title: Inter- and intra-fraction motion in stereotactic body radiotherapy for spinal and paraspinal tumours using cone-beam CT and positional correction in six degrees of freedom.
abstract: BACKGROUND
  Stereotactic body radiotherapy (SBRT) for spinal tumours delivers high doses per fraction to targets in close proximity to neural tissue. With steep dose gradients, small changes in position can confer significant dosimetric impact on adjacent structures. We analysed positioning error in consecutively treated patients on a strict image-guidance protocol with online correction in 6 degrees of freedom (6-DOF).
  
  
  METHODS
  Set-up error, residual error post-correction and intra-fraction motion for 30 courses of spinal SBRT in 27 patients were assessed using cone-beam CT. Positional error was corrected in x, y and z translational planes and rotational axes using a robotic couch, applying 2 mm and 2° action levels. Linear mixed-effects model assessed whether positional error was influenced by factors such as vertebral level, immobilisation device and treatment duration.
  
  
  RESULTS
  Sixty-two fractions were delivered with 225 image registrations. Median treatment duration was significantly longer for patients treated with static-field intensity-modulated radiotherapy compared with volumetric-modulated arc treatment--40 min versus 28 min, respectively (P = 0.01). Across all fractions, the median residual positional error after initial correction was greatest in the x translational plane (0.5 mm; 95% confidence interval (CI) 0.3-0.6) and y rotational axis (0.25°; 95% CI 0.1-0.3). Median intra-fraction error was also greatest in the x-plane (0.7 mm; 95% CI 0.5-1.0) and y-axis (0.4°; 95% CI 0.2-0.5).
  
  
  CONCLUSIONS
  With strict immobilisation, image-guidance and 6-DOF correction, our current practice of applying 3-mm planning margins for target volumes and critical structures appears safe. Lower image-guidance action thresholds plus verification with end-to-end testing would be recommended before further reducing margins.
authors.affiliation: Department of Radiation Oncology, Princess Alexandra Hospital, Brisbane, Queensland, Australia.
authors.forename: Renee
authors.initials: R
authors.lastname: Finnigan
authors.affiliation: Department of Radiation Oncology, Princess Alexandra Hospital, Brisbane, Queensland, Australia.
authors.forename: Brock
authors.initials: B
authors.lastname: Lamprecht
authors.affiliation: Department of Radiation Oncology, Princess Alexandra Hospital, Brisbane, Queensland, Australia.
authors.forename: Tamara
authors.initials: T
authors.lastname: Barry
authors.affiliation: Centre for Experimental Haematology, University of Queensland School of Medicine, Translational Research Institute, Brisbane, Queensland, Australia.
authors.forename: Kimberley
authors.initials: K
authors.lastname: Jones
authors.affiliation: Department of Radiation Oncology, Princess Alexandra Hospital, Brisbane, Queensland, Australia.
authors.forename: Joshua
authors.initials: J
authors.lastname: Boyd
authors.affiliation: University of Queensland School of Medicine, Princess Alexandra Hospital, Brisbane, Queensland, Australia.
authors.forename: Andrew
authors.initials: A
authors.lastname: Pullar
authors.affiliation: University of Queensland School of Medicine, Princess Alexandra Hospital, Brisbane, Queensland, Australia.
authors.forename: Bryan
authors.initials: B
authors.lastname: Burmeister
authors.affiliation: University of Queensland School of Medicine, Princess Alexandra Hospital, Brisbane, Queensland, Australia.
authors.forename: Matthew
authors.initials: M
authors.lastname: Foote
mesh_terms: D016477:Artifacts; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D019583:Dose Fractionation, Radiation; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods; D007103:Immobilization / Q000379:methods; D009038:Motion; D009068:Movement; D056888:Patient Positioning / Q000379:methods; D011856:Radiographic Image Enhancement / Q000379:methods; D016634:Radiosurgery / Q000379:methods*; D011879:Radiotherapy Dosage; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013125:Spinal Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D016896:Treatment Outcome
keywords: cone-beam computed tomography; image-guided radiotherapy; radiosurgery; spine; stereotactic