(a) Axial T1 fat-saturated MRI shows soft tissue mass with peripheral high signal and internal low signal in the muscle anterior to the right shoulder. (b) Axial contrast-enhanced CT scan shows a peripherally enhancing soft tissue mass located in the muscle, anterior to the right shoulder. Calcification is noted in the mass (arrow). (c) Macroscopically, the tumor is composed of tan−white tissue with gritty zones corresponding to bone formation. On hematoxylin and eosin, a neoplastic proliferation of atypical polygonal to spindle cells producing the malignant osteoid is evident ((d) 100× magnification, (e) 200× magnification). A strong immunohistochemical nuclear expression of SATB2 in neoplastic cells demonstrates the osteogenic differentiation and osteoid matrix production of the neoplasm ((f) 200× magnification).