The pathology of extrapulmonary small cell carcinoma.

Extrapulmonary small cell carcinomas (EPSCCs) are uncommon malignant neoplasms with a reported incidence of 0.1% to 0.4% in the United States. Since their first description in 1930, they have been seen in nearly every organ system. Like their more common pulmonary counterparts, EPSCCs are thought to arise from a multipotential stem cell. However, there is recent molecular evidence that small cell elements may arise as a late-stage phenomenon in the genetic progression of more organ-typical carcinomas. The morphologic, immunohistochemical, and ultrastructural features are similar to those described in pulmonary small cell carcinomas (PSCCs). The differential diagnosis of EPSCC includes PSCC, other neuroendocrine tumors, small round blue cell tumors, metastatic melanoma, lymphoma, and poorly differentiated non-small cell carcinomas. Molecular alterations reported to occur in EPSCCs include abnormalities described in PSCC and changes found in carcinomas more typically encountered in the organ from which they arise. In this article we discuss the pathology of EPSCC with a review of theories of histogenesis, sites of occurrence, diagnostic features, differential diagnosis, molecular alterations, and clinical behavior.