title: Inter- and intrafraction variability in liver position in non-breath-hold stereotactic body radiotherapy.
abstract: OBJECTIVE
  The inter- and intrafraction variability of liver position was assessed in patients with liver cancer treated with kilovoltage cone-beam computed tomography (CBCT)-guided stereotactic body radiotherapy.
  
  
  METHODS
  A total of 314 CBCT scans obtained in the treatment position immediately before and after each fraction were evaluated from 29 patients undergoing six-fraction, non-breath-hold stereotactic body radiotherapy for unresectable liver cancer. Off-line, the CBCT scans were sorted into 10 bins, according to the phase of respiration. The liver position (relative to the vertebral bodies) was measured using rigid alignment of the exhale CBCT liver with the exhale planning CT liver, following the alignment of the vertebrae. The interfraction liver position change was measured by comparing the pretreatment CBCT scans, and the intrafraction change was measured from the CBCT scans obtained immediately before and after each fraction.
  
  
  RESULTS
  The mean amplitude of liver motion for all patients was 1.8 mm (range, 0.1-5.7), 8.0 mm (range, 0.1-18.8), and 4.3 mm (range 0.1-12.1) in the medial-lateral (ML), craniocaudal (CC), and anteroposterior (AP) directions, respectively. The mean absolute ML, CC, and AP interfraction changes in liver position were 2.0 mm (90th percentile, 4.2), 3.5 mm (90th percentile, 7.3), and 2.3 mm (90th percentile, 4.7). The mean absolute intrafraction ML, CC, and AP changes were 1.3 mm (90th percentile, 2.9), 1.6 mm (90th percentile, 3.6), and 1.5 mm (90th percentile, 3.1), respectively. The interfraction changes were significantly larger than the intrafraction changes, with a CC systematic error of 2.9 and 1.1 mm, respectively. The intraobserver reproducibility (sigma, n = 29 fractions) was 1.3 mm in the ML, 1.4 mm in the CC, and 1.6 mm in the AP direction.
  
  
  CONCLUSIONS
  Interfraction liver position changes relative to the vertebral bodies are an important source of geometric uncertainty, providing a rationale for prefraction soft-tissue image guidance. The intrafraction change in liver position from the beginning to the end of each fraction was small for most patients.
authors.affiliation: Princess Margaret Hospital, Toronto, ON, Canada.
authors.forename: Robert B
authors.initials: RB
authors.lastname: Case
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
authors.forename: Douglas J
authors.initials: DJ
authors.lastname: Moseley
authors.forename: John
authors.initials: J
authors.lastname: Kim
authors.forename: Kristy K
authors.initials: KK
authors.lastname: Brock
authors.forename: Laura A
authors.initials: LA
authors.lastname: Dawson
mesh_terms: D054893:Cone-Beam Computed Tomography; D019583:Dose Fractionation, Radiation; D006801:Humans; D008099:Liver / Q000000981:diagnostic imaging / Q000601:surgery; D008113:Liver Neoplasms / Q000000981:diagnostic imaging / Q000601:surgery; D009068:Movement; D015588:Observer Variation; D016634:Radiosurgery / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D012119:Respiration; D015656:Respiratory Mechanics; D013131:Spine / Q000000981:diagnostic imaging