title: Positioning reproducibility with and without rotational corrections for 2 head and neck immobilization systems.
abstract: OBJECTIVE
  The purpose of this study was to evaluate the impact of offline rotational corrections and assess intrafraction motion for head and neck (H&N) cancer patients immobilized with and without a custom neck cushion.
  
  
  METHODS
  Fifty H&N cancer patients were immobilized and imaged with pretreatment and posttreatment cone beam computed tomography (CBCT) for each treatment fraction. Of these patients, 25 had a custom neck cushion added to their immobilization. Each CBCT was registered to the simulation computed tomography offline. Registrations were performed with automatching tools and a matching volume of interest that consisted of a 5-mm expansion around the mandible, occipital bone, C1/C2, and C7/T1. To determine positioning accuracy, the registration was inspected to confirm these bony anatomy structures were contained within a 3- or 5-mm expansion of the simulation position. If not, the registration was repeated with rotational corrections included and re-evaluated. For each fraction, intrafraction motion was also quantified through the difference between the pretreatment and posttreatment CBCT registration coordinates.
  
  
  RESULTS
  For translational registrations, the bony anatomy in pretreatment imaging was outside the 3-mm or 5-mm expansion structure, respectively, for 49% and 15% of fractions on average for patients without a custom headrest and for 48% and 13% of fractions on average for patients with a custom headrest. The addition of rotational corrections reduced these numbers to 21% and 4% and to 28% and 6%, respectively. Intrafraction motion was significantly lower for patients immobilized with the addition of a custom neck cushion: 1.0 ± 0.5 mm compared with 1.8 ± 1.6 mm for patients with the standard headrest only (P = .02). This was reflected in posttreatment positioning accuracy, which was significantly reduced in the case of the standard headrest compared with pretreatment imaging (P values of < .001 to .048).
  
  
  CONCLUSIONS
  Rotational corrections significantly improved pretreatment patient positioning accuracy (P < .001). Intrafraction motion was reduced significantly through the addition of a custom neck cushion and resulted in an increase in posttreatment positioning accuracy for these patients.
authors.affiliation: Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota.
authors.forename: Lorraine
authors.initials: L
authors.lastname: Courneyea
authors.affiliation: Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota.
authors.forename: John
authors.initials: J
authors.lastname: Mullins
authors.affiliation: Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota.
authors.forename: Michelle
authors.initials: M
authors.lastname: Howard
authors.affiliation: Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota.
authors.forename: Robert
authors.initials: R
authors.lastname: Foote
authors.affiliation: Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota.
authors.forename: Yolanda
authors.initials: Y
authors.lastname: Garces
authors.affiliation: Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota.
authors.forename: Daniel
authors.initials: D
authors.lastname: Ma
authors.affiliation: Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota.
authors.forename: Chris
authors.initials: C
authors.lastname: Beltran
authors.affiliation: Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota.
authors.forename: Debra
authors.initials: D
authors.lastname: Brinkmann
authors.affiliation: Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota. Electronic address: Pafundi.Deanna@mayo.edu.
authors.forename: Deanna
authors.initials: D
authors.lastname: Pafundi
mesh_terms: D054893:Cone-Beam Computed Tomography; D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D007103:Immobilization / Q000295:instrumentation / Q000379:methods*; D058958:Organs at Risk; D056888:Patient Positioning; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061106:Radiotherapy Setup Errors / Q000517:prevention & control*; D061089:Radiotherapy, Image-Guided / Q000379:methods*