Table 3.

Markers considered to be useful for the routine immunohistochemical characterization of otherwise difficult to classify thymomas and thymic carcinomas.³

Marker	Cellular and subcellular targets of mediastinal tumors
Cytokeratins	Epithelial cells of normal thymus, thymomasa, thymic carcinomas, neuroendocrine tumors, many germ cell tumors, rare sarcomas and dendritic cell tumors; metatases to the mediastinum
Cytokeratin 19	Epithelial cells of normal thymus, thymomasa and thymic carcinomas
Cytokeratin 20	Negative in normal thymus and thymomas. May be positive in rare thymic adenocarcinomas, teratomas or metastases
P63	Nuclei of normal and neoplastic thymic epithelial cells, squamous epithelial cells (e.g. in teratoma, metastasis), primary mediastinal large B-cell lymphoma
P40	Nuclei of normal and neoplastic thymic epithelial cells, squamous epithelial cells (e.g. in teratoma; metastasis)
TdT	Immature T cells of normal thymus, >90% of thymomas and neoplastic T cells of T lymphoblastic lymphoma
CD5	Immature and mature T cells of thymus and >90% of thymomas Neoplastic T cells of many T lymphoblastic lymphomas Epithelial cells in70% of thymic carcinomasb
CD20	Normal and neoplastic B cells Epithelial cells in 50% of cases of type A and AB thymoma
CD117	Epithelial cells in 80% of thymic carcinomas Neoplastic cells in most seminomas

^aBeware of rare cytokeratin-negative thymomas (that typically maintain expression of p63/p40)²¹;

^bbeware of adenocarcinoma metastases to the mediastinum that can be CD5+ as well