A 16-year-old boy presents with pain around the right knee. Plain radiograph shows an aggressive-looking lesion in the distal femoral metaphysis with a permeative lytic pattern, cortical destruction, and a periosteal reaction showing multiple layers of new bone formation concentric to the cortex (onion-skin periostitis). No calcified matrix is visible. What is the most likely diagnosis?
- A Osteosarcoma
- B Osteomyelitis
- C Langerhans cell histiocytosis
- D Ewing sarcoma ✓
Explanation
Ewing sarcoma classically arises in the diaphysis or metadiaphysis of long bones (femur most common) in children and adolescents aged 5-25 years. Its characteristic radiographic appearance includes a permeative lytic lesion with poorly defined zone of transition, cortical destruction, and lamellated (onion-skin) periosteal reaction representing interrupted repetitive periosteal responses to tumour growth. Soft tissue mass is present in most cases. Osteosarcoma shows osteoid matrix production (cloudlike opacity, Codman's triangle), predominantly in the metaphysis. Osteomyelitis may mimic but is associated with clinical signs of infection. LCH produces well-defined lytic lesions often with a hole-within-a-hole appearance.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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