Case-of-the-Month - February 1997

Diagnosis

The features are characteristic of ossifying fibromyxoid tumor of soft parts (2-5), a rare, slowly growing tumor which arise in subcutaneous tissue, or in deeper muscle, of upper and lower extremities, and occasionally on the trunk or in the head and neck region. Histologically, a characteristic feature is the presence of mature bone or osteoid in the thick fibrous capsule and its septa-like extensions. Within the tumor bone formation is rare but stromal osteoid is occasionally seen. In spite of the name, about 20% of examples lack any bone, as in the current case, in which the whole capsule was sampled. The sheets and cords of bland, uniform smallish cells and fibromyxoid stroma, occasionally with hypercellular areas, are also characteristic, but give rise to difficulties in diagnosis. Most cases are S100 positive and some also have the other markers noted above. Ossifying fibromyxoid tumors frequently recur although metastasis is very rare. However, atypical and aggressive variants have recently been described (6).

Comments

This case illustrates the value of electron microscopy in diagnosing myxoid tumors. In particular, the technique assists distinction of ossifying fibromyxoid tumor, a locally recurrent lesion, from extraskeletal myxoid chondrosarcoma (1) , which is also S100 protein positive, but is a sarcoma with metastatic potential. Electron microscopy also helps to exclude other diagnoses suggested by the immunohistochemistry, including myoepithelial and smooth muscle tumors, and clear cell sarcoma. Finally, electron microscopy can suggest possible histogenesis (5). Since this tumor has ultrastructural features (interdigiatating processes, external lamina) suggestive of nerve sheath differentiation, and is S100 protein positive, it is at present regarded as a variant of schwannoma, possibly of low grade malignancy.

References

1. De Blois D, Wang S, Kay S. Microtubular aggregates within endoplasmic reticulum: an unusual ultrastructural feature of extraskeletal myxoid chondrosarcoma. Hum Pathol 1986; 17: 469- 475.
2. Enzinger FM, Weiss S, Liang CY. Ossifying fibromyxoid tumor of soft parts. A clinicopathologic analysis of 59 cases. Am J Surg Pathol 1989; 13: 817-827.
3. Miettinen M. Ossifying fibromyxoid tumor of soft parts. Additional observations of a distinctive soft tissue tumor. Am J Clin Pathol 1991; 95: 32-39.
4. Schofield JB, Krausz T, Stamp GWH, Fletcher CDM, Fisher C, Azzopardi J. Ossifying fibromyxoid tumour of soft parts: immunohistochemical and ultrastructural analysis. Histopathology 1993; 22: 101-112.
5. Fisher C, Hedges M, Weiss SW. Ossifying fibromyxoid tumor with stromal cyst formation and ribosome-lamella complexes. Ultrastructural Pathol 1994; 18: 593-600.
6. Kilpatrick SE, Ward WG, Mozes M, Miettinen M, Fukunaga M, Fletcher CDM. Atypical and malignant variants of ossifying fibromyxoid tumor. Clinicopathologic analysis of six cases. Am J Surg Pathol 1995; 19: 1039-1046.

Light Microscopy	Electron Microscopy
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