The next day, an incisional biopsy was performed. Histological study of the mass showed a predominant population of large polygonal cells and a smaller number of admixed multinucleate giant cells (Figure 3). Both the mononuclear and multinuclear cells had a moderate to large amount of lightly eosinophilic cytoplasm. The nuclei in the mononuclear and multinucleate cells were slightly enlarged and irregular, but significant hyperchromasia was not present and nucleoli were inconspicuous. The mitotic rate was low at fewer than 2 mitoses per 10 high-power fields. The lesion infiltrated skeletal muscle. Stains for fungi and mycobacteria were negative. The mononuclear and multinucleate cells were strongly positive on a vimentin stain. Actin, desmin, S100, factor VIII, and cytokeratin stains all were negative.