title: A dual centre study of setup accuracy for thoracic patients based on cone-beam CT data.
abstract: OBJECTIVE
  To compare setup uncertainties at two different institutions by using identical imaging and analysis techniques for thoracic patients with different fixation equipments.
  
  
  METHODS
  Patient registration results from Cone-Beam CT (CBCT) scans of 174 patients were evaluated (1068 CBCT scans). Patients were fixated using a standard or custom made fixation at Royal Marsden Hospital and Odense University Hospital, respectively. Five imaging protocols were retrospectively simulated to compare the fixation equipments. Systematic and random setup uncertainties were calculated to estimate sufficient treatment margins.
  
  
  RESULTS
  The setup uncertainties are of similar sizes at the two institutions and there is no observable drift in the precision of the fixation equipments during the treatment course. When a correcting imaging protocol is performed there is a significant increase of the systematic setup uncertainties in between imaging fractions. A margin reduction of ≥0.2 cm can be achieved for patients with peak-to-peak respiration amplitudes of ≥1.2 cm when changing from 4D-CT to Active Breathing Coordinator™ (ABC).
  
  
  CONCLUSIONS
  The setup uncertainties at the two institutions are the same despite different fixation equipments. Hence margins cannot be reduced by changing fixating equipment.
authors.affiliation: Institute of Clinical Research, University of Southern Denmark, Odense, Denmark. tine.bjoern.nielsen@ouh.regionsyddanmark.dk
authors.forename: Tine B
authors.initials: TB
authors.lastname: Nielsen
authors.forename: Vibeke N
authors.initials: VN
authors.lastname: Hansen
authors.forename: Jonas
authors.initials: J
authors.lastname: Westberg
authors.forename: Olfred
authors.initials: O
authors.lastname: Hansen
authors.forename: Carsten
authors.initials: C
authors.lastname: Brink
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D019583:Dose Fractionation, Radiation; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional; D008175:Lung Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D008297:Male; D056888:Patient Positioning; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061106:Radiotherapy Setup Errors / Q000517:prevention & control*; D012189:Retrospective Studies