2522, Australia.
authors: Kirbie
authors: K
authors: Warrener
authors: School of Physics, University of Western Australia, Crawley, Western Australia 6009, Australia and Department of Radiation Oncology, Sir Charles Gairdner Hospital, Perth, Western Australia 6009, Australia.
authors: Benjamin
authors: B
authors: Hug
authors: School of Physics, University of Sydney, Darlington, New South Wales 2008, Australia.
authors: Paul
authors: P
authors: Liu
authors: Chris O'Brien Lifehouse, Radiation Oncology, Sydney, New South Wales 2050, Australia.
authors: Anna
authors: A
authors: Ralston
authors: School of Physics, University of Western Australia, Crawley, Western Australia 6009, Australia and Department of Radiation Oncology, Sir Charles Gairdner Hospital, Perth, Western Australia 6009, Australia.
authors: Martin A
authors: MA
authors: Ebert
authors: School of Physics, University of Sydney, Darlington, New South Wales 2008, Australia.
authors: David R
authors: DR
authors: McKenzie
authors: School of Physics, University of Sydney, Darlington, New South Wales 2008, Australia and Chris O'Brien Lifehouse, Radiation Oncology, Sydney, New South Wales 2050, Australia.
authors: Natalka
authors: N
authors: Suchowerska
mesh_terms: D000388:Air; D009010:Monte Carlo Method; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011874:Radiometry / Q000295:instrumentation*; D011880:Radiotherapy Planning, Computer-Assisted