title: CyberKnife beam output factor measurements: A multi-site and multi-detector study.
abstract: OBJECTIVE
  New promising detectors are available for measuring small field size output factors (OFs). This study focused on a multicenter evaluation of two new generation detectors for OF measurements on CyberKnife systems.
  
  
  METHODS
  PTW-60019 microDiamond and W1 plastic scintillation detector (PSD) were used to measure OFs on eight CyberKnife units of various generations for 5-60mm fixed cones. MicroDiamond and PSD OF were compared to routinely used silicon diodes data corrected applying published Monte Carlo (MC) factors. PSD data were corrected for Čerenkov Light Ratio (CLR). The uncertainties related to CLR determination were estimated.
  
  
  RESULTS
  Considering OF values averaged over all centers, the differences between MC corrected diode and the other two detectors were within 1.5%. MicroDiamond exhibited an over-response of 1.3% at 7.5mm and a trend inversion at 5mm with a difference of 0.2%. This behavior was consistent among the different units. OFs measured by PSD slightly under-responded compared to MC corrected diode for the smaller cones and the differences were within 1%. The observed CLR variability was 2.5% and the related variation in OF values was 1.9%.
  
  
  CONCLUSIONS
  This study indicates that CyberKnife microDiamond OF require corrections below 2%. The results are enhanced by the consistency observed among different units. Scintillator shows a good agreement to MC corrected diode but CLR determination remains critical requiring further investigations. The results emphasized the value of a multi-center validation over a single center approach.
authors: Department of Medical Physics and Radiation Oncology, IFCA, I-50139 Firenze, Italy. Electronic address: l.masi@giomi.com.
authors: Laura
authors: L
authors: Masi
authors: Medical Physics Unit of Radiation Oncology Dept, Azienda Sanitaria Firenze, I-50012 Firenze, Italy.
authors: Serenella
authors: S
authors: Russo
authors: Department of Radiation Oncology, Ospedale di Vicenza, I-36100 Vicenza, Italy.
authors: Paolo
authors: P
authors: Francescon
authors: Department of Medical Physics and Radiation Oncology, IFCA, I-50139 Firenze, Italy.
authors: Raffaela
authors: R
authors: Doro
authors: Radiotherapy Department, Mater Dei Hospital, Città di Bari Hospital spa, I-70124 Bari, Italy.
authors: Maria Cristina
authors: MC
authors: Frassanito
authors: Radiotherapy Department, Istituto Neurologico Besta, I-20133 Milano, Italy.
authors: Maria Luisa
authors: ML
authors: Fumagalli
authors: Medical Physics Unit of Radiation Oncology Dept., Humanitas Research Hospital, I-20089 Milano, Italy.
authors: Giacomo
authors: G
authors: Reggiori
authors: INFN-Dipartimento di Ingegneria Industriale, Università di Roma 'Tor Vergata', I-00133 Roma, Italy.
authors: Marco
authors: M
authors: Marinelli
authors: Radiotherapy Department, Centro Diagnostico Italiano, I-20147 Milano, Italy.
authors: Irene
authors: I
authors: Redaelli
authors: Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti, ENEA-INMRI C R Casaccia, I-00123 Roma, Italy.
authors: Maria
authors: M
authors: Pimpinella
authors: INFN-Dipartimento di Ingegneria Industriale, Università di Roma 'Tor Vergata', I-00133 Roma, Italy.
authors: Gianluca
authors: G
authors: Verona Rinati
authors: Medical Physics Unit, A.O.U. Policlinico G. Martino, Messina, Italy.
authors: Carmelo
authors: C
authors: Siragusa
authors: Radiotherapy Department, Istituto Europeo Oncologia, I-20141 Milano, Italy.
authors: Sabrina
authors: S
authors: Vigorito
authors: Medical Physics Unit of Radiation Oncology Dept., Humanitas Research Hospital, I-20089 Milano, Italy.
authors: Pietro
authors: P
authors: Mancosu
mesh_terms: D018130:Diamond; D009010:Monte Carlo Method; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*
chemical_list: D018130:Diamond
keywords: CyberKnife output factors; Multicenter evaluation; PTW-60019 microDiamond detector; W1 scintillation detector