Microscopic features of the case. A. The tumor formed by dense and loose cell regions. The dense regions were composed of plexiform and nodular oval- to spindle-shaped tumor cells. The loose regions were composed of fibrosis. Bone trabeculae erosion was observed. B. There were relatively clear boundaries of dense and loose cell regions, both of which exhibited abundant capillary vessels. Muscle infiltration was confirmed by the few residual muscle cells in the tumor (arrow). C. The loose region was made up of two components. One was a fibrotic area with fibrinoid degeneration, and the other was composed of sparse stellate-shaped cells with myxoid background. D. The large mononucleate cells and multinucleate giant cells were randomly-distributed among the oval-to-spindle cells, with abundant capillary vessels in the mesenchyme. The osteoclast-like multinucleate giant cells were abundant and scattered randomly. E. The oval-to-spindle cells and giant cells aggregated to form non-necrotizing-like granulomas lesions, with abundant small vascular-formation. F. The sponginess hemangioma regions were also observed in the dense tumor cell nodules. G. The thick-wall vessels with periendothelial proliferation were congregated in some areas. This pattern always appeared in the loose region. H. The big vessels showed periendothelial proliferation and protruding into the lumen, as well. Stellate-shaped cells were sporadically distributed in the loose region. I. The big thin type of vessel was seen in some areas. (H&E stain. Magnification: A: 10×, B-I: 40×).