title: Setup uncertainties of anatomical sub-regions in head-and-neck cancer patients after offline CBCT guidance.
abstract: OBJECTIVE
  To quantify local geometrical uncertainties in anatomical sub-regions during radiotherapy for head-and-neck cancer patients.
  
  
  METHODS
  Local setup accuracy was analyzed for 38 patients, who had received intensity-modulated radiotherapy and were regularly scanned during treatment with cone beam computed tomography (CBCT) for offline patient setup correction. In addition to the clinically used large region of interest (ROI), we defined eight ROIs in the planning CT that contained rigid bony structures: the mandible, larynx, jugular notch, occiput bone, vertebrae C1-C3, C3-C5, and C5-C7, and the vertebrae caudal of C7. By local rigid registration to successive CBCT scans, the local setup accuracy of each ROI was determined and compared with the overall setup error assessed with the large ROI. Deformations were distinguished from rigid body movements by expressing movement relative to a reference ROI (vertebrae C1-C3).
  
  
  RESULTS
  The offline patient setup correction protocol using the large ROI resulted in residual systematic errors (1 SD) within 1.2 mm and random errors within 1.5 mm for each direction. Local setup errors were larger, ranging from 1.1 to 3.4 mm (systematic) and 1.3 to 2.5 mm (random). Systematic deformations ranged from 0.4 mm near the reference C1-C3 to 3.8 mm for the larynx. Random deformations ranged from 0.5 to 3.6 mm.
  
  
  CONCLUSIONS
  Head-and-neck cancer patients show considerable local setup variations, exceeding residual global patient setup uncertainty in an offline correction protocol. Current planning target volume margins may be inadequate to account for these uncertainties. We propose registration of multiple ROIs to drive correction protocols and adaptive radiotherapy to reduce the impact of local setup variations.
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands.
authors.forename: Simon
authors.initials: S
authors.lastname: van Kranen
authors.forename: Suzanne
authors.initials: S
authors.lastname: van Beek
authors.forename: Coen
authors.initials: C
authors.lastname: Rasch
authors.forename: Marcel
authors.initials: M
authors.lastname: van Herk
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
mesh_terms: D000465:Algorithms; D002574:Cervical Vertebrae / Q000000981:diagnostic imaging; D054893:Cone-Beam Computed Tomography / Q000379:methods; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D006801:Humans; D007103:Immobilization / Q000379:methods*; D007601:Jugular Veins / Q000000981:diagnostic imaging; D007830:Larynx / Q000000981:diagnostic imaging; D008334:Mandible / Q000000981:diagnostic imaging; D009068:Movement; D009777:Occipital Bone / Q000000981:diagnostic imaging; D011187:Posture; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated; D013997:Time Factors