title: Interobserver and intermodality variability in GTV delineation on simulation CT, FDG-PET, and MR Images of Head and Neck Cancer.
abstract: OBJECTIVE
  To compare the interobserver and intermodality differences in image-based identification of head and neck primary site gross tumor volumes (GTV). Modalities compared include: contrast-enhanced CT, F-18 fluorodeoxyglucose positron emission tomography (PET/CT) and contrast-enhanced MRI.
  
  
  METHODS
  Fourteen patients were simulated after immobilization for all 3 imaging modalities (CT, PET/CT, MRI). Three radiation oncologists (RO) contoured GTVs as seen on each modality. The GTV was contoured first on the contrast-enhanced CT (considered the standard), then on PET/CT, and finally on post-contrast T1 MRI. Interobserver and intermodality variability were analyzed by volume, intersection, union, and volume overlap ratio (VOR).
  
  
  RESULTS
  Analysis of RO contours revealed the average volume for CT-, PET/CT-, and MRI-derived GTVs were 45cc, 35cc and 49cc, respectively. In 93% of cases PET/CT-derived GTVs had the smallest volume and in 57% of cases MRI-derived GTVs had the largest volume. CT showed the largest variation in target definition (standard deviation amongst observers 35%) compared to PET/CT (28%) and MRI (27%). The VOR was largest (indicating greatest interobserver agreement) in PET/CT (46%), followed by MRI (36%), followed by CT (34%). For each observer, the least agreement in GTV definition occurred between MRI & PET/CT (average VOR = 41%), compared to CT & PET/CT (48%) and CT & MRI (47%).
  
  
  CONCLUSIONS
  A nonsignificant interobserver difference in GTVs for each modality was seen. Among three modalities, CT was least consistent, while PET/CT-derived GTVs had the smallest volumes and were most consistent. MRI combined with PET/CT provided the least agreement in GTVs generated. The significance of these differences for head & neck cancer is important to explore as we move to volume-based treatment planning based on multi-modality imaging as a standard method for treatment delivery.
authors.affiliation: Department of Radiation Oncology, University of Iowa Hospitals and Clinics, Iowa City, Iowa, USA.
authors.forename: Carryn M
authors.initials: CM
authors.lastname: Anderson
authors.affiliation: Department of Radiology, University of Iowa Hospitals and Clinics, Iowa City, Iowa, USA.
authors.forename: Wenqing
authors.initials: W
authors.lastname: Sun
authors.affiliation: Department of Biostatistics, University of Iowa Hospitals and Clinics, Iowa City, Iowa, USA.
authors.forename: John M
authors.initials: JM
authors.lastname: Buatti
authors.affiliation: Department of Human Oncology, University of Wisconsin, Madison, Wisconsin, USA.
authors.forename: Joan E
authors.initials: JE
authors.lastname: Maley
authors.forename: Bruno
authors.initials: B
authors.lastname: Policeni
authors.forename: Sarah L
authors.initials: SL
authors.lastname: Mott
authors.forename: John E
authors.initials: JE
authors.lastname: Bayouth