Small cell carcinoma with unknown primary (SCUP) is an uncommon cancer. Histologically, SCUP has been included with extrapulmonary small cell carcinoma (EPSCC) that is defined as a biopsy proven small cell carcinoma in a nonpulmonary primary site. SCUP is usually diagnosed in the lymph nodes, liver, brain, or bone. This tumor is a distinct clinicopathologic entity from small cell carcinoma of the lung. Generally, the clinical course is aggressive and often recurrent, and it has poor prognosis. The identification and differential diagnosis of this tumor can be made with the morphology of neuroendocrine features, immunohistochemistry with chromogranin A, neuron-specific enolase, synaptophysin and CD56, and cytogenetic findings. We treated a case of aggressive SCUP diagnosed by bone marrow biopsy in a 68-year-old man. There was no evidence of tumor in the lung. The patient died one month after without receiving any therapy.

Keywords: Small cell carcinoma of unknown primary (SCUP), Extrapulmonary small cell carcinoma (EPSCC), CD56