A and B, Anteroposterior and lateral radiographs show osteosarcoma in left distal femur of 31-year-old man. C and D, Low-power and high-power photomicrographs show malignant spindle cells producing osteoid. After preoperative chemotherapy, patient underwent wide resection and endoprosthetic reconstruction. E, Photograph of resected specimen and implant. F, Photograph of implant in situ. G and H, Postoperative anteroposterior and lateral radiographs. I, Photomicrograph showing complete tumor necrosis.