## Anatomical Location of Ewing Sarcoma **Key Point:** Ewing sarcoma characteristically arises in the **diaphysis** (shaft) of long bones, particularly the femur, tibia, and fibula. This is a distinguishing feature from osteosarcoma, which typically arises in the metaphysis. ### Site Predilection | Bone Location | Ewing Sarcoma | Osteosarcoma | Clinical Significance | |---------------|---------------|--------------|----------------------| | **Diaphysis** | **Most common (50–60%)** | Rare | Central location in bone shaft | | **Metaphysis** | Less common (30–40%) | **Most common** | Epiphyseal side of growth plate | | **Epiphysis** | Rare | Very rare | Articular end of bone | ### Common Sites in Order of Frequency 1. **Femur** (40% of all cases) — distal and proximal femoral diaphysis 2. **Tibia** (20%) — proximal tibial diaphysis 3. **Fibula** (10%) 4. **Pelvis** (10–15%) — often presents late due to deep location 5. **Humerus** (5%) **High-Yield:** The diaphyseal origin of Ewing sarcoma in a young patient (10–20 years old) with a long bone lesion is a classic teaching point. Metaphyseal osteosarcoma in the same age group is the key differential diagnosis. **Clinical Pearl:** Pelvic Ewing sarcomas are often diagnosed at an advanced stage because deep pelvic location delays symptom recognition and clinical presentation. **Mnemonic:** **"EWING = Diaphyseal"** (central shaft), whereas **"OSTEO = Metaphyseal"** (near the growth plate). [cite:Robbins 10e Ch 26] 
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