Occult Primary Pulmonary Adenocarcinoma Presenting With Multistation Nodal Disease: A Case Report

Occult primary lung cancer is an uncommon presentation in which metastatic disease is histologically and immunophenotypically compatible with a pulmonary origin despite the absence of a detectable lung lesion on CT or fluorodeoxyglucose PET/CT. We report a 55-year-old man who presented with a clinically suspicious left supraclavicular lymph node and an Eastern Cooperative Oncology Group (ECOG) performance status of 1. Excisional lymph node biopsy demonstrated metastatic moderately differentiated adenocarcinoma with glandular, trabecular, and cribriform architecture and mucin production. The tumor showed diffuse strong expression of cytokeratin 7 and thyroid transcription factor 1, whereas cytokeratin 20, CDX2, GATA3, P504S, and cytokeratin 19 were not expressed, supporting a pulmonary origin. Baseline imaging demonstrated hypermetabolic cervical, bilateral supraclavicular, left axillary, and diffuse mediastinal lymphadenopathy without a detectable pulmonary primary or definite visceral or osseous metastases. Epidermal growth factor receptor testing was negative, anaplastic lymphoma kinase expression was absent, and the programmed death-ligand 1 tumor proportion score was below 1%. The patient received six cycles of cisplatin 80 mg/m² and paclitaxel 200 mg/m² intravenously on day 1 of a 21-day cycle. Marked nodal regression after three cycles prompted three additional cycles. At the end of six cycles, imaging showed only small residual nodes, no emerging lung primary, and no new metastatic sites. The patient remained in good general condition with a performance status of 1; documented toxicities were cisplatin-related hypoacusis and grade 1 lower-limb sensory neuropathy attributed to paclitaxel. Maintenance therapy was discussed, but the patient opted for surveillance; subsequent follow-up imaging demonstrated a sustained response without disease progression. This case illustrates the value of integrated histopathology, immunohistochemistry, molecular testing, and serial imaging in diagnosing and managing pulmonary adenocarcinoma with an occult primary site.