A 38-year-old man reports a slowly enlarging, painless mass behind the lower end of the thigh for 2 years. Radiograph shows a densely mineralised mass applied to the posterior cortex of the distal femur, separated from the shaft by a thin radiolucent line. Biopsy shows a well-differentiated fibro-osseous tumour. What is the diagnosis?

• A Conventional intramedullary osteosarcoma
• B Periosteal osteosarcoma
• C Myositis ossificans traumatica
• D Parosteal (juxtacortical) osteosarcoma ✓

Explanation

Parosteal osteosarcoma is a low-grade variant arising on the bone surface, classically the posterior aspect of the distal femur in adults in the third to fourth decade. It grows slowly, appears as a dense ossified mass with a cleavage plane between it and the cortex, and carries a much better prognosis than conventional osteosarcoma. Periosteal osteosarcoma differs by being chondroblastic, diaphyseal, and intermediate grade.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.