## Anatomical Distribution of Ewing Sarcoma **Key Point:** The distal femur and proximal tibia (periarticular region around the knee) account for approximately 50% of all Ewing sarcomas, making this the most common site of origin. ### Distribution Pattern | Anatomical Site | Frequency | Notes | | --- | --- | --- | | **Distal femur + proximal tibia** | ~50% | Most common; periarticular | | Pelvis | ~20% | Second most common; worse prognosis | | Proximal femur | ~10% | Femoral diaphysis | | Proximal humerus | ~5% | Upper limb involvement | | Other sites | ~15% | Fibula, spine, ribs, pelvis | **High-Yield:** Ewing sarcoma typically arises in the **diaphysis or metadiaphyseal region** of long bones, in contrast to osteosarcoma (which favors the metaphysis). This is a key distinguishing radiological feature. ### Clinical Significance - **Lower limb involvement:** ~90% of cases (femur, tibia, fibula) - **Pelvic origin:** Associated with delayed diagnosis due to deep location and larger tumor size at presentation - **Age correlation:** Peak incidence 10–20 years; occurs during periods of rapid bone growth **Clinical Pearl:** A young patient with a diaphyseal lesion around the knee should raise suspicion for Ewing sarcoma; the combination of age, location, and radiological pattern is highly suggestive. 
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