It is a tumor of adults, commoner in men, occurring in the subcutis or muscle mostly of the extremities. The tumor has a thick fibrous capsule from which septa extend in, giving a partially lobulated pattern. The stroma is myxoid and focally fibrosed or hyaline, in places suggesting osteoid formation. Mitotic activity is usually minimal and there is no necrosis or vascular invasion. Atypical variants have been described.
Some cases recur, and an occasional metastasis has been reported.
The majority of cells are strongly positive for S 100 protein and for NSE, and some have GFAP positivity. No reactivity is detected with HMB45, or with antibodies to cytokeratin, EMA, or neurofilament. However, in some examples of this tumor a few cells have desmin and actin positivity. Electron microscopy shows the tumor cells to have rather short cytoplasmic processes with occasional cell membrane thickening suggestive of attempted junction formation but no definite desmosomes. Many cells have a discontinuous, thick external lamina which is focally reduplicated into the stroma.