title: Nonrigid patient setup errors in the head-and-neck region.
abstract: OBJECTIVE
  To investigate the magnitude and clinical relevance of relative motion/nonrigid setup errors in the head-and-neck (H&N) region.
  
  
  METHODS
  Eleven patients with tumors in the H&N region were immobilized in thermoplastic head masks. Patient positioning was verified using a kilovoltage cone-beam CT (kv CBCT) prior to 100 treatment fractions. Five different regions of interest (ROIs) were selected for automatic image registration of planning CT and verification CBCT: (1) the whole volume covering planning CT and CBCT, (2) the skull, (3) the mandible, (4) C1-C3, and (5) C4-C6. Differences were calculated describing relative motion between the ROIs.
  
  
  RESULTS
  The 3-D patient setup error was 3.2 mm +/- 1.7 mm based on registration of the whole volume. No systematic relative motion (group mean errors <0.5 mm and <0.5 degrees ) between planning and treatment for any ROI was observed. Mobility was largest for the skull and the mandible relative to C4-C6 with 3-D displacements of 4.7 mm +/- 2.5 mm and 4.4 mm +/- 2.5 mm. Relative rotations were largest around the left-right axis (nodding) between C1-C3 and C4-C6 with maximum 11 degrees . No time trend of relative motion was observed. Margins for compensation of relative motion ranged between 5 mm and 10 mm.
  
  
  CONCLUSIONS
  The simplification of the patient as a rigid body was shown to result in significant errors due to relative motion in the H&N region. Margins for compensation of relative motion exceeded margins for compensation of patient positioning errors.
authors.affiliation: Department of Radiation Oncology, Julius Maximilians University, Würzburg, Germany.
authors.forename: Buelent
authors.initials: B
authors.lastname: Polat
authors.forename: Juergen
authors.initials: J
authors.lastname: Wilbert
authors.forename: Kurt
authors.initials: K
authors.lastname: Baier
authors.forename: Michael
authors.initials: M
authors.lastname: Flentje
authors.forename: Matthias
authors.initials: M
authors.lastname: Guckenberger
mesh_terms: D016477:Artifacts; D001822:Body Burden; D054893:Cone-Beam Computed Tomography / Q000295:instrumentation*; D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D007103:Immobilization / Q000295:instrumentation*; D008397:Masks; D011835:Radiation Protection; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation*