Gross and histologic pathology of the soft tissue mass. With regard to gross pathology, the soft tissue mass was pink, and exhibited a necrotic portion on its surface (A). Most of the lesion was infracted, so the nuclei of cells had disappeared. Some parts were composed of areas of hemorrhage with red blood cells (hematoxylin and eosin staining; original magnification ×200) (B). The mass was surrounded by collagenous tissue and synovial lining. In the viable area of the tumor, histology findings included multinucleated giant cells, macrophages, foam cells, and fibroblasts with hemosiderin pigment deposits in macrophages and in the extracellular space (hematoxylin and eosin staining; original magnification ×200) (C).