title: Deformable image registration for adaptive radiation therapy of head and neck cancer: accuracy and precision in the presence of tumor changes.
abstract: OBJECTIVE
  To compare deformable image registration (DIR) accuracy and precision for normal and tumor tissues in head and neck cancer patients during the course of radiation therapy (RT).
  
  
  METHODS
  Thirteen patients with oropharyngeal tumors, who underwent submucosal implantation of small gold markers (average 6, range 4-10) around the tumor and were treated with RT were retrospectively selected. Two observers identified 15 anatomical features (landmarks) representative of normal tissues in the planning computed tomography (pCT) scan and in weekly cone beam CTs (CBCTs). Gold markers were digitally removed after semiautomatic identification in pCTs and CBCTs. Subsequently, landmarks and gold markers on pCT were propagated to CBCTs, using a b-spline-based DIR and, for comparison, rigid registration (RR). To account for observer variability, the pair-wise difference analysis of variance method was applied. DIR accuracy (systematic error) and precision (random error) for landmarks and gold markers were quantified. Time trend of the precisions for RR and DIR over the weekly CBCTs were evaluated.
  
  
  RESULTS
  DIR accuracies were submillimeter and similar for normal and tumor tissue. DIR precision (1 SD) on the other hand was significantly different (P<.01), with 2.2 mm vector length in normal tissue versus 3.3 mm in tumor tissue. No significant time trend in DIR precision was found for normal tissue, whereas in tumor, DIR precision was significantly (P<.009) degraded during the course of treatment by 0.21 mm/week.
  
  
  CONCLUSIONS
  DIR for tumor registration proved to be less precise than that for normal tissues due to limited contrast and complex non-elastic tumor response. Caution should therefore be exercised when applying DIR for tumor changes in adaptive procedures.
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, the Netherlands. Electronic address: a.mencarelli@nki.nl.
authors.forename: Angelo
authors.initials: A
authors.lastname: Mencarelli
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, the Netherlands.
authors.forename: Simon Robert
authors.initials: SR
authors.lastname: van Kranen
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, the Netherlands.
authors.forename: Olga
authors.initials: O
authors.lastname: Hamming-Vrieze
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, the Netherlands.
authors.forename: Suzanne
authors.initials: S
authors.lastname: van Beek
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, the Netherlands; Department of Radiation Oncology, Amsterdam Medical Centre, Amsterdam, the Netherlands.
authors.forename: Coenraad Robert
authors.initials: CR
authors.lastname: Nico Rasch
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, the Netherlands.
authors.forename: Marcel
authors.initials: M
authors.lastname: van Herk
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, the Netherlands.
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
mesh_terms: D000465:Algorithms; D000704:Analysis of Variance; D059925:Anatomic Landmarks / Q000000981:diagnostic imaging*; D054893:Cone-Beam Computed Tomography; D057918:Fiducial Markers; D006046:Gold; D006801:Humans; D015588:Observer Variation; D058958:Organs at Risk / Q000000981:diagnostic imaging*; D009959:Oropharyngeal Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy*; D011873:Radiology Information Systems; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation / Q000379:methods
chemical_list: D006046:Gold