title: Assessment of residual error in liver position using kV cone-beam computed tomography for liver cancer high-precision radiation therapy.
abstract: OBJECTIVE
  To evaluate the residual error in liver position using breath-hold kilovoltage (kV) cone-beam computed tomography (CT) following on-line orthogonal megavoltage (MV) image-guided breath-hold liver cancer conformal radiotherapy.
  
  
  METHODS
  Thirteen patients with liver cancer treated with 6-fraction breath-hold conformal radiotherapy were investigated. Before each fraction, orthogonal MV images were obtained during exhale breath-hold, with repositioning for offsets>3 mm, using the diaphragm for cranio-caudal (CC) alignment and vertebral bodies for medial-lateral (ML) and anterior posterior (AP) alignment. After repositioning, repeat orthogonal MV images, orthogonal kV fluoroscopic movies, and kV cone-beam CTs were obtained in exhale breath-hold. The cone-beam CT livers were registered to the planning CT liver to obtain the residual setup error in liver position.
  
  
  RESULTS
  After repositioning, 78 orthogonal MV image pairs, 61 orthogonal kV image pairs, and 72 kV cone-beam CT scans were obtained. Population random setup errors (sigma) in liver position were 2.7 mm (CC), 2.3 mm (ML), and 3.0 mm (AP), and systematic errors (Sigma) were 1.1 mm, 1.9 mm, and 1.3 mm in the superior, medial, and posterior directions. Liver offsets>5 mm were observed in 33% of cases; offsets>10 mm and liver deformation>5 mm were observed in a minority of patients.
  
  
  CONCLUSIONS
  Liver position after radiation therapy guided with MV orthogonal imaging was within 5 mm of planned position in the majority of patients. kV cone-beam CT image guidance should improve accuracy with reduced dose compared with orthogonal MV image guidance for liver cancer radiation therapy.
authors.affiliation: Radiation Medicine Program, Princess Margaret Hospital, Department of Radiation Oncology, University of Toronto, Toronto, Ontario, Canada.
authors.forename: Maria A
authors.initials: MA
authors.lastname: Hawkins
authors.forename: Kristy K
authors.initials: KK
authors.lastname: Brock
authors.forename: Cynthia
authors.initials: C
authors.lastname: Eccles
authors.forename: Douglas
authors.initials: D
authors.lastname: Moseley
authors.forename: David
authors.initials: D
authors.lastname: Jaffray
authors.forename: Laura A
authors.initials: LA
authors.lastname: Dawson
mesh_terms: D006528:Carcinoma, Hepatocellular / Q000000981:diagnostic imaging / Q000532:radiotherapy; D018281:Cholangiocarcinoma / Q000000981:diagnostic imaging / Q000532:radiotherapy; D005471:Fluoroscopy / Q000379:methods; D006801:Humans; D008099:Liver / Q000000981:diagnostic imaging*; D008113:Liver Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D009068:Movement; D011187:Posture; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D012119:Respiration; D014057:Tomography, X-Ray Computed / Q000379:methods*