Optimal delineation of the clinical target volume for thymomas in the post-resection setting: A multi-center study

Background: The definition of the clinical target volume (CTV) for post-operative radiotherapy (PORT) for thymoma is largely unexplored. The aim of this study was to analyze 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 CTVs 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 of 4.5 (+/- 2.2) cm. Collaborative delineation resulted in significantly smaller volumes compared to RTO (mean 57.1 +/- 58.6 cm(3), p < 0.001). A mean volume of 18.9 (+/- 38.1) cm(3) was included in joint contours, but missed by RTO. Conversely, a mean volume of 55.7 (+/- 39.9) cm(3) was included in RTO's delineations, but not in the joint delineations. Conclusions: To the best of our knowledge, this is the first study investigating CTV definition in thymoma. We demonstrated a significant variability between RTO and surgeons. Joint delineation prompted revisions in smaller CTV as well as favoring the surgeons' judgement, suggesting that surgeons provided relevant insight into other risk areas than RTO. We recommend a multidisciplinary approach to PORT for thymomas in clinical practice. (C) 2021 The Author(s). Published by Elsevier B.V.