## Distinguishing Osteosarcoma from Ewing's Sarcoma ### Key Radiological Features **Key Point:** The **sunburst (radiating) periosteal reaction** is the most characteristic and discriminating radiological finding of osteosarcoma. This reflects aggressive tumor growth breaking through the cortex with periosteal new bone formation along blood vessels. ### Comparison Table | Feature | Osteosarcoma | Ewing's Sarcoma | | --- | --- | --- | | **Periosteal reaction** | Sunburst, spiculated (CHARACTERISTIC) | Onion-skin, lamellated | | **Location** | Metaphysis (distal femur, proximal tibia) | Diaphysis or metadiaphysis | | **Age** | 10–25 years (peak 15–20) | 10–30 years (peak 15–20) | | **Codman's triangle** | Present | Present | | **Matrix mineralization** | Osteoid/bone (dense) | Minimal | | **Cortical destruction** | Extensive | Variable | ### Why Sunburst is the Best Discriminator **High-Yield:** While both tumors occur in adolescents and both can show Codman's triangle and metaphyseal involvement, the **sunburst (radiating spicule) pattern** is virtually pathognomonic for osteosarcoma. It represents aggressive periosteal reaction with tumor osteoid formation along Haversian and Volkmann canals. **Clinical Pearl:** Ewing's sarcoma classically shows an **onion-skin (lamellated) periosteal reaction**, reflecting concentric layers of periosteal new bone—a distinctly different appearance from the radiating spicules of osteosarcoma. ### Pathological Basis Osteosarcoma is an **osteoid-producing malignancy**; the tumor itself mineralizes bone matrix, creating the dense, aggressive appearance. Ewing's sarcoma is a small round cell tumor with minimal osteoid production, hence less periosteal ossification. **Mnemonic:** **SUNN** = **S**unburst = **U**nequivocal **N**ew bone = **N**ot Ewing's. 
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