title: Setup accuracy of the Novalis ExacTrac 6DOF system for frameless radiosurgery.
abstract: OBJECTIVE
  Stereotactic radiosurgery using frame-based positioning is a well-established technique for the treatment of benign and malignant lesions. By contrast, a new trend toward frameless systems using image-guided positioning techniques is gaining mainstream acceptance. This study was designed to measure the detection and positioning accuracy of the ExacTrac/Novalis Body (ET/NB) for rotations and to compare the accuracy of the frameless with the frame-based radiosurgery technique.
  
  
  METHODS
  A program was developed in house to rotate reference computed tomography images. The angles measured by the system were compared with the known rotations. The accuracy of ET/NB was evaluated with a head phantom with seven lead beads inserted, mounted on a treatment couch equipped with a robotic tilt module, and was measured with a digital water level and portal films. Multiple hidden target tests (HTT) were performed to measure the overall accuracy of the different positioning techniques for radiosurgery (i.e., frameless and frame-based with relocatable mask or invasive ring, respectively).
  
  
  RESULTS
  The ET/NB system can detect rotational setup errors with an average accuracy of 0.09° (standard deviation [SD] 0.06°), 0.02° (SD 0.07°), and 0.06° (SD 0.14°) for longitudinal, lateral, and vertical rotations, respectively. The average positioning accuracy was 0.06° (SD 0.04°), 0.08° (SD 0.06°), and 0.08° (SD 0.07°) for longitudinal, lateral and vertical rotations, respectively. The results of the HTT showed an overall three-dimensional accuracy of 0.76 mm (SD 0.46 mm) for the frameless technique, 0.87 mm (SD 0.44 mm) for the relocatable mask, and 1.19 mm (SD 0.45 mm) for the frame-based technique.
  
  
  CONCLUSIONS
  The study showed high detection accuracy and a subdegree positioning accuracy. On the basis of phantom studies, the frameless technique showed comparable accuracy to the frame-based approach.
authors.affiliation: Department of Radiotherapy, Universitair Ziekenhuis Brussel, Vrije Universiteit Brussel, Brussels, Belgium. thierry.gevaert@uzbrussel.be
authors.forename: Thierry
authors.initials: T
authors.lastname: Gevaert
authors.forename: Dirk
authors.initials: D
authors.lastname: Verellen
authors.forename: Koen
authors.initials: K
authors.lastname: Tournel
authors.forename: Nadine
authors.initials: N
authors.lastname: Linthout
authors.forename: Samuel
authors.initials: S
authors.lastname: Bral
authors.forename: Benedikt
authors.initials: B
authors.lastname: Engels
authors.forename: Christine
authors.initials: C
authors.lastname: Collen
authors.forename: Tom
authors.initials: T
authors.lastname: Depuydt
authors.forename: Michael
authors.initials: M
authors.lastname: Duchateau
authors.forename: Truus
authors.initials: T
authors.lastname: Reynders
authors.forename: Guy
authors.initials: G
authors.lastname: Storme
authors.forename: Mark
authors.initials: M
authors.lastname: De Ridder
mesh_terms: D056888:Patient Positioning / Q000592:standards; D019047:Phantoms, Imaging; D016634:Radiosurgery / Q000379:methods* / Q000592:standards; D061106:Radiotherapy Setup Errors / Q000517:prevention & control; D061089:Radiotherapy, Image-Guided / Q000379:methods* / Q000592:standards; D012399:Rotation; D012984:Software; D014057:Tomography, X-Ray Computed / Q000592:standards