title: Diffusion weighted MRI in head-and-neck cancer: geometrical accuracy.
abstract: BACKGROUND
  The aim of this study is to assess the geometric accuracy of diffusion weighted (DW)-MRI by quantification of geometric distortions in the gross tumor volume (GTV) in head and neck (HN) cancer.
  
  
  METHODS
  A retrospective analysis was performed on the data of 23 patients (with 24 lesions). For these patients, magnetic field maps and DW-MRI were acquired. The magnetic field maps were converted to voxel displacement maps. GTV delineations were transferred onto these voxel displacement maps and the voxel shifts in the GTV were analyzed.
  
  
  RESULTS
  The median shift was 3.2mm and the maximal posterior and anterior shifts were up to 15.0 and 26.0mm respectively. The range of shifts varied from 11.8 to 25.6mm. The percentage of GTV voxels that showed a shift of at least 6mm was found to be 23.2%.
  
  
  CONCLUSIONS
  Current DW-MRI images of HN tumors show severe distortions up to centimeters, which restrict the use of DW-MRI scans for GTV definition in RT treatment planning.
authors.affiliation: Department of Radiotherapy, University Medical Center Utrecht, The Netherlands. Electronic address: T.Schakel@umcutrecht.nl.
authors.forename: Tim
authors.initials: T
authors.lastname: Schakel
authors.forename: Johannes M
authors.initials: JM
authors.lastname: Hoogduin
authors.forename: Chris H J
authors.initials: CH
authors.lastname: Terhaard
authors.forename: Marielle E P
authors.initials: ME
authors.lastname: Philippens
mesh_terms: D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods*; D006258:Head and Neck Neoplasms / Q000473:pathology* / Q000532:radiotherapy; D006801:Humans; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012189:Retrospective Studies
keywords: Diffusion; Geometrical accuracy; Head-and-neck cancer; MRI; Radiotherapy