title: FDG-PET-CT reduces the interobserver variability in rectal tumor delineation.
abstract: OBJECTIVE
  Previously, we showed a good correlation between pathology and an automatically generated PET-contour in rectal cancer. This study analyzed the effect of the use of PET-CT scan on the interobserver variation in GTV definition in rectal cancer and the influence of PET-CT on treatment volumes.
  
  
  METHODS
  Forty two patients diagnosed with rectal cancer underwent an FDG-PET-CT for radiotherapy planning. An automatic contour was created on PET-scan using the source-to-background ratio. The GTV was delineated by 5 observers in 3 rounds: using CT and MRI, using CT, MRI and PET and using CT, MRI and PET auto-contour. GTV volumes were compared and concordance indices (CI) were calculated. Since the GTV is only a small portion of the treatment volume in rectal cancer, a separate analysis was performed to evaluate the influence of PET on the definition of the CTV used in daily clinical practice and the caudal extension of the treatment volumes.
  
  
  RESULTS
  GTV volumes based on PET were significantly smaller. CIs increased significantly using PET and the best interobserver agreement was observed using PET auto-contours. Furthermore, we found that in up to 29% of patients the CTV based on PET extended outside the CTV used in clinical practice. The caudal border of the treatment volume can be tailored using PET-scan in low seated tumors. Influence of PET on the position of the caudal border was most pronounced in high seated tumors.
  
  
  CONCLUSIONS
  PET-CT increases the interobserver agreement in the GTV definition in rectal cancer, helps to avoid geographical misses and allows tailoring the caudal border of the treatment volume.
authors.affiliation: Department of Radiation Oncology, MAASTRO, GROW - School for Oncology and Developmental Biology, Maastricht University Medical Centre, The Netherlands. jeroen.buijsen@maastro.nl
authors.forename: Jeroen
authors.initials: J
authors.lastname: Buijsen
authors.forename: Jørgen
authors.initials: J
authors.lastname: van den Bogaard
authors.forename: Hiska
authors.initials: H
authors.lastname: van der Weide
authors.forename: Stephanie
authors.initials: S
authors.lastname: Engelsman
authors.forename: Ruud
authors.initials: R
authors.lastname: van Stiphout
authors.forename: Marco
authors.initials: M
authors.lastname: Janssen
authors.forename: Geerard
authors.initials: G
authors.lastname: Beets
authors.forename: Regina
authors.initials: R
authors.lastname: Beets-Tan
authors.forename: Philippe
authors.initials: P
authors.lastname: Lambin
authors.forename: Guido
authors.initials: G
authors.lastname: Lammering
mesh_terms: D019788:Fluorodeoxyglucose F18; D006801:Humans; D064847:Multimodal Imaging / Q000379:methods*; D015588:Observer Variation; D049268:Positron-Emission Tomography; D019275:Radiopharmaceuticals; D011880:Radiotherapy Planning, Computer-Assisted; D012004:Rectal Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology; D014057:Tomography, X-Ray Computed
chemical_list: D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18