International reproducibility study of thymic epithelial tumors staging: pT stage is an issue. proposals for improvement. A RYTHMIC/ITMIG study

•Reproducibility between expert pathologists to evaluate pT of thymomas is problematic.•Thymomas may grow with their “capsule” adhering to rather than invading into lung or pericardium.•Mediastinal pleura is difficult to recognize on microscopy.•Recognizing the outer pericardial layer from the fibrous part is important for pericardial invasion diagnosis.•Highly contrasted elastic stains as well as immunohistochemical markers that highlight mesothelial and tumor cells can aid in the evaluation of lung invasion.