title: Magnitude of observer error using cone beam CT for prostate interfraction motion estimation: effect of reducing scan length or increasing exposure.
abstract: OBJECTIVE
  Cone beam CT (CBCT) enables soft-tissue registration to planning CT for position verification in radiotherapy. The aim of this study was to determine the interobserver error (IOE) in prostate position verification using a standard CBCT protocol, and the effect of reducing CBCT scan length or increasing exposure, compared with standard imaging protocol.
  
  
  METHODS
  CBCT images were acquired using a novel 7 cm length image with standard exposure (1644 mAs) at Fraction 1 (7), standard 12 cm length image (1644 mAs) at Fraction 2 (12) and a 7 cm length image with higher exposure (2632 mAs) at Fraction 3 (7H) on 31 patients receiving radiotherapy for prostate cancer. Eight observers (two clinicians and six radiographers) registered the images. Guidelines and training were provided. The means of the IOEs were compared using a Kruzkal-Wallis test. Levene's test was used to test for differences in the variances of the IOEs and the independent prostate position.
  
  
  RESULTS
  No significant difference was found between the IOEs of each image protocol in any direction. Mean absolute IOE was the greatest in the anteroposterior direction. Standard deviation (SD) of the IOE was the least in the left-right direction for each of the three image protocols. The SD of the IOE was significantly less than the independent prostate motion in the anterior-posterior (AP) direction only (1.8 and 3.0 mm, respectively: p = 0.017). IOEs were within 1 SD of the independent prostate motion in 95%, 77% and 96% of the images in the RL, SI and AP direction.
  
  
  CONCLUSIONS
  Reducing CBCT scan length and increasing exposure did not have a significant effect on IOEs. To reduce imaging dose, a reduction in CBCT scan length could be considered without increasing the uncertainty in prostate registration. Precision of CBCT verification of prostate radiotherapy is affected by IOE and should be quantified prior to implementation.
  
  
  CONCLUSIONS
  This study shows the importance of quantifying the magnitude of IOEs prior to CBCT implementation.
authors.affiliation: 1 Royal Marsden NHS Foundation Trust, Sutton, UK.|2 Institute of Cancer Research, Sutton, UK.
authors.forename: Helen A
authors.initials: HA
authors.lastname: McNair
authors.affiliation: 1 Royal Marsden NHS Foundation Trust, Sutton, UK.|2 Institute of Cancer Research, Sutton, UK.
authors.forename: Emma J
authors.initials: EJ
authors.lastname: Harris
authors.affiliation: 1 Royal Marsden NHS Foundation Trust, Sutton, UK.
authors.forename: Vibeke N
authors.initials: VN
authors.lastname: Hansen
authors.affiliation: 1 Royal Marsden NHS Foundation Trust, Sutton, UK.
authors.forename: Karen
authors.initials: K
authors.lastname: Thomas
authors.affiliation: 3 Department of Physics, St Luke's Cancer Centre, The Royal County Hospital NHS Foundation Trust, Guilford, Surrey.
authors.forename: Christopher
authors.initials: C
authors.lastname: South
authors.affiliation: 1 Royal Marsden NHS Foundation Trust, Sutton, UK.|2 Institute of Cancer Research, Sutton, UK.
authors.forename: Shaista
authors.initials: S
authors.lastname: Hafeez
authors.affiliation: 1 Royal Marsden NHS Foundation Trust, Sutton, UK.|2 Institute of Cancer Research, Sutton, UK.
authors.forename: Robert
authors.initials: R
authors.lastname: Huddart
authors.affiliation: 1 Royal Marsden NHS Foundation Trust, Sutton, UK.|2 Institute of Cancer Research, Sutton, UK.
authors.forename: David P
authors.initials: DP
authors.lastname: Dearnaley
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006801:Humans; D008297:Male; D009038:Motion; D015588:Observer Variation; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D015203:Reproducibility of Results