title: Reproducibility of the MRI-defined spinal cord position in stereotactic radiotherapy for spinal oligometastases.
abstract: OBJECTIVE
  To establish the reproducibility of the MRI-defined spinal cord position within the spinal canal.
  
  
  METHODS
  We acquired T1- and T2-weighted MRI scans of 15 volunteers on spine levels C7, T8 or L2. The scan protocol was repeated several times for different postures and time intervals. We determined the spinal cord shift (LR, AP, CC) using a rigid, grey value, vertebral body registration, followed by a spinal cord registration. We tested the sensitivity of our method, introducing artificial spinal cord shifts by varying the size and direction of the water-fat-shift (WFS) of the MR sequences.
  
  
  RESULTS
  The spinal cord position on MRI is reproducible within approximately 0.2mm SD (LR, AP) and 0.7mm SD (CC) when reproducing the posture on the same day, as well as several weeks later. However, when comparing different postures, shifts of ∼1.5mm were found. Varying the WFS difference between scans (0.6-3.0mm) induced equivalent virtual spinal cord shifts (0.5-2.5mm).
  
  
  CONCLUSIONS
  Displacement of the spinal cord inside the spinal canal may occur as a result of posture change. Considering the total geometric accuracy of spine SBRT, MRI-defined spinal cord position is sufficiently reproducible and requires no addition to the typical setup-and-intrafraction motion PRV margin if posture is identical throughout the RT process.
authors.affiliation: Department of Radiation Oncology, Netherlands Cancer Institute, Amsterdam, The Netherlands. Electronic address: a.v.mourik@nki.nl.
authors.forename: Anke M
authors.initials: AM
authors.lastname: van Mourik
authors.affiliation: Department of Radiation Oncology, Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
authors.affiliation: Department of Radiation Oncology, Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: Tineke
authors.initials: T
authors.lastname: Vijlbrief
authors.affiliation: Department of Radiation Oncology, Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: Luc
authors.initials: L
authors.lastname: Dewit
authors.affiliation: Department of Radiation Oncology, Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: Eugène M
authors.initials: EM
authors.lastname: Damen
authors.affiliation: Department of Radiation Oncology, Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: Peter
authors.initials: P
authors.lastname: Remeijer
authors.affiliation: Department of Radiation Oncology, Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: Uulke A
authors.initials: UA
authors.lastname: van der Heide
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D016634:Radiosurgery / Q000379:methods*; D015203:Reproducibility of Results; D013116:Spinal Cord; D013120:Spinal Cord Neoplasms / Q000473:pathology / Q000556:secondary
keywords: Geometric accuracy; MRI; Margins; Spine SBRT