Observer variation in contouring gross tumor volume in patients with poorly defined non-small-cell lung tumors on CT: the impact of 18FDG-hybrid PET fusion

Int J Radiat Oncol Biol Phys. 2001 Nov 15;51(4):923-31. doi: 10.1016/s0360-3016(01)01722-9.


Purpose: To quantify interobserver variation in gross tumor volume (GTV) localization using CT images for patients with non-small-cell lung carcinoma and poorly defined tumors on CT and to determine whether variability would be reduced if coregistered 2-[18F]fluoro-2-deoxy-d-glucose (FDG)-hybrid positron emission tomography (PET) with CT images were used.

Methods and materials: Prospectively, 30 patients with non-small-cell lung carcinoma had CT and FDG-hybrid PET examinations in radiation treatment position on the same day. Images were coregistered using eight fiducial markers. Guidelines were established for contouring GTVs. Three radiation oncologists performed localization independently. The coefficient of variation was used to assess interobserver variability.

Results: The size of the GTV defined showed great variation among observers. The mean ratios of largest to smallest GTV were 2.31 and 1.56 for CT only and for CT/FDG coregistered data, respectively. The addition of PET reduced this ratio in 23 of 30 cases and increased it in 7. The mean coefficient of variation for GTV based on the combined modalities was significantly smaller (p < 0.01) than that for CT data only.

Conclusions: High observer variability in CT-based definition of the GTV can occur. A more consistent definition of the GTV can often be obtained if coregistered FDG-hybrid PET images are used.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Aged
  • Aged, 80 and over
  • Carcinoma, Non-Small-Cell Lung / diagnostic imaging*
  • Carcinoma, Non-Small-Cell Lung / secondary
  • Female
  • Fluorodeoxyglucose F18*
  • Humans
  • Lung Neoplasms / diagnostic imaging*
  • Lymphatic Metastasis / diagnostic imaging
  • Male
  • Neoplasm Staging
  • Observer Variation
  • Prospective Studies
  • Radiopharmaceuticals*
  • Tomography, Emission-Computed / methods*
  • Tomography, X-Ray Computed / methods*


  • Radiopharmaceuticals
  • Fluorodeoxyglucose F18