title: Precision of image-guided radiotherapy (IGRT) in six degrees of freedom and limitations in clinical practice.
abstract: OBJECTIVE
  To evaluate the precision of image-guided radiotherapy (IGRT) using cone-beam computed tomography (CB-CT) for volume imaging and a robotic couch for correcting setup errors in six degrees of freedom.
  
  
  METHODS
  47 consecutive patients with 372 fractions were classified according to whether a patient fixation device was used (pat(fix): n = 28) or not (pat(non-fix): n = 19). Prior to treatment a CB-CT was acquired and translational and rotational setup errors were corrected online without an action level using a robotic couch (HexaPOD). A second CB-CT was acquired after the correction process and after treatment in 134 and 238 fractions, respectively.
  
  
  RESULTS
  In 17 fractions (4.6%) rotational errors > 3 degrees exceeded the motion range of the HexaPOD. Errors (3D vector) after the correction process were significantly smaller for pat(fix) compared to pat(non-fix) (p < 0.001): 0.9 mm +/- 0.5 mm and 1.6 mm +/- 0.8 mm, respectively. For pat(non-fix) the correction of rotational errors resulted in displacements of the patients on the angled couch of 0.6 mm/1 degree. Intrafractional motion further decreased precision in pat(non-fix) but not in pat(fix).
  
  
  CONCLUSIONS
  Very high precision in cranial and extracranial treatment of immobilized patients was demonstrated. Without application of adequate immobilization the correction of rotational errors and intrafractional patient motion significantly decreased the accuracy of the online correction protocol.
authors.affiliation: Department of Radiation Oncology, Julius Maximilians University of Wuerzburg, Josef-Schneider-Strasse 11, 97080 Würzburg, Germany. guckenberg_m@klinik.uni-wuerzburg.de
authors.forename: Matthias
authors.initials: M
authors.lastname: Guckenberger
authors.forename: Juergen
authors.initials: J
authors.lastname: Meyer
authors.forename: Juergen
authors.initials: J
authors.lastname: Wilbert
authors.forename: Kurt
authors.initials: K
authors.lastname: Baier
authors.forename: Otto
authors.initials: O
authors.lastname: Sauer
authors.forename: Michael
authors.initials: M
authors.lastname: Flentje
mesh_terms: D016477:Artifacts; D019583:Dose Fractionation, Radiation; D004867:Equipment Design; D006258:Head and Neck Neoplasms / Q000532:radiotherapy; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000295:instrumentation*; D021621:Imaging, Three-Dimensional / Q000295:instrumentation*; D007103:Immobilization / Q000295:instrumentation; D008297:Male; D009369:Neoplasms / Q000532:radiotherapy*; D009862:Online Systems; D010386:Pelvic Neoplasms / Q000532:radiotherapy; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation*; D012371:Robotics / Q000295:instrumentation*; D013125:Spinal Neoplasms / Q000532:radiotherapy / Q000556:secondary; D014057:Tomography, X-Ray Computed / Q000295:instrumentation*