A 28-year-old woman presents with a 6-month history of progressive left knee pain and mild swelling. She reports no history of trauma. On examination, there is mild effusion and restricted range of motion. X-ray shows a lytic lesion in the distal femur extending to the articular surface, with a thin sclerotic rim. MRI reveals a heterogeneous mass with areas of hemorrhage and hemosiderin deposition. The lesion is eccentrically located in the metaphyseal-diaphyseal region. What is the most likely diagnosis?