Primary Hepatoid Adenocarcinoma of the Lung With Signet Ring Cell Differentiation and Widespread Osseous Metastases: A Case Report

Pulmonary hepatoid adenocarcinoma (HAL) is an extremely rare and highly aggressive extrahepatic adenocarcinoma that exhibits hepatocellular carcinoma-like morphology and hepatocytic immunophenotype, often leading to diagnostic delay due to nonspecific respiratory presentations. We report a male patient in his 50s with a long history of heavy tobacco use who presented with a persistent cough initially treated as pneumonia without clinical improvement. Over subsequent weeks, he developed progressive dyspnea, weight loss, fatigue and diffuse bone pain. Computed tomography demonstrated left upper-lobe consolidation with pleural effusion and underlying collapse, mediastinal lymphadenopathy, and multiple lytic skeletal lesions consistent with metastatic disease, while triphasic abdominal imaging showed no focal hepatic lesions. Laboratory testing revealed mild anaemia with leukocytosis and thrombocytosis, markedly elevated carcinoembryonic antigen and normal alpha-fetoprotein. Tru-cut biopsy showed poorly differentiated adenocarcinoma with prominent signet ring cell differentiation. Immunohistochemistry demonstrated diffuse CK7 and HepPar-1 positivity with focal CDX2 positivity and negativity for TTF-1, CK20, p63, and D2-40, supporting a diagnosis of primary pulmonary hepatoid adenocarcinoma. The patient received 3 cycles of gemcitabine plus carboplatin but experienced rapid clinical deterioration. This case underscores that HAL may mimic nonresolving pneumonia and that normal AFP does not exclude the diagnosis; early biopsy and a targeted immunohistochemical panel are essential for timely recognition of this lethal entity.