A 12-year-old girl presented with progressive right knee pain at rest of 4-months duration. There was tenderness and swelling over the right distal thigh. Plain radiograph (Fig. 1) showed a lytic lesion over the medial supracondylar region of the right femur with a wide zone of transition, aggressive periosteal reaction, and cortical erosion. Axial T1-weighted, post-contrast MRI (Fig. 2) showed an enhancing medullary lesion eroding the medial cortex with a large extraosseous soft-tissue component (arrow). Histology obtained via core needle biopsy yielded high-grade osteosarcoma. CT thorax and bone scan showed no distant metastases. She underwent neoadjuvant chemotherapy, followed by wide resection and reconstruction with a megaprosthesis and finally adjuvant chemotherapy.