Primary cutaneous myxoid spindle cell squamous cell carcinoma: a clinicopathologic study and review of the literature.

Mucocutaneous squamous cell carcinoma (SCC) may rarely exhibit intracellular mucin production. Extracellular mucin production is an even rarer finding in SCC that is not well documented in the literature. Here, we report six cases of primary cutaneous and mucocutaneous SCC with prominent extracellular stromal mucin deposition and an epithelial spindle cell component. We propose the term 'yxoid spindle cell SCC' (MSC SCC) to describe the histologic characteristics of these six cases. We also propose a set of histologic and immunohistochemical findings for distinguishing MSC SCC from primary cutaneous and metastatic spindle cell neoplasms including other sarcomatoid carcinomas, myxoid sarcomas and the spindle cell variant of atypical fibroxanthoma (AFX). The criteria can also help discern MSC SCC from spindle cell melanomas, which may rarely show a prominent myxoid stroma. Given the small numbers of cases reported to date, the presence of prominent myxoid stroma in primary cutaneous spindle cell SCC has unknown prognostic significance at this time.