title: Magnetic resonance imaging for prostate bed radiotherapy planning: An inter- and intra-observer variability study.
abstract: BACKGROUND
  We assessed the inter- and intra-observer variability in contouring the prostate bed for radiation therapy planning using MRI compared with computed tomography (CT).
  
  
  METHODS
  We selected 15 patients with prior radical prostatectomy. All had CT and MRI simulation for planning purposes. Image fusions were done between CT and MRI. Three radiation oncologists with several years of experience in treating prostate cancer contoured the prostate bed first on CT and then on MRI. Before contouring, each radiation oncologist had to review the Radiation Therapy Oncology Group guidelines for postoperative external beam radiotherapy. The agreement between volumes was calculated using the Dice similarity coefficient (DSC). Analysis was done using the Matlab software. The DSC was compared using non-parametric statistical tests.
  
  
  RESULTS
  Contouring on CT alone showed a statistically significant (P = 0.001) higher similarity between observers with a mean DSC of 0.76 (standard deviation ± 0.05) compared with contouring on MRI with a mean of 0.66 (standard deviation ± 0.05). Mean intra-observer variability between CT and MRI was 0.68, 0.75 and 0.78 for the three observers. The clinical target volume was 19-74% larger on CT than on MRI. The intra-observer difference in clinical target volume between CT and MRI was statistically significant in two observers and non-significant in the third one (P = 0.09).
  
  
  CONCLUSIONS
  We found less inter-observer variability when contouring on CT than on MRI. Radiation Therapy Oncology Group contouring guidelines are based on anatomical landmarks readily visible on CT. These landmarks are more inter-observer dependent on MRI. Therefore, present contouring guidelines might not be applicable to MRI planning.
authors.affiliation: Department of Radiation Oncology, Centre hospitalier de l'Université de Montréal (CHUM), Hôpital Notre-Dame, Montreal, Québec, Canada.
authors.forename: Maroie
authors.initials: M
authors.lastname: Barkati
authors.affiliation: Department of Radiation Oncology, Centre hospitalier de l'Université de Montréal (CHUM), Hôpital Notre-Dame, Montreal, Québec, Canada.
authors.forename: Dany
authors.initials: D
authors.lastname: Simard
authors.affiliation: Department of Radiation Oncology, Centre hospitalier de l'Université de Montréal (CHUM), Hôpital Notre-Dame, Montreal, Québec, Canada.
authors.forename: Daniel
authors.initials: D
authors.lastname: Taussky
authors.affiliation: Department of Radiation Oncology, Centre hospitalier de l'Université de Montréal (CHUM), Hôpital Notre-Dame, Montreal, Québec, Canada.
authors.forename: Guila
authors.initials: G
authors.lastname: Delouya
mesh_terms: D059925:Anatomic Landmarks / Q000000981:diagnostic imaging; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D015588:Observer Variation; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods*
keywords: MRI; inter-observer variability; planning; prostate bed; radiotherapy