A large lytic intramedullary soft tissue mass involving the proximal shaft of the ulna, eroding the posterior cortex and extending into the overlying soft tissues. Multiple cystic spaces filled with blood are seen with extensive fluid-fluid levels. MRI findings are indicative of an aggressive pathology, likely telangiectatic osteosarcoma. Differential diagnosis includes aneurysmal bone cyst. Points against this being an aneurysmal bone cyst include:

• age of the patient.
• lack of a distinct sclerotic rim/margin.
• erosion of the cortex, and 
• extension into the soft tissues.

Final diagnosis was given as telangiectatic osteosarcoma.

Diagnosis was confirmed on histopathology.