Improving Observer Variability in Target Delineation for Gastro-oesophageal Cancer-the Role of ^18Ffluoro-2-deoxy-d-glucose Positron Emission Tomography/Computed Tomography

Aim: To evaluate the effect of the addition of fused positron emission tomography-computed tomography (PET-CT) imaging vs computed tomography alone in the identification of the gross tumour volume (GTV) in patients with gastro-oesophageal carcinoma. Materials and methods: Ten patients with gastro-oesophageal cancer referred for radiation therapy underwent both ^18Ffluoro-2-deoxy-d-glucose-PET (FDG-PET) and computed tomography in the treatment position. Image sets were anonymised and co-registered. Six radiation oncologists independently defined the GTV, first using the computed tomography data alone supplemented by standardised clinical and diagnostic imaging information, and second, using co-registered computed tomography and FDG-PET data (PET-CT). The standard deviation for both GTV length and volume (excluding involved lymph nodes) was taken as a measurement of inter-observer and intra-observer variability. Computer software that calculates volume overlap between contours was also used to generate an observer agreement index to compare intra- and inter-observer variability. Results: The addition of FDG-PET imaging decreased the median standard deviation for tumour length from 10 mm (range 8.1-33.3, mean 12.4 mm) for computed tomography alone to 8 mm (range 4.4-18.1, mean 8.1 mm) for PET-CT (P = 0.02). Eight of the 10 patients showed an increase in volume of overlap between observers with the addition of FDG-PET imaging to the contouring process (P = 0.05). The average observer agreement index in PET-CT was 72.7% compared with 69.1% when using computed tomography alone. There was significantly less intra-observer variability in all measures when PET-CT was used. The median standard deviation in length improved from 5.3 to 1.8 mm, the median standard deviation in volume improved from 4.5 to 3 cm³ and the median observer agreement index improved from 76.2 to 78.7% when computed tomography alone was compared with PET-CT. The corresponding P values were 0.001, 0.033 and 0.022, respectively. Conclusions: The addition of FDG-PET to computed tomography-based planning for the identification of primary tumour GTV in patients with gastro-oesophageal carcinoma decreases both inter-observer and intra-observer variability.