- Reactive process of unknown etiology
- Most common on the limbs and trunk of young to middle-aged adults
- Presents as a rapidly growing, painful subcutaneous nodule
- Local recurrence uncommonly occurs.
- Well-circumscribed proliferation of interlacing fascicles of plump spindle cells ("tissue culture-like fibroblasts")
- Mitotically active
- Loose myxoid and collagenous background, highly variable in degree of collagenization and myxoid change
- Multiple branching thin-walled blood vessels are interspersed throughout the lesion.
- Focal hemorrhage and lymphocytic inflammation are often seen.
- Foamy histiocytes and osteoclast-like giant cells are sometimes present.
- Immunohistochemically, the spindle cells are positive for MSA and calponin, but negative for desmin, consistent with myofibroblasts.