title: Quality assessment of frameless fractionated stereotactic radiotherapy using cone beam computed tomography.
abstract: OBJECTIVE
  A quality assessment of intracranial stereotactic radiotherapy was performed using cone beam computed tomography (CBCT). Setup errors were analyzed for two groups of patients: (1) those who were positioned using a frameless SonArray (FSA) system and immobilized with a bite plate and thermoplastic (TP) mask (the bFSA group); and (2) those who were positioned by room laser and immobilized using a TP mask (the mLAS group).
  
  
  METHODS
  A quality assurance phantom was used to study the system differences between FSA and CBCT. The quality assessment was performed using an Elekta Synergy imager (XVI) (Elekta Oncology Systems, Norcross, GA) and an On-Board Imager (OBI) (Varian Medical Systems, Palo Alto, CA) for 25 patients. For the first three fractions, and weekly thereafter, the FSA system was used for patient positioning, after which CBCT was performed to obtain setup errors.
  
  
  RESULTS
  (1) Phantom tests: The mean differences in the isocenter displacements for the two systems was 1.2 ± 0.7 mm. No significant variances were seen between the XVI and OBI units (p~0.208). (2)Patient tests: The mean of the displacements between FSA and CBCT were independent of the CBCT system used; mean setup errors for the bFSA group were smaller (1.2 mm) than those of the mLAS group (3.2 mm) (p < 0.005). For the mLAS patients, the 90th percentile and the maximum rotational displacements were 3° and 5°, respectively. A 4-mm drift in setup accuracy occurred over the treatment course for 1 bFSA patient.
  
  
  CONCLUSIONS
  System differences of less than 1 mm between CBCT and FSA were seen. Error regression was observed for the bFSA patients, using CBCT (up to 4 mm) during the treatment course. For the mLAS group, daily CBCT imaging was needed to obtain acceptable setup accuracies.
authors.affiliation: Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, Florida 32610-0385, USA.
authors.forename: Lee-Cheng
authors.initials: LC
authors.lastname: Peng
authors.forename: Darren
authors.initials: D
authors.lastname: Kahler
authors.forename: Sanjiv
authors.initials: S
authors.lastname: Samant
authors.forename: Jonathan
authors.initials: J
authors.lastname: Li
authors.forename: Robert
authors.initials: R
authors.lastname: Amdur
authors.forename: Jatinder R
authors.initials: JR
authors.lastname: Palta
authors.forename: Chihray
authors.initials: C
authors.lastname: Liu
mesh_terms: D002138:Calibration; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006801:Humans; D007103:Immobilization / Q000295:instrumentation / Q000379:methods; D009068:Movement; D056888:Patient Positioning / Q000295:instrumentation / Q000379:methods*; D019047:Phantoms, Imaging; D011786:Quality Control; D016634:Radiosurgery / Q000379:methods* / Q000592:standards*; D015203:Reproducibility of Results