title: Gamma knife output factor measurements using VIP polymer gel dosimetry.
abstract: OBJECTIVE
  Water equivalent polymer gel dosimeters and magnetic resonance imaging were employed to measure the output factors of the two smallest treatment fields available in a Gamma Knife model C radiosurgery unit, those formed employing the 4 and 8 mm final collimator helmets.
  
  
  METHODS
  Three samples of the VIP normoxic gel formulation were prepared and irradiated so that a single shot of the field whose output factor is to be measured and a single shot of the reference 18 mm field were delivered in each one. Emphasis is given to the development and benchmarking of a refined data processing methodology of reduced uncertainty that fully exploits the 3D dose distributions registered in the dosimeters.
  
  
  RESULTS
  Polymer gel results for the output factor of the 8 mm collimator helmet are found to be in close agreement with the corresponding value recommended by the vendor (0.955 +/- 0.007 versus 0.956, respectively). For the 4 mm collimator helmet, however, polymer gel results suggest an output factor 3% lower than the value recommended by the vendor (0.841 +/- 0.009 versus 0.870, respectively).
  
  
  CONCLUSIONS
  A comparison with corresponding measurements published in the literature indicates that output factor results of this work are in agreement with those obtained using dosimetric systems which, besides fine spatial resolution and lack of angular and dose rate dependence of the dosimeter's response, share with polymer gels the favorable characteristic of minimal radiation field perturbation.
authors.affiliation: Medical Physics Laboratory, Medical School, University of Athens, 75 Mikras Asias, 115 27 Athens, Greece. armouts@phys.uoa.gr
authors.forename: A
authors.initials: A
authors.lastname: Moutsatsos
authors.forename: L
authors.initials: L
authors.lastname: Petrokokkinos
authors.forename: P
authors.initials: P
authors.lastname: Karaiskos
authors.forename: P
authors.initials: P
authors.lastname: Papagiannis
authors.forename: E
authors.initials: E
authors.lastname: Georgiou
authors.forename: K
authors.initials: K
authors.lastname: Dardoufas
authors.forename: P
authors.initials: P
authors.lastname: Sandilos
authors.forename: M
authors.initials: M
authors.lastname: Torrens
authors.forename: E
authors.initials: E
authors.lastname: Pantelis
authors.forename: I
authors.initials: I
authors.lastname: Kantemiris
authors.forename: L
authors.initials: L
authors.lastname: Sakelliou
authors.forename: I
authors.initials: I
authors.lastname: Seimenis
mesh_terms: D003198:Computer Simulation; D005782:Gels / Q000528:radiation effects; D016014:Linear Models; D008279:Magnetic Resonance Imaging / Q000379:methods; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011108:Polymers / Q000528:radiation effects; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D016634:Radiosurgery; D011879:Radiotherapy Dosage; D035501:Uncertainty
chemical_list: D005782:Gels; D011108:Polymers