title: Inter- and intrafractional localisation errors in cone-beam CT guided stereotactic radiation therapy of tumours in the liver and lung.
abstract: BACKGROUND
  Localisation errors in cone-beam CT (CBCT) guided stereotactic body radiation therapy (SBRT) were evaluated and compared to positioning using the external coordinates of a stereotactic body frame (SBF) alone. Possible correlations to patient- or treatment-specific factors such as body mass index (BMI), planning time, treatment delivery time, and distance between tumour and spinal cord were explored to determine whether they influenced on the benefit of image-guidance.
  
  
  METHODS
  A total of 34 patients received SBRT (3 fractions) for tumours in the liver (15 patients) or the lung (19 patients). Immobilisation and positioning was obtained with a SBF. Pre- and post-treatment CBCT scans were registered with the bony anatomy of the planning CT to find inter- and intrafractional patient positioning errors (PPE). For lung tumour patients, matching was also performed on the tumours to find the tumour positioning errors (TPE) and baseline shifts relative to bony anatomy.
  
  
  RESULTS
  The mean inter- and intrafractional 3D vector PPE was 4.5 ± 2.7 mm (average ± SD) and 1.5 ± 0.6 mm, respectively, for the combined group of patients. For lung tumours, the interfractional misalignment was 5.6 ± 1.8 mm. The baseline shift was 3.9 ± 2.0 mm. Intrafractional TPE and baseline shifts were 2.1 ± 0.7 mm and 1.9 ± 0.6 mm, respectively. The magnitude of interfractional baseline shift was closely correlated with the distance between the tumour and the spinal cord. Intrafractional errors were independent of patient BMI, age or gender.
  
  
  CONCLUSIONS
  Image-guidance reduced setup errors considerably. The study demonstrated the benefit of CBCT-guidance regardless of patient specific factors such as BMI, age or gender. Protection of the spinal cord was facilitated by the correlation between the tumour position relative to the spinal cord and the magnitude of baseline shift.
authors.affiliation: Department of Medical Physics, Aarhus University Hospital, Aarhus, Denmark. esbeworm@rm.dk.
authors.forename: Esben S
authors.initials: ES
authors.lastname: Worm
authors.forename: Anders T
authors.initials: AT
authors.lastname: Hansen
authors.forename: Jørgen B
authors.initials: JB
authors.lastname: Petersen
authors.forename: Ludvig P
authors.initials: LP
authors.lastname: Muren
authors.forename: Lars H
authors.initials: LH
authors.lastname: Præstegaard
authors.forename: Morten
authors.initials: M
authors.lastname: Høyer
mesh_terms: D001650:Bile Duct Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000556:secondary / Q000601:surgery; D001653:Bile Ducts, Intrahepatic / Q000000981:diagnostic imaging / Q000473:pathology; D002289:Carcinoma, Non-Small-Cell Lung / Q000000981:diagnostic imaging / Q000601:surgery; D018281:Cholangiocarcinoma / Q000000981:diagnostic imaging / Q000473:pathology / Q000556:secondary / Q000601:surgery; D015331:Cohort Studies; D054893:Cone-Beam Computed Tomography / Q000379:methods* / Q000592:standards; D003951:Diagnostic Errors; D019583:Dose Fractionation, Radiation; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D021621:Imaging, Three-Dimensional / Q000379:methods; D008113:Liver Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000556:secondary / Q000601:surgery; D008175:Lung Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000556:secondary / Q000601:surgery; D008297:Male; D016634:Radiosurgery / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D047368:Tumor Burden / Q000528:radiation effects