Thoracic solitary fibrous tumors with small cell features: A clinicopathological and immunohistochemical analysis of 5 cases.

Five cases of thoracic solitary fibrous tumor (SFT) with small cell features are presented mimicking a neuroendocrine neoplasm. The patients were four men and one woman aged 43 to 74 years who presented with symptoms of chest pain, cough, dyspnea or hemoptysis. Two tumors were intrapulmonary neoplasms, while three were pleural-based. Grossly, the tumors ranged in size from 4 to 6 cm and were white and solid; in two tumors necrosis was apparent. Histologically, they were characterized by a cellular proliferation composed of small cells with round nuclei and inconspicuous nucleoli. The cellular proliferation in some areas had a subtle nested pattern, while in other areas the tumor showed extensive sclerosis and small vessel proliferation. Cellular pleomorphism was not marked and the mitotic activity varied from 1 to 5 mitotic figures per 10 high power fields. Microscopically, necrosis was observed in two cases and focally present in one. Immunohistochemical stains showed tumors cells universally negative for pancytokeratin; in the two pulmonary cases, focal staining for synaptophysin, CD56, and INSM1 was observed. The unexpected lack of expression of pancytokeratin led to additional analysis revealing positive staining with CD34 and STAT6 confirming a diagnosis of SFT. Clinical follow-up showed tumor recurrence in one patient while three patients remained alive and well after a period of 12 to 20 months. The current cases highlight an unusual variant of SFT that may be confused with other small cell tumor entities, such as neuroendocrine or neuroectodermal tumors, especially when originating in the thoracic cavity.