Ossifying fibromyxoid tumor of soft parts. Additional observations of a distinctive soft tissue tumor.

The author studied four subcutaneous soft tissue tumors, similar to those recently described by Enzinger and associates (Am J Surg Pathol 1989;13:817) by the name "ossifying fibromyxoid tumor," by immunohistochemistry and electron microscopy to further understand the cellular nature of this lesion. The four tumors were composed of uniform round cells often surrounded by a lacunar space. The tumors often contained a peripheral zone of metaplastic bone. The cellularity was high, but the mitotic rate was low, suggesting a benign or borderline nature of the lesion. Longer follow-up was available for three cases, showing recurrence-free survival times of 11, 8, and 3 years. Immunohistochemistry studies revealed that all tumors were strongly positive for S-100 protein and focally positive for Leu-7, whereas melanoma-specific marker HMB45 was negative. Vimentin was the main type of intermediate filament protein, and one case also contained scattered glial fibrillary acidic protein-positive cells. Epithelial markers (keratins, epithelial membrane antigen), desmin, and muscle actins were negative. Electron microscopic examination showed partial, sometimes reduplicated, basal lamina surrounding many cells. Complex cell processes were also present. No myofilaments were found. The immunohistochemical and electron microscopic results may suggest that this tumor has Schwann's cell differentiation.