title: Validation of a pretreatment delivery quality assurance method for the CyberKnife Synchrony system.
abstract: OBJECTIVE
  To evaluate the geometric and dosimetric accuracies of the CyberKnife Synchrony respiratory tracking system (RTS) and to validate a method for pretreatment patient-specific delivery quality assurance (DQA).
  
  
  METHODS
  An EasyCube phantom was mounted on the ExacTrac gating phantom, which can move along the superior-inferior (SI) axis of a patient to simulate a moving target. The authors compared dynamic and static measurements. For each case, a Gafchromic EBT3 film was positioned between two slabs of the EasyCube, while a PinPoint ionization chamber was placed in the appropriate space. There were three steps to their evaluation: (1) the field size, the penumbra, and the symmetry of six secondary collimators were measured along the two main orthogonal axes. Dynamic measurements with deliberately simulated errors were also taken. (2) The delivered dose distributions (from step 1) were compared with the planned ones, using the gamma analysis method. The local gamma passing rates were evaluated using three acceptance criteria: 3% local dose difference (LDD)/3 mm, 2%LDD/2 mm, and 3%LDD/1 mm. (3) The DQA plans for six clinical patients were irradiated in different dynamic conditions, to give a total of 19 cases. The measured and planned dose distributions were evaluated with the same gamma-index criteria used in step 2 and the measured chamber doses were compared with the planned mean doses in the sensitive volume of the chamber.
  
  
  RESULTS
  (1) A very slight enlargement of the field size and of the penumbra was observed in the SI direction (on average <1 mm), in line with the overall average CyberKnife system error for tracking treatments. (2) Comparison between the planned and the correctly delivered dose distributions confirmed the dosimetric accuracy of the RTS for simple plans. The multicriteria gamma analysis was able to detect the simulated errors, proving the robustness of their method of analysis. (3) All of the DQA clinical plans passed the tests, both in static and dynamic conditions. No statistically significant differences were found between static and dynamic cases, confirming the high degree of accuracy of the Synchrony RTS.
  
  
  CONCLUSIONS
  The presented methods and measurements verified the mechanical and dosimetric accuracy of the Synchrony RTS. Their method confirms the fact that the RTS, if used properly, is able to treat a moving target with great precision. By combining PinPoint ion chamber, EBT3 films, and gamma evaluation of dose distributions, their DQA method robustly validated the effectiveness of CyberKnife and Synchrony system.
authors.affiliation: Medical Physics Unit, CNAO Foundation-National Centre for Oncological Hadron Therapy, Pavia I-27100, Italy and Medical Physics Unit, IEO-European Institute of Oncology, Milan I-20141, Italy.
authors.forename: E
authors.initials: E
authors.lastname: Mastella
authors.affiliation: Medical Physics Unit, IEO-European Institute of Oncology, Milan I-20141, Italy.
authors.forename: S
authors.initials: S
authors.lastname: Vigorito
authors.affiliation: Medical Physics Unit, IEO-European Institute of Oncology, Milan I-20141, Italy.
authors.forename: E
authors.initials: E
authors.lastname: Rondi
authors.affiliation: Department of Radiation Oncology, IEO-European Institute of Oncology, Milan I-20141, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Piperno
authors.affiliation: Department of Radiation Oncology, IEO-European Institute of Oncology, Milan I-20141, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Ferrari
authors.affiliation: Department of Radiation Oncology, IEO-European Institute of Oncology, Milan I-20141, Italy.
authors.forename: E
authors.initials: E
authors.lastname: Strata
authors.affiliation: Department of Radiation Oncology, IEO-European Institute of Oncology, Milan I-20141, Italy.
authors.forename: D
authors.initials: D
authors.lastname: Rozza
authors.affiliation: Department of Radiation Oncology, IEO-European Institute of Oncology, Milan I-20141, Italy and Department of Oncology and Hematology Oncology, University of Milan, Milan I-20122, Italy.
authors.forename: B A
authors.initials: BA
authors.lastname: Jereczek-Fossa
authors.affiliation: Medical Physics Unit, IEO-European Institute of Oncology, Milan I-20141, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Cattani
mesh_terms: D005373:Film Dosimetry / Q000379:methods; D006801:Humans; D008113:Liver Neoplasms / Q000601:surgery; D008953:Models, Anatomic; D009038:Motion; D019047:Phantoms, Imaging; D011785:Quality Assurance, Health Care / Q000379:methods*; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D012119:Respiration; D012371:Robotics