An 83-year-old man presented with supraclavicular and mediastinal lymph nodes swelling and elevated serum levels of neuron-specific enolase (NSE), pro-gastrin-releasing peptide (pro-GRP), and cytokeratin fragment (CYFRA). He underwent supraclavicular lymph node dissection. The pathological diagnosis was metastatic lymph node neuroendocrine carcinoma. The initial diagnosis was small cell lung carcinoma c-TxN3M0 III B with an unknown primary site, because fluorine-18-fluorodeoxyglucose positron emission tomography (FDG-PET) had revealed increased uptake in the neck and mediastinal lymphadenopathy, but no significant intrapulmonary uptake. However, computed tomography (CT) of the chest had detected a lesion, which was assumed to be a vessel, in the right lower lung. The patient underwent radiotherapy, and CT of the chest 1 month later revealed a partial response of the lymph nodes. However, at the same time, disease recurred in the skin adjacent to the site of supraclavicular lymph node dissection, and the lesion in the right lower lung enlarged. We suspected that this intrapulmonary lesion was the primary site. Metastasis to cervical and mediastinal lymph nodes from an unknown primary carcinoma is rare, and the primary site should be determined so that appropriate treatment can be performed. If the primary site cannot be determined with the initial examination, regular follow-up examinations with CT, magnetic resonance imaging, and FDG-PET should be performed.
