BACKGROUND: The detailed definition of the clinical target volume (CTV) for post-operative radiotherapy (PORT) for thymoma is largely unexplored. The aim of this study was to analyse the difference in CTV delineation between radiation oncologists (RTO) and surgeons. METHODS: This retrospective multi-center study enrolled 31 patients who underwent PORT for a thymoma from five hospitals. Three CTVs were delineated per patient: one CTV by the RTO, one CTV by the surgeon (blinded to the results of the RTO) and a joint CTV after collaboration. Volumes (cm(3)), Hausdorff distances (HD) and Dice similarity coefficients (DSC) were analyzed. RESULTS: RTO delineated significantly bigger CTV than surgeons (mean: 93.9±63.1, versus 57.9±61.3 cm(3), P=0.003). Agreement was poor between RO and surgeons, with a low mean DSC (0.34±0.21) and high mean HD (4.5±2.2 cm). Collaborative delineation resulted in significantly smaller volumes compared to RTO (P
