Cutaneous sarcomas and sarcomatoid neoplasms of the skin.

Cutaneous sarcomas and sarcoma-like lesions pose special problems for pathologists. These lesions manifest a great deal of clinical and histological overlap, and they may be virtually impossible to separate from one another on conventional microscopy. Spindle-cell squamous carcinoma (SCSC), sarcomatoid malignant melanoma (SCMM), and fibrohistiocytic tumors, such as "atypical fibroxanthoma" (AFX) and superficial malignant fibrous histiocytoma (SMFH), comprise the group of neoplasms in this category that pose the greatest diagnostic difficulty. All may or may not show attachments between dermal tumor cells and the overlying epidermis; all may be composed of varying proportions of fusiform and pleomorphic cells; and all may demonstrate the presence of "divergent" differentiation into bone, cartilage, or myogenous tissues. Electron microscopy and immunohistology usually are required to identify these tumors with certainty. The usefulness in doing so is supported by differing biological behaviors in this context. SCMM shows the greatest tendency for recurrence and metastasis, followed in relative order by SMFH, SCSC, and AFX.