Tumour delineation in oesophageal cancer - A prospective study of delineation in PET and CT with and without endoscopically placed clip markers.

Purpose: The objective was to analyse the value of F-18-fluorodesoxyglucose (FDG)-positron emission tomography/computed tomography (PET/CT) for delineation of the Gross Tumour Volumes (GTVs) in primary radiotherapy of oesophageal cancer.

Method: 20 consecutive and prospective patients (13 men, 7 women) underwent FDG-PET/CT for initial staging and radiation treatment planning. After endoscopy-guided clipping of the tumour another CT study was acquired. The CT and the FDG-PET/CT were registered with a rigid and a non-rigid registration algorithm to compare the overlap between GTV contours defined with the following methods: manual GTV definition in (1) the CT image of the FDG-PET/CT, (2) the PET image of the FDG-PET/CT, (3) the CT study based on endoscopic clips (CT clip), and (4) in the PET-data using different semi-automatic PET segmentation algorithms including a gradient-based algorithm. The absolute tumour volumes, tumour length in cranio-caudal direction, as well as the overlap with the reference volume (CT-clip) were compared for all lesions and separately for proximal/distal tumours.

Results: In 6 of the patients, FDG-PET/CT discovered previously unknown tumour locations, which resulted in either altered target volumes (n=3) or altered intent of treatment from curative to palliative (n=3) by upstaging to stage IV. For tumour segmentation a large variability between all algorithms was found. For the absolute tumour volumes with CT-clip as reference, no single PET-based segmentation algorithm performed better compared to using the manual CT delineation alone. The best correlation was found between the CT-clip and the gradient based segmentation algorithm (PET-edge, R(2)=0.84) as well as the manual CT-delineation (CT-manual R(2)=0.89). Non-rigid registration between CT and image FDG-PET/CT did not decrease variability between segmentation methods compared to rigid registration statistically significant. For the analysis of tumour length no homogeneous correlation was found.

Conclusion: Whereas FDG-PET was highly relevant for staging purposes, CT imaging with clipping of the tumour extension remains the gold standard for GTV delineation.