A 16-year-old boy presents with a 3-month history of pain and swelling in the mid-shaft of the femur. Radiographs show a mixed lytic and sclerotic lesion with periosteal reaction. Which single feature best distinguishes Ewing sarcoma from osteosarcoma in this clinical scenario?

Explanation

## Distinguishing Ewing Sarcoma from Osteosarcoma ### Key Anatomical Difference **Key Point:** Location in the bone is the single most reliable discriminator between Ewing sarcoma and osteosarcoma. | Feature | Ewing Sarcoma | Osteosarcoma | |---------|---------------|---------------| | **Location** | Diaphysis (shaft) or diametaphysis | Metaphysis (around knee joint) | | **Age** | 10–20 years (peak 15–16) | 10–25 years (peak 15–20) | | **Periosteal reaction** | Onion-skin (lamellated) or sunburst | Sunburst or Codman triangle | | **Alkaline phosphatase** | Normal or mildly elevated | Markedly elevated | | **Histology** | Small round blue cells, no osteoid | Malignant osteoid production | ### Why Location Matters **High-Yield:** Ewing sarcoma arises in the **diaphysis (shaft)** of long bones, whereas osteosarcoma arises in the **metaphysis (end region)** near the knee joint. This anatomical distinction is the most reliable single feature on plain radiographs and is present in >90% of cases. **Clinical Pearl:** The classic teaching is "Ewing in the shaft, osteo at the knee." A lesion in the mid-diaphysis of the femur is far more likely to be Ewing sarcoma than osteosarcoma. ### Why Other Features Are Less Discriminatory - **Onion-skin periosteal reaction:** Both tumors can show this; it is not specific to Ewing sarcoma. - **Age:** Both present in the second decade; overlap is substantial. - **Elevated LDH and alkaline phosphatase:** Both tumors elevate these markers; osteosarcoma typically shows higher alkaline phosphatase, but this is not a reliable single discriminator. [cite:Robbins 10e Ch 26]