Which of the following is the most common site of origin for Ewing sarcoma in the femur?

Explanation

## Anatomical Location of Ewing Sarcoma **Key Point:** Ewing sarcoma characteristically arises in the **diaphysis (shaft)** of long bones, most commonly the femur. This is a key distinguishing feature from osteosarcoma, which favors the metaphysis. ### Site Predilection in Ewing Sarcoma | Anatomical Region | Frequency | Notes | |-------------------|-----------|-------| | **Diaphysis** | ~50% (most common) | Central shaft location; may extend into metaphysis | | **Metadiaphyseal** | ~40% | Involves both diaphysis and metaphysis | | **Metaphysis only** | ~10% | Rare; more suggestive of osteosarcoma | ### Bone Distribution - **Femur:** 40% of all Ewing sarcomas - **Pelvis:** 20–25% (often presents late due to deep location) - **Tibia/fibula:** 15–20% - **Humerus:** 10% - Other long bones: 5–10% **High-Yield:** The **diaphyseal location in a long bone** is a classic exam point that helps differentiate Ewing sarcoma from osteosarcoma (metaphyseal) and giant cell tumor (epiphyseal). **Mnemonic:** **DEWS** — Diaphysis, Ewing sarcoma; Metaphysis, osteosarcoma (Epiphysis, giant cell tumor; Shaft, Ewing). **Clinical Pearl:** Although Ewing sarcoma typically arises in the diaphysis, it frequently extends into the metaphysis and soft tissues, making accurate localization on imaging essential for surgical planning.