## Anatomical Origin of Ewing Sarcoma **Key Point:** Ewing sarcoma arises in the **diaphysis (shaft)** of long bones, classically in the femur, tibia, fibula, and humerus. This distinguishes it from osteosarcoma, which typically arises in the metaphysis. ### Characteristic Sites | Feature | Ewing Sarcoma | Osteosarcoma | | --- | --- | --- | | **Primary site** | Diaphysis (shaft) | Metaphysis (near growth plate) | | **Most common bone** | Femur (40%) | Distal femur, proximal tibia | | **Age group** | 10–20 years | 10–25 years | | **Periosteal reaction** | Onion-skin (lamellated) | Sunburst, Codman triangle | **High-Yield:** The **diaphyseal origin** is a key diagnostic feature that helps differentiate Ewing sarcoma from osteosarcoma on imaging. Ewing sarcoma arises in the medullary cavity of the diaphysis and can extend into the metaphysis and epiphysis, but the primary site of origin is always the shaft. **Clinical Pearl:** Ewing sarcoma typically presents in the second decade of life (peak 10–20 years) with pain, swelling, and systemic symptoms (fever, malaise). The diaphyseal location and aggressive periosteal reaction (onion-skin pattern) on X-ray are classic radiological findings. ### Why Diaphysis? The exact reason for diaphyseal predilection is not fully understood, but it may relate to the rapid bone turnover and metabolic activity in the diaphyseal region during adolescence.
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