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.