A Histological Conundrum With a Distinctive Biochemical Marker: A Case Report of Primary Pulmonary Choriocarcinoma.

Primary pulmonary choriocarcinoma (PPC) is a rare, malignant germ cell neoplasm with a poor prognosis that is difficult to diagnose. Measurement of human chorionic gonadotropin (hCG), which is secreted by PPCs, facilitates the detection of PPCs. We describe a 69-year-old man who presented with shortness of breath and was found to have a large lung tumor. Histological examination of the biopsied right lung mass revealed tumor cells with enlarged pleomorphic nuclei, irregular nuclear contour, and conspicuous nucleoli. Immunostaining of the tumor cells was positive for MNF-116 and patchily positive for p40, while negative for leukocyte common antigen (LCA (CD45)), S100, smooth muscle actin (SMA), thyroid transcription factor 1 (TTF-1), and synaptophysin. While such features mimicked squamous cell carcinoma, an extremely elevated serum total hCG level of 24544 IU/L prompted further immunohistochemical staining for choriocarcinomas. The tumor was positive for spalt-like transcription factor 4 (SALL4) and GATA-binding protein 3 (GATA3), which established the diagnosis of PPC. This case illustrates the histopathological difficulties in diagnosing PPC and the utility of serum hCG measurement in the detection of PPC. Serum hCG measurement is useful as an initial investigation for possible PPC in patients with non-small cell lung tumors, which should be followed by immunohistochemical staining for markers of choriocarcinoma.