title: Spine stereotactic body radiotherapy utilizing cone-beam CT image-guidance with a robotic couch: intrafraction motion analysis accounting for all six degrees of freedom.
abstract: OBJECTIVE
  To evaluate the residual setup error and intrafraction motion following kilovoltage cone-beam CT (CBCT) image guidance, for immobilized spine stereotactic body radiotherapy (SBRT) patients, with positioning corrected for in all six degrees of freedom.
  
  
  METHODS
  Analysis is based on 42 consecutive patients (48 thoracic and/or lumbar metastases) treated with a total of 106 fractions and 307 image registrations. Following initial setup, a CBCT was acquired for patient alignment and a pretreatment CBCT taken to verify shifts and determine the residual setup error, followed by a midtreatment and posttreatment CBCT image. For 13 single-fraction SBRT patients, two midtreatment CBCT images were obtained. Initially, a 1.5-mm and 1° tolerance was used to reposition the patient following couch shifts which was subsequently reduced to 1 mm and 1° degree after the first 10 patients.
  
  
  RESULTS
  Small positioning errors after the initial CBCT setup were observed, with 90% occurring within 1 mm and 97% within 1°. In analyzing the impact of the time interval for verification imaging (10 ± 3 min) and subsequent image acquisitions (17 ± 4 min), the residual setup error was not significantly different (p > 0.05). A significant difference (p = 0.04) in the average three-dimensional intrafraction positional deviations favoring a more strict tolerance in translation (1 mm vs. 1.5 mm) was observed. The absolute intrafraction motion averaged over all patients and all directions along x, y, and z axis (± SD) were 0.7 ± 0.5 mm and 0.5 ± 0.4 mm for the 1.5 mm and 1 mm tolerance, respectively. Based on a 1-mm and 1° correction threshold, the target was localized to within 1.2 mm and 0.9° with 95% confidence.
  
  
  CONCLUSIONS
  Near-rigid body immobilization, intrafraction CBCT imaging approximately every 15-20 min, and strict repositioning thresholds in six degrees of freedom yields minimal intrafraction motion allowing for safe spine SBRT delivery.
authors.affiliation: Department of Radiation Oncology, Sunnybrook Health Sciences Centre, University of Toronto, Toronto, Ontario, Canada.
authors.forename: Derek
authors.initials: D
authors.lastname: Hyde
authors.forename: Fiona
authors.initials: F
authors.lastname: Lochray
authors.forename: Renee
authors.initials: R
authors.lastname: Korol
authors.forename: Melanie
authors.initials: M
authors.lastname: Davidson
authors.forename: C Shun
authors.initials: CS
authors.lastname: Wong
authors.forename: Lijun
authors.initials: L
authors.lastname: Ma
authors.forename: Arjun
authors.initials: A
authors.lastname: Sahgal
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D057918:Fiducial Markers; D006801:Humans; D007103:Immobilization / Q000379:methods; D009068:Movement; D058958:Organs at Risk / Q000000981:diagnostic imaging / Q000528:radiation effects; D011832:Radiation Injuries / Q000517:prevention & control; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods; D061106:Radiotherapy Setup Errors / Q000517:prevention & control; D011881:Radiotherapy, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D012371:Robotics / Q000295:instrumentation / Q000379:methods*; D013116:Spinal Cord / Q000000981:diagnostic imaging / Q000528:radiation effects; D013125:Spinal Neoplasms / Q000000981:diagnostic imaging / Q000556:secondary / Q000601:surgery*