A 16-year-old boy presents with a 3-month history of pain and swelling in the mid-shaft of the femur. Imaging shows a diaphyseal lesion with periosteal reaction. Biopsy reveals small, round blue cells with high mitotic activity. Which single feature best distinguishes Ewing sarcoma from osteosarcoma?

Explanation

## Anatomical Location: The Gold Standard Discriminator **Key Point:** The most reliable single feature distinguishing Ewing sarcoma from osteosarcoma is their **anatomical origin within the bone**. ### Location Comparison | Feature | Ewing Sarcoma | Osteosarcoma | |---------|---------------|---------------| | **Primary site** | Diaphysis (shaft) | Metaphysis (end) | | **Long bones** | Femur, tibia, humerus | Distal femur, proximal tibia, proximal humerus | | **Typical age** | 10–20 years | 10–25 years (peak 15–19) | | **Histology** | Small round blue cells | Malignant osteoid production | **High-Yield:** Ewing sarcoma = **diaphyseal** origin; osteosarcoma = **metaphyseal** origin. This is the single most consistent and diagnostically useful distinction. ### Why Location Matters Diaphyseal tumours in a teenager with small round cells = Ewing sarcoma until proven otherwise. This anatomical rule is taught in every major orthopedic textbook and is the first step in differential diagnosis on imaging. **Clinical Pearl:** When you see a long bone tumour in a child, ask "Where in the bone?" before anything else. Location narrows the differential dramatically. ### Other Differences (Not the Best Single Discriminator) - **Periosteal reaction:** Ewing shows "onion-skin" (lamellated); osteosarcoma shows "sunburst" — these overlap and are less reliable than location - **Histology:** Ewing produces small round blue cells; osteosarcoma produces malignant osteoid — both are definitive but require biopsy - **Prognosis:** Both now respond to neoadjuvant chemotherapy; prognosis is similar (~70% 5-year survival with modern protocols) [cite:Robbins 10e Ch 26]