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

Abstract 

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 10mm (range 8.1–33.3, mean 12.4mm) for computed tomography alone to 8mm (range 4.4–18.1, mean 8.1mm) 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 1.8mm, the median standard deviation in volume improved from 4.5 to 3cm³ 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.

Key words: FDG-PET, gastro-oesophageal cancer, inter-observer variability, intra-observer variability, oesophageal cancer, radiotherapy planning