A 28-year-old man presents with a slowly growing firm mass fixed to the posterior aspect of the distal femur. X-ray shows a densely ossified lobulated mass attached broadly to the posterior femoral cortex with a thin radiolucent line separating it from the underlying cortex, and no medullary continuity. The most likely diagnosis, also the variant of osteosarcoma with the best prognosis, is:

• A Parosteal (juxtacortical) osteosarcoma ✓
• B Periosteal osteosarcoma
• C High-grade surface osteosarcoma
• D Myositis ossificans traumatica

Explanation

Parosteal osteosarcoma is a low-grade tumour arising on the external surface of bone, classically the posterior distal femoral metaphysis in young adults. It forms a dense ossified mass attached by a broad base, separated from the cortex by a lucent cleft, and lacks medullary continuity, which distinguishes it from osteochondroma. Being low grade, it has the best prognosis among osteosarcomas. Periosteal osteosarcoma is intermediate grade and produces cartilage rather than dense bone.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.