## Most Common Site of Ewing Sarcoma **Key Point:** Ewing sarcoma characteristically arises in the **diaphysis (shaft)** of long bones, particularly the femur. This is a distinctive feature that helps differentiate it from osteosarcoma, which favours the metaphysis. ### Anatomical Predilection | Site | Frequency | Notes | | --- | --- | --- | | **Diaphysis** | ~50% | Most common; central location in shaft | | Metadiaphyseal | ~40% | Extends from diaphysis into metaphysis | | Metaphysis only | ~10% | Rare; osteosarcoma more typical here | | Epiphysis | <5% | Very uncommon | **High-Yield:** The **femur is the single most common bone affected** (25–30% of all Ewing sarcomas), followed by pelvis (20–25%) and tibia (15–20%). Within the femur, the diaphysis is the classic site. ### Clinical Pearl The diaphyseal origin of Ewing sarcoma contrasts sharply with **osteosarcoma**, which arises in the **metaphysis** near the knee joint. This anatomical distinction is a key diagnostic clue on imaging and helps guide differential diagnosis in a young patient with a femoral lesion. ### Imaging Features Supporting Diaphyseal Origin 1. **Onion-skin (lamellated) periosteal reaction** — concentric layers of new bone 2. **Wide zone of transition** — poorly demarcated margins 3. **Soft tissue mass** — often large and extends beyond bone 4. **Codman triangle** — may be present but less prominent than in osteosarcoma **Mnemonic: DIAPHYSIS for Ewing** — **D**iaphysis is the **D**istinctive site for **E**wing sarcoma (vs. metaphysis for osteosarcoma). [cite:Robbins 10e Ch 26]
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