Ossifying subcutaneous tumor with myofibroblastic differentiation: a variant of ossifying fibromyxoid tumor of soft parts?

Immunohistochemical, ultrastructural, and flow cytometric studies were performed on an ossifying soft-tissue tumor, presumed to be a variant of ossifying fibromyxoid tumor of soft parts, which was located in the subcutis of the left buttock of a 76 year old Japanese woman. Histologically, this was a benign-looking spindle, oval, or round cell lesion, having a fibrous capsule with a discontinuous rim of bone as seen in typical cases. However, this lesion was also characterized by a high degree of cellular proliferation in storiform and whorl arrangements, extensive ossification, osteoid and metaplastic bone formation and absence of myxoid features. In an immunohistochemical study using formalin-fixed, paraffin-embedded sections, many tumor cells expressed vimentin, S-100 protein, Leu-7, neuron specific enolase, and desmin. Ultrastructurally, this neoplasm consisted of fibroblast-like cells and myofibroblast-like cells. This tumor had an aneuploid DNA content. No recurrence has been observed for 16 months. These results suggest that the neoplastic cells may show the phenotypic expressions of myofibroblast and also osteogenic differentiation.