The highly aggressive nature, the location and the age group of the patient narrow the differential diagnosis of this lesion to either osteosarcoma or Ewing sarcoma, however the osseous matrix in the lesion and the age of the patient (older than 10 years) as well as the eccenteric metaphyseal location favor osteosarcoma.

Osteosarcoma is a malignant bone forming tumor and the second most common primary bone tumor after multiple myeloma. It accounts for about 20% of all primary bone tumors. Its primary form occurs in young patients (10-20 years). Its secondary form occurs in the elderly, usually secondary to malignant degeneration of Paget disease, extensive bone infarcts or post radiotherapy. Patients usually present with bone pain, occasionally accompanied by a soft-tissue mass or swelling. They typically occur at the metadiaphysis of tubular bones in the appendicular skeleton. Commonest site is around the knee.

Ewing sarcoma is the second most common primary bone tumor of childhood after osteosarcoma. Typically occurs in children (especially before 10 years old), and has a slight male predilection. Presentation is non-specific with local pain being by far the most common symptom. As far as location within long bones, the tumor is almost always metadiaphyseal or diaphyseal. Commonest site is around the knee.