A 14-year-old girl presents with a 6-week history of pain and swelling in the right lower limb. Imaging reveals a mixed lytic-sclerotic lesion in the femoral shaft with onion-skin periosteal reaction and soft tissue mass. Which is the most common bone affected by Ewing sarcoma?

Explanation

## Most Common Bone Site in Ewing Sarcoma **Key Point:** The **femur** is the most common bone affected by Ewing sarcoma, accounting for approximately **40%** of all cases. The tibia and fibula are the second most common sites (combined ~20%), followed by the humerus. ### Distribution of Ewing Sarcoma by Bone | Bone | Frequency | Notes | | --- | --- | --- | | **Femur** | ~40% | Most common; diaphysis of femoral shaft | | **Tibia** | ~15–20% | Second most common; diaphysis | | **Fibula** | ~5–10% | Often with tibia involvement | | **Humerus** | ~10% | Upper extremity; diaphysis | | **Pelvis** | ~10–15% | Worst prognosis; late diagnosis | | **Foot/Ankle** | ~5% | Rare; includes calcaneus, talus | **High-Yield:** The **femur → tibia → fibula → humerus** sequence represents the descending order of frequency. Femoral involvement is tested frequently because it is the single most common site and often presents with a classic clinical picture of pain and swelling in the thigh. **Clinical Pearl:** Ewing sarcoma of the femur typically presents in adolescents (10–20 years) with progressive pain and swelling in the thigh. Systemic symptoms (fever, weight loss, malaise) are common, and the tumor can be quite large at presentation due to its location in a large bone. ### Why the Femur? The femur is the largest long bone in the body with the highest metabolic activity during adolescence. This may explain the predilection for Ewing sarcoma in this location.