This case is an osteosarcoma. The point here is to demonstrate the radiographic appearance of the tumor and to hint at the characteristic balance between the amount of tumor inside and outside the bone that may be seen in osteosarcoma. Why do we care? Because these features might help distinguish this tumor from the other tumor that is likely in this patient and this location, Ewing's sarcoma. As it happens, Ewing's sarcoma usually does not have this type of periosteal reaction. Ewing's may also have a much more "lopsided" appearance. There may be a small intraosseous tumor and a large soft tissue mass. In fact, the abnormality within the bone may be minimally visible in Ewing's tumor even while the mass outside the bone is very large. Bone tumors do not follow the "rules" and they do not always look they way they "should". Nevertheless, this case might help you decide between an osteosarcoma and a Ewing's tumor someday.