Which lesion is associated with radiolucent, extraarticular, geographic appearance?

• A. Osteomyelitis
• B. Osteosarcoma
• C. Ewing sarcoma
• D. Unicameral bone cyst

Answer: (D)

A radiolucent, well-defined, geographic lesion that lies away from the joint is most consistent with a unicameral bone cyst. These simple bone cysts are fluid-filled cavities in the metaphysis of long bones, commonly in children and adolescents. On X-ray they appear as a single, round or oval radiolucent area with a thin sclerotic rim—a geographic border—and are typically entirely intraosseous and extrarticular, not crossing into the joint.

Others would usually show more aggressive features: osteomyelitis can be radiolucent but often has irregular margins with periosteal reaction or sequestra; osteosarcoma tends to have mixed lytic-sclerotic destruction with aggressive periosteal reaction; Ewing sarcoma is typically permeative with onion-skin periostitis.