title: Commissioning of the Leksell Gamma Knife®Icon™.
abstract: OBJECTIVE
  The Leksell Gamma Knife (LGK) Icon has been recently introduced to provide Gamma Knife technology with frameless stereotactic treatments which use an additional cone-beam CT (CBCT) imaging system and a motion tracking system (IFMM, Intra-Fraction Motion Management). The system was commissioned for the treatment unit itself as well as the imaging system.
  
  
  METHODS
  The LGK Icon was calibrated using an A1SL ionization chamber. EBT3 radiochromic films were employed to independently check the machine calibration, to measure the relative output factors (ROFs) and to collect dose distributions. Coincidence between CBCT isocenter and radiological focus was evaluated by means of EBT3 films. CBCT image quality was investigated in terms of spatial resolution, contrast-to-noise ratio (CNR), and uniformity for the two presets available (low dose and high dose). Computed Tomography Dose Index (CTDI) was also measured for both presets.
  
  
  RESULTS
  The absolute dose rate of the LGK Icon was 3.86 ± 0.09 Gy/min. This result was confirmed by EBT3 readings. ROF were found to be 0.887 ± 0.035 and 0.797 ± 0.032 for the 8 mm and 4 mm collimators, respectively, which are within 2% of the Monte Carlo-derived ROF values. Excellent agreement was found between calculated and measured dose distribution with the gamma pass rate >95% of points for the nine dose distributions analyzed with 3%/1 mm criteria. CBCT isocenter was found to be within 0.2 mm with respect to radiological focus. Image quality parameters were found to be well within the manufacturer's specifications with the high-dose preset being superior in terms of CNR and uniformity. CTDI values were 2.41 mGy and 6.32 mGy, i.e. -3.6% and 0.3% different from the nominal values for the low-dose and high-dose presets, respectively.
  
  
  CONCLUSIONS
  The LGK Icon was successfully commissioned for clinical use. The use of the EBT3 to characterize the treatment unit was demonstrated to be feasible. The CBCT imaging system operates well within the manufacturer's specifications and provides good geometrical accuracy.
authors.affiliation: Institute of Radiation Physics, Lausanne University Hospital, Lausanne, Switzerland.
authors.forename: Michele
authors.initials: M
authors.lastname: Zeverino
authors.affiliation: Institute of Radiation Physics, Lausanne University Hospital, Lausanne, Switzerland.
authors.forename: Maud
authors.initials: M
authors.lastname: Jaccard
authors.affiliation: Institute of Radiation Physics, Lausanne University Hospital, Lausanne, Switzerland.
authors.forename: David
authors.initials: D
authors.lastname: Patin
authors.affiliation: Institute of Radiation Physics, Lausanne University Hospital, Lausanne, Switzerland.
authors.forename: Nick
authors.initials: N
authors.lastname: Ryckx
authors.affiliation: Institute of Radiation Physics, Lausanne University Hospital, Lausanne, Switzerland.
authors.forename: Maud
authors.initials: M
authors.lastname: Marguet
authors.affiliation: Department of Neurosurgery and Gamma Knife Center, Lausanne University Hospital, Lausanne, Switzerland.
authors.forename: Constantin
authors.initials: C
authors.lastname: Tuleasca
authors.affiliation: Department of Radiation Oncology, Lausanne University Hospital, Lausanne, Switzerland.
authors.forename: Luis
authors.initials: L
authors.lastname: Schiappacasse
authors.affiliation: Department of Radiation Oncology, Lausanne University Hospital, Lausanne, Switzerland.
authors.forename: Jean
authors.initials: J
authors.lastname: Bourhis
authors.affiliation: Department of Neurosurgery and Gamma Knife Center, Lausanne University Hospital, Lausanne, Switzerland.
authors.forename: Marc
authors.initials: M
authors.lastname: Levivier
authors.affiliation: Institute of Radiation Physics, Lausanne University Hospital, Lausanne, Switzerland.
authors.forename: Francois O
authors.initials: FO
authors.lastname: Bochud
authors.affiliation: Institute of Radiation Physics, Lausanne University Hospital, Lausanne, Switzerland.
authors.forename: Raphaël
authors.initials: R
authors.lastname: Moeckli
mesh_terms: D002138:Calibration; D054893:Cone-Beam Computed Tomography / Q000295:instrumentation / Q000379:methods; D005373:Film Dosimetry; D006257:Head / Q000000981:diagnostic imaging / Q000601:surgery; D006801:Humans; D008953:Models, Anatomic; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D016634:Radiosurgery / Q000295:instrumentation* / Q000379:methods; D011879:Radiotherapy Dosage; D014867:Water
chemical_list: D014867:Water
keywords: 
  CBCT
  ; 
  CTDI
  ; Leksell Gamma Knife® Icon™; film dosimetry; small field dosimetry