title: Fractionated stereotactic radiotherapy for skull base tumors: analysis of treatment accuracy using a stereotactic mask fixation system.
abstract: BACKGROUND
  To assess the accuracy of fractionated stereotactic radiotherapy (FSRT) using a stereotactic mask fixation system.
  
  
  METHODS
  Sixteen patients treated with FSRT were involved in the study. A commercial stereotactic mask fixation system (BrainLAB AG) was used for patient immobilization. Serial CT scans obtained before and during FSRT were used to assess the accuracy of patient immobilization by comparing the isocenter position. Daily portal imaging were acquired to establish day to day patient position variation. Displacement errors along the different directions were calculated as combination of systematic and random errors.
  
  
  RESULTS
  The mean isocenter displacements based on localization and verification CT imaging were 0.1 mm (SD 0.3 mm) in the lateral direction, 0.1 mm (SD 0.4 mm) in the anteroposterior, and 0.3 mm (SD 0.4 mm) in craniocaudal direction. The mean 3D displacement was 0.5 mm (SD 0.4 mm), being maximum 1.4 mm. No significant differences were found during the treatment (P=0.4). The overall isocenter displacement as calculated by 456 anterior and lateral portal images were 0.3 mm (SD 0.9 mm) in the mediolateral direction, -0.2 mm (SD 1 mm) in the anteroposterior direction, and 0.2 mm (SD 1.1 mm) in the craniocaudal direction. The largest displacement of 2.7 mm was seen in the cranio-caudal direction, with 95% of displacements<2 mm in any direction.
  
  
  CONCLUSIONS
  The results indicate that the setup error of the presented mask system evaluated by CT verification scans and portal imaging are minimal. Reproducibility of the isocenter position is in the best range of positioning reproducibility reported for other stereotactic systems.
authors.affiliation: Department of Radiation Oncology, Sant' Andrea Hospital, University La Sapienza, via di Grottarossa 1035-1039, 00189, Rome, Italy. gminniti@ospedalesantandrea.it
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Minniti
authors.forename: Maurizio
authors.initials: M
authors.lastname: Valeriani
authors.forename: Enrico
authors.initials: E
authors.lastname: Clarke
authors.forename: Marco
authors.initials: M
authors.lastname: D'Arienzo
authors.forename: Michelangelo
authors.initials: M
authors.lastname: Ciotti
authors.forename: Roberto
authors.initials: R
authors.lastname: Montagnoli
authors.forename: Francesca
authors.initials: F
authors.lastname: Saporetti
authors.forename: Riccardo Maurizi
authors.initials: RM
authors.lastname: Enrici
mesh_terms: D004867:Equipment Design / Q000295:instrumentation; D006801:Humans; D016634:Radiosurgery / Q000295:instrumentation*; D011880:Radiotherapy Planning, Computer-Assisted; D015203:Reproducibility of Results; D019292:Skull Base Neoplasms / Q000000981:diagnostic imaging / Q000601:surgery*; D014057:Tomography, X-Ray Computed