A 28-year-old man presents with a painful swelling over the distal femur. Plain radiographs show a metaphyseal lesion with a "sunburst" periosteal reaction and cortical destruction. Which imaging feature best distinguishes osteosarcoma from Ewing sarcoma?

Explanation

## Distinguishing Osteosarcoma from Ewing Sarcoma ### Key Imaging Differences **Key Point:** The zone of transition and margin characteristics are the most reliable radiographic discriminators between these two aggressive bone tumors, both of which occur in young patients. | Feature | Osteosarcoma | Ewing Sarcoma | |---------|--------------|---------------| | **Zone of transition** | Wide (>5 cm) | Narrow (<5 cm) | | **Margins** | Mixed lytic and sclerotic | Permeative, ill-defined | | **Periosteal reaction** | Sunburst, Codman triangle | Onion-skin (lamellated) | | **Location** | Metaphysis (distal femur, proximal tibia) | Diaphysis/metadiaphysis | | **Age** | 10–25 years | 10–20 years | | **Cortical destruction** | Aggressive, rapid | Aggressive, rapid | ### Why Zone of Transition Matters **High-Yield:** Osteosarcoma typically presents with a **wide zone of transition** — the tumor margin blends gradually with normal bone over a distance >5 cm, often with a mixed pattern of lytic (bone destruction) and sclerotic (reactive bone formation) changes. This reflects the tumor's rapid but somewhat organized growth. Ewing sarcoma, by contrast, shows a **narrow, permeative zone of transition** — the tumor infiltrates bone with ill-defined, aggressive margins and minimal reactive bone formation. This reflects its rapid, diffuse spread through the medullary canal. ### Periosteal Reactions **Clinical Pearl:** Both tumors can show aggressive periosteal reactions: - **Osteosarcoma:** Sunburst (radiating spicules) and Codman triangle (elevated periosteum with new bone) are classic but not pathognomonic. - **Ewing sarcoma:** Onion-skin (concentric lamellated) periosteal reaction is more characteristic, though less commonly seen than in osteosarcoma. **Warning:** Do not rely on periosteal reaction alone — both patterns can overlap. The **zone of transition and margin definition** are more discriminatory. ### Clinical Context **Key Point:** Location also helps: - **Osteosarcoma:** Metaphyseal (around the knee in 50% of cases). - **Ewing sarcoma:** Diaphyseal or metadiaphyseal (femoral shaft). However, imaging margin characteristics remain the single best radiographic discriminator. [cite:Robbins 10e Ch 26]