Which feature best distinguishes osteosarcoma from Ewing's sarcoma in a 16-year-old boy presenting with a painful metaphyseal lesion of the distal femur?

Explanation

## Distinguishing Radiographic Features **Key Point:** The sunburst (radiating spiculated) pattern of periosteal new bone formation is the hallmark radiographic finding of osteosarcoma and is rarely seen in Ewing's sarcoma. ### Comparative Radiographic Findings | Feature | Osteosarcoma | Ewing's Sarcoma | |---------|--------------|------------------| | **Periosteal reaction** | Sunburst (spiculated), Codman's triangle | Onion-skin (lamellated), concentric | | **Bone destruction** | Mixed lytic-sclerotic | Purely lytic | | **Soft tissue mass** | Often present | Less prominent | | **Cortical breakthrough** | Early and extensive | Later presentation | **High-Yield:** The sunburst pattern reflects aggressive periosteal new bone formation along blood vessels (Sharpey's fibers), creating radiating spicules perpendicular to the cortex — pathognomonic for osteosarcoma. ### Key Distinguishing Points **Clinical Pearl:** While both are metaphyseal lesions in adolescents, the radiographic pattern is the most reliable imaging discriminator before histology. **Mnemonic:** **SARCOMA** — **S**unburst = **S**arcoma (osteosarcoma); **O**nion-skin = **O**ther (Ewing's). ## Why This Matters The sunburst pattern indicates rapid, aggressive periosteal response and is so characteristic that it should immediately raise suspicion for osteosarcoma on plain films, prompting urgent MRI and biopsy [cite:Robbins 10e Ch 26].