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

•Interobserver variability in delineation of the oesophageal GTV can be considerable.•Delineation variation is mainly located at the cranial and caudal border.•PET significantly influences the delineated GTV in oesophageal cancer.•The impact of PET to CT on observer variation of the GTV is limited.•Accurate GTV delineation is essential for results of radiation boost-strategies.