Post-treatment histological section of the surgical specimen. The diffuse proliferation of short-spindle-shaped cells arranged in a storiform pattern (mimicking benign fibrous histiocytoma) involving bone formation is shown. Clusters of foam cells are also seen. Both the stromal cells and osteoclast-like giant cells have disappeared. (hematoxylin-eosin stain; A, ×100; B, ×200).