title: Determination of output factors for the Leksell gamma knife using ion chamber, thermoluminescence detectors and films.
abstract: OBJECTIVE
  In stereotactic radiosurgery by the Leksell gamma knife, small fields of 4-18 mm in diameter are used. The difficulties associated with the dosimetry of small radiosurgical beams are well known. The output factors for small field sizes show a strong field size dependence, with rapidly decreasing output factors as the diameter of the field decreases. The main purpose of this study was to determine output factors of 18, 14, 8 and 4 mm collimators for Leksell gamma knife and to compare them with the values given by the manufacturer.
  
  
  METHODS
  The relative output factors of the 18, 14 8 and 4 mm collimators for model B Leksell gamma knife were measured. The output factor measurements were prformed using a PTW 0.125 cc ion chamber, G200 thermoluminescence detectors (TLDs), KodakXV and Kodak ERD2 films.
  
  
  RESULTS
  For 14 mm collimator, output measurements used with an ion chamber, TLD, Kodak XV film and Kodak ERD2 film were in agreement with the manufacturer's estimates within 1%. For 8 and 4 mm collimators, the best agreement with values given by the manufacturer were obtained by Kodak EDR2 films (2% and 5%, respectively).
  
  
  CONCLUSIONS
  The measured output factors are in good agreement with the values recommended by the manufacturer for 18, 14 and 8 mm collimators. No good agreement was found for 4 mm collimator.
authors.affiliation: Oncology Institute of Istanbul University, Istanbul, Turkey. haticebilge@yahoo.com
authors.forename: H
authors.initials: H
authors.lastname: Bilge
authors.forename: Z
authors.initials: Z
authors.lastname: Osen
authors.forename: O
authors.initials: O
authors.lastname: Senkesen
authors.forename: H
authors.initials: H
authors.lastname: Kucucuk
authors.forename: A
authors.initials: A
authors.lastname: Cakir
authors.forename: M
authors.initials: M
authors.lastname: Sengoz
mesh_terms: D006801:Humans; D016634:Radiosurgery / Q000295:instrumentation* / Q000379:methods; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods; D013819:Thermoluminescent Dosimetry / Q000295:instrumentation* / Q000379:methods