title: Repositioning accuracy of fractionated stereotactic irradiation: assessment of isocentre alignment for different dental fixations by using sequential CT scanning.
abstract: OBJECTIVE
  To quantify the accuracy and reproducibility of patient repositioning in fractionated stereotactic conformal radiotherapy (SCRT) using dental fixations in conjunction with a stereotactic head mask.
  
  
  METHODS
  One hundred and fourteen verification CT scans were performed on 57 patients in order to check set-up alignment. The first scan was done immediately after the first treatment. Twelve patients were checked for alignment accuracy with weekly CT scans over a period of 3-6 weeks, all others had 1-2 scans. Two different dental fixations were used in combination with a non-invasive mask system: an upper jaw support (35 patients) and a customised bite-block (17 patients). Five patients were treated with no additional fixation. Co-registration to the planning CT was used to assess alignment of the isocentre to the reference markers. Additionally, the intra-operator variability of image co-registration was assessed.
  
  
  RESULTS
  There was a significant improvement of the overall alignment in using the bite-block instead of the upper jaw support (P<0.001). The mean deviation was for the bite-block 2.2+/-1.1 mm (1 SD), for the upper jaw support 3.3+/-1.8 mm and 3.7+/-2.8 mm for the mask alone. Overall isocentre deviations independent of the method of fixation were 2.8 mm (1.7 mm, 1 SD). Displacements in CC direction were significantly less for the bite-block compared to the upper jaw support (P=0.03). The addition of an upper jaw support significantly reduced lateral rotations compared to the mask system alone (P=0.03). The intra-operator variability of image co-registration was 1.59+/-0.49 mm (1 SD).
  
  
  CONCLUSIONS
  The reproducibility of patient positioning using a re-locatable head mask system combined with a bite-block is within the reported range for similar devices and is preferable to a simple upper jaw support. In order to further reduce the margin for the planning target volume an intra-oral dental fixation is recommended.
authors.affiliation: Department of Radiation Oncology, University Hospital, Zurich, Switzerland.
authors.forename: Brigitta G
authors.initials: BG
authors.lastname: Baumert
authors.forename: Peter
authors.initials: P
authors.lastname: Egli
authors.forename: Stephan
authors.initials: S
authors.lastname: Studer
authors.forename: Cary
authors.initials: C
authors.lastname: Dehing
authors.forename: J Bernard
authors.initials: JB
authors.lastname: Davis
mesh_terms: D001932:Brain Neoplasms / Q000532:radiotherapy*; D003748:Dental Equipment; D019583:Dose Fractionation, Radiation; D004867:Equipment Design; D006801:Humans; D007103:Immobilization / Q000295:instrumentation*; D008397:Masks; D009064:Mouth Protectors; D020266:Radiotherapy, Conformal / Q000295:instrumentation* / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012984:Software; D013238:Stereotaxic Techniques; D014057:Tomography, X-Ray Computed