Utility of immunohistochemistry in separating thymic neoplasms from germ cell tumors and metastatic lung cancer involving the anterior mediastinum.

A variety of primary and metastatic neoplasms can involve the anterior mediastinum and have similar radiographic or histologic appearances. We report an immunohistochemical (IHC) panel that includes CD5, placental-like alkaline phosphatase (PLAP), thyroid transcription factor 1 (TTF-1), cytokeratin (CK) 7, CK20, CK5/6, and CD57 for the separation of thymic neoplasms from germ cell tumors (GCTs) and metastatic lung malignancies. One hundred cases representing different types of neoplasms involving the anterior mediastinum were retrieved and consisted of 20 pulmonary adenocarcinomas (PAs), 20 pulmonary large cell carcinomas (PLCs), 20 pulmonary squamous cell carcinomas (PSCCs), 20 thymomas (Ths), 10 invasive thymomas (IThs), and 10 thymic carcinomas (TCs). Fifteen cases of GCTs were also included in the study. CD5 was positive in 17 of 20 PAs, 11 of 20 PLCs, 2 of 15 GCTs, and 9 of 10 TCs. PLAP was positive in 5 of 20 PAs, 2 of 20 PLCs, 1 of 20 PSCCs, and 11 of 15 GCTs. TTF-1 was positive in 18 of 20 PAs, 10 of 20 PLCs, and 2 of 20 PSCCs. CK7 was positive in 20 of 20 PAs, 15 of 20 PLCs, 8 of 20 PSCCs, 5 of 15 GCTs, 11 of 20 Ths, 7 of 10 IThs, and 4 of 10 TCs. CK20 was positive only in 2 of 20 PAs. CK5/6 was positive in all cases of PSCC, Th, ITh, and TC, in 9 of 20 PLCs, and 1 of 15 GCTs. CD57 was positive in 9 of 20 PAs, 3 of 20 PLCs, 8 of 15 GCTs, 13 of 20 Ths, and 6 of 10 IThs. In conclusion, CD5 had low specificity for TC (60%) because it also stained 85% of PA and 55% of PLC. CK5/6 was a sensitive but not a specific IHC marker for the epithelial component of thymic neoplasms (100%); TTF-1 had a good sensitivity (90%) and excellent specificity (100%) for distinguishing PA from thymic neoplasms. An IHC panel consisting of CD5, CK5/6, PLAP, TTF-1, and CD57 is useful for the separation of thymic neoplasms from other epithelial malignancies involving the mediastinum.