A 32-year-old man presents with a slowly growing, mildly painful mass behind the knee. X-ray shows a dense ossified mass attached to the posterior surface of the distal femur with a broad base and a lucent cleavage line separating it from the underlying cortex, without medullary continuity. Biopsy shows well-formed trabeculae of mature bone with bland fibroblastic stroma. The most likely diagnosis is:

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

Explanation

Parosteal osteosarcoma is a low-grade surface osteosarcoma occurring in adults in the third and fourth decades, classically on the posterior distal femoral metaphysis. It forms a broad-based ossified mass on the cortical surface with a radiolucent cleavage plane, and unlike conventional osteosarcoma it lacks medullary continuity and carries a much better prognosis with wide resection. Myositis ossificans shows zonal maturation and a history of trauma, while periosteal chondrosarcoma produces cartilage matrix.

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

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Written and medically reviewed by the StethoPrep medical team.