title: On-line cone beam CT image guidance for vocal cord tumor targeting.
abstract: OBJECTIVE
  We are developing a technique for highly focused vocal cord irradiation in early glottic carcinoma to optimally treat a target volume confined to a single cord. This technique, in contrast with the conventional methods, aims at sparing the healthy vocal cord. As such a technique requires sub-mm daily targeting accuracy to be effective, we investigate the accuracy achievable with on-line kV-cone beam CT (CBCT) corrections.
  
  
  METHODS
  CBCT scans were obtained in 10 early glottic cancer patients in each treatment fraction. The grey value registration available in X-ray volume imaging (XVI) software (Elekta, Synergy) was applied to a volume of interest encompassing the thyroid cartilage. After application of the thus derived corrections, residue displacements with respect to the planning CT scan were measured at clearly identifiable relevant landmarks. The intra- and inter-observer variations were also measured.
  
  
  RESULTS
  While before correction the systematic displacements of the vocal cords were as large as 2.4+/-3.3mm (cranial-caudal population mean+/-SD Sigma), daily CBCT registration and correction reduced these values to less than 0.2+/-0.5mm in all directions. Random positioning errors (SD sigma) were reduced to less than 1mm. Correcting only for translations and not for rotations did not appreciably affect this accuracy. The residue random displacements partly stem from intra-observer variations (SD=0.2-0.6mm).
  
  
  CONCLUSIONS
  The use of CBCT for daily image guidance in combination with standard mask fixation reduced systematic and random set-up errors of the vocal cords to <1mm prior to the delivery of each fraction dose. Thus, this facilitates the high targeting precision required for a single vocal cord irradiation.
authors.affiliation: Radiation Oncology, Division of Medical Physics, Erasmus MC-Daniel den Hoed Cancer Center, Rotterdam, The Netherlands. s.osman@erasmusmc.nl
authors.forename: Sarah O S
authors.initials: SO
authors.lastname: Osman
authors.forename: Hans C J
authors.initials: HC
authors.lastname: de Boer
authors.forename: Eleftheria
authors.initials: E
authors.lastname: Astreinidou
authors.forename: Anne
authors.initials: A
authors.lastname: Gangsaas
authors.forename: Ben J M
authors.initials: BJ
authors.lastname: Heijmen
authors.forename: Peter C
authors.initials: PC
authors.lastname: Levendag
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000295:instrumentation / Q000379:methods*; D005260:Female; D006801:Humans; D007822:Laryngeal Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D008297:Male; D015588:Observer Variation; D009862:Online Systems; D011832:Radiation Injuries / Q000517:prevention & control; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D011881:Radiotherapy, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated; D012494:Sampling Studies; D014827:Vocal Cords / Q000000981:diagnostic imaging / Q000528:radiation effects*