Gross tumour delineation on computed tomography and positron emission tomography-computed tomography in oesophageal cancer: A nationwide study

Abstract

Background and purpose: Accurate delineation of the primary tumour is vital to the success of radiotherapy and even more important for successful boost strategies, aiming for improved local control in oesophageal cancer patients. Therefore, the aim was to assess delineation variability of the gross tumour volume (GTV) between CT and combined PET-CT in oesophageal cancer patients in a multi-institutional study.

Materials and methods: Twenty observers from 14 institutes delineated the primary tumour of 6 cases on CT and PET-CT fusion. The delineated volumes, generalized conformity index (CIgen) and standard deviation (SD) in position of the most cranial/caudal slice over the observers were evaluated. For the central delineated region, perpendicular distance between median surface GTV and each individual GTV was evaluated as in-slice SD.

Results: After addition of PET, mean GTVs were significantly smaller in 3 cases and larger in 1 case. No difference in CIgen was observed (average 0.67 on CT, 0.69 on PET-CT). On CT cranial-caudal delineation variation ranged between 0.2 and 1.5 cm SD versus 0.2 and 1.3 cm SD on PET-CT. After addition of PET, the cranial and caudal variation was significantly reduced in 1 and 2 cases, respectively. The in-slice SD was on average 0.16 cm in both phases.

Conclusion: In some cases considerable GTV delineation variability was observed at the cranial-caudal border. PET significantly influenced the delineated volume in four out of six cases, however its impact on observer variation was limited.

Keywords: AJCC, American Joint Committee on Cancer; CIgen, generalized conformity index; Chemoradiotherapy; EGJ, oesophageal-gastric junction; FDG-PET/CT; GTV delineation; GTV, gross tumour volume; Interobserver variability; Oesophageal cancer; SD, standard deviation; SUV, standardized uptake volume; dCRT, definitive chemoradiation; nCR, neoadjuvant chemoradiation.

Fig. 1

Delineation variation in case 1 for 20 observers. 1A (phase 1). The GTV of the primary tumour was delineated by 20 observers on CT, with available clinical and diagnostic information. 1B (phase 2). After fusion of the FDG-PET scan, the GTV was adjusted based on the visual interpretation of the FDG-PET scan. The colour wash image indicates the intensity of the SUV signal. This case contained satellite lesions proximal of the tumour. Although the presence of satellite lesions was included in the clinical information, these were included in the GTV by only 9 and 11 of the 20 observers in phase 1 and 2 respectively.

Fig. 2

Local surface distance variation (1SD) over the 20 observers projected on the median surface of the 6 patients. AP = anterior-posterior view; PA = posterior-anterior view.