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low-grade osteosarcoma arising on the surface of bone without involvement of the underlying marrow, usually occurring as a heavily ossified mass of the distal femur in women in the third and fourth decades of life.
parosteal osteosarcomaAn uncommon (4% of all osteosarcomas (OS)) indolent, painless OS of patients age 30 to 60, which arises in the metaphysis and is confined to the surface long bones (posterior distal femur, tibia, humerus; rarely hands or mandible). Parosteal OS lacks intramedullary involvement; it has mature lamellar bone, fibrous stroma, minimally increased cellularity, atypia and increased mitotic activity. Up to 70% arise on the surface of the distal femur. Recurrent parosteal OSs may dedifferentiate.
Prominent extracortical calcified mass encircling bone.
Generally good—80% 5-year survival; recurrence with dedifferentiation significantly worsens the prognosis.