Radiation-associated osteosarcoma may develop when more than 30Gy is given. There is a latency of more than 3 years after radiation therapy, on average 11-14 years. It may mimic osteonecrosis and primary recurrence. Imaging feature of osteosarcoma includes bone-forming tumor with soft tissue mass and interval growth. Osteonecrosis on the other hand usually shows osteopenia and heterogeneous bone density with an absence of a soft tissue mass and usually develops 1-3 years after radiation therapy with no significant change over time.