A 16-year-old boy presents with knee pain and swelling. Plain radiograph shows an eccentric lytic lesion in the distal femoral metaphysis with a narrow zone of transition, a thin sclerotic rim, and no periosteal reaction or soft-tissue extension. MRI confirms a fluid-filled lesion that does not cross the physis. What is the most likely diagnosis?

• A Osteosarcoma
• B Simple bone cyst
• C Aneurysmal bone cyst
• D Non-ossifying fibroma ✓

Explanation

Non-ossifying fibroma is a developmental cortical defect most common in the metaphysis of long bones in adolescents, appearing as an eccentric lytic lesion with a thin sclerotic rim, narrow zone of transition, and no periosteal reaction—indicating a benign, non-aggressive process. It does not cross the physis. Simple bone cysts are central, not eccentric. Aneurysmal bone cysts show fluid-fluid levels on MRI and often have a more aggressive appearance. Osteosarcoma is the main differential for distal femur lesions but shows periosteal reaction and soft-tissue mass.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.