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MICROSCOPIC DESCRIPTION:
The tissue submitted for pathology was fragmented, firm, and tan with some paler areas. Histopathologically, the lesion was cellular and ill-circumscribed (Fig. 4). It extended into adjacent bone, tendon, and skeletal muscle (Fig. 5). The cellular areas were composed of uniform-appearing round to oval cells with a prominent but bland nucleus and eosinophilic or amphophilic cytoplasm (Fig. 6, Fig.7). Nucleoli were often conspicuous. Mitotic figures were rare, and atypical mitotic figures were absent. Intermingled were clusters of osteoclast-like multinucleated giant cells (Fig 8), foamy macrophages, and sparse foci of extra- and intracellular brown pigment (Fig. 9). Prussian-blue stain identified this pigment as hemosiderin deposits (Fig. 10). The numbers of nuclei in the giant cells ranged from 5-35, the vast majority possessing less than 20. Their morphologic features differed considerably from the features of the nuclei contained in the mononuclear cells. The areas of lesser cellularity showed ill-formed aggregates of eosinophilic material reminiscent of osteoid and/or collagen (Fig. 11). No well-formed trabecular patterns or areas of conspicuous osteoblastic rimming were identified. No necrosis, recent hemorrhage, spindle cells, a cartilaginous component, or areas of storiform growth were found. Immunohistochemical staining was non-contributory.
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