Head and neck tumor segmentation in PET/CT: The HECKTOR challenge

•The paper describes the first challenge on head and neck tumor segmentation in PET/CT.•Training (n=201, 4 centers) and test sets (n=53, 1 unseen center) amount to 254 cases.•All ground truth segmentations underwent cleaning to ensure quality and homogeneity.•The winning team obtained a DSC of 0.759, showing a larg improvement over the baseline.•Additional post-challenge analyses (e.g. false positives analysis, ranking stability).