title: Performance of a novel repositioning head frame for gamma knife perfexion and image-guided linac-based intracranial stereotactic radiotherapy.
abstract: OBJECTIVE
  To evaluate the geometric positioning and immobilization performance of a vacuum bite-block repositioning head frame (RHF) system for Perfexion (PFX-SRT) and linac-based intracranial image-guided stereotactic radiotherapy (SRT).
  
  
  METHODS
  Patients with intracranial tumors received linac-based image-guided SRT using the RHF for setup and immobilization. Three hundred thirty-three fractions of radiation were delivered in 12 patients. The accuracy of the RHF was estimated for linac-based SRT with online cone-beam CT (CBCT) and for PFX-SRT with a repositioning check tool (RCT) and offline CBCT. The RCT's ability to act as a surrogate for anatomic position was estimated through comparison to CBCT image matching. Immobilization performance was evaluated daily with pre- and postdose delivery CBCT scans and RCT measurements.
  
  
  RESULTS
  The correlation coefficient between RCT- and CBCT-reported displacements was 0.59, 0.75, 0.79 (Right, Superior, and Anterior, respectively). For image-guided linac-based SRT, the mean three-dimensional (3D) setup error was 0.8 mm with interpatient (Sigma) and interfraction (sigma) variations of 0.1 and 0.4 mm, respectively. For PFX-SRT, the initial, uncorrected mean 3D positioning displacement in stereotactic coordinates was 2.0 mm, with Sigma = 1.1 mm and sigma = 0.8 mm. Considering only RCT setups <1mm (PFX action level) the mean 3D positioning displacement reduced to 1.3 mm, with Sigma = 0.9 mm and sigma = 0.4 mm. The largest contributing systematic uncertainty was in the superior-inferior direction (mean displacement = -0.5 mm; Sigma = 0.9 mm). The largest mean rotation was 0.6 degrees in pitch. The mean 3D intrafraction motion was 0.4 +/- 0.3 mm.
  
  
  CONCLUSIONS
  The RHF provides excellent immobilization for intracranial SRT and PFX-SRT. Some small systematic uncertainties in stereotactic positioning exist and must be considered when generating PFX-SRT treatment plans. The RCT provides reasonable surrogacy for internal anatomic displacement.
authors.affiliation: Princess Margaret Hospital, Toronto, Canada. Mark.Ruschin@rmp.uhn.on.ca
authors.forename: Mark
authors.initials: M
authors.lastname: Ruschin
authors.forename: Nazanin
authors.initials: N
authors.lastname: Nayebi
authors.forename: Per
authors.initials: P
authors.lastname: Carlsson
authors.forename: Kevin
authors.initials: K
authors.lastname: Brown
authors.forename: Messeret
authors.initials: M
authors.lastname: Tamerou
authors.forename: Winnie
authors.initials: W
authors.lastname: Li
authors.forename: Normand
authors.initials: N
authors.lastname: Laperriere
authors.forename: Arjun
authors.initials: A
authors.lastname: Sahgal
authors.forename: Young-Bin
authors.initials: YB
authors.lastname: Cho
authors.forename: Cynthia
authors.initials: C
authors.lastname: Ménard
authors.forename: David
authors.initials: D
authors.lastname: Jaffray
mesh_terms: D001932:Brain Neoplasms / Q000601:surgery*; D054893:Cone-Beam Computed Tomography / Q000379:methods; D016371:Cranial Irradiation / Q000295:instrumentation*; D019583:Dose Fractionation, Radiation; D006257:Head; D006801:Humans; D007103:Immobilization / Q000295:instrumentation* / Q000379:methods; D008488:Medical Illustration; D056888:Patient Positioning / Q000295:instrumentation*; D011446:Prospective Studies; D016634:Radiosurgery / Q000295:instrumentation* / Q000379:methods; D011881:Radiotherapy, Computer-Assisted / Q000295:instrumentation / Q000379:methods; D014618:Vacuum