Focal positivity of immunohistochemical markers for pulmonary squamous cell carcinoma in primary pulmonary choriocarcinoma: A histopathological study.

Cytokeratin 5/6 (CK5/6), p63, and p40 are commonly used as immunohistochemical markers for squamous cell carcinoma (SqCC) of the lung. To elucidate their positivity in primary pulmonary choriocarcinoma (PPC), the present study examined 4 PPCs, including 1 surgically removed PPC and 3 postmortem PPCs. All PPCs consisted of nested cytotrophoblastic tumor cells and occasional syncytiotrophoblastic tumor cells although 1 surgically removed PPC was markedly affected by pre-operative therapy-associated necrosis and 3 postmortem PPCs coexisted with adenocarcinoma. In 1 surgical case, a pre-operative biopsy specimen of PPC contained a few polygonal tumor cells, which mimicked SqCC and exhibited focal p40+ features. Nuclear p63+ and p40+ features of cytotrophoblast-like polygonal tumor cells were focally observed in 3 PPCs (75%) and 2 PPCs (50%), respectively. CK5/6+ trophoblastic tumor cells were focally identified in 1 PPC. Additionally, in 2 other PPCs, CK5/6+ tumor cells were scattered in choriocarcinomatous areas, but possible intermingling of CK5/6+ adenocarcinoma cells could not be ruled out. The results emphasized that PPCs could mimic SqCC morphologically and immunohistochemically, although PPC was an extremely rare neoplasm. Surgical pathologists should be aware of this diagnostic pitfall when encountering a few squamous marker-positive polygonal tumor cells within hemorrhagic necrotic biopsy specimens from lung tumors.