## Distinguishing Osteosarcoma from Ewing Sarcoma ### Histopathological Hallmark **Key Point:** The defining feature of osteosarcoma is the production of osteoid and/or bone by the malignant mesenchymal cells themselves — this is the single most reliable histological discriminator. ### Comparative Table | Feature | Osteosarcoma | Ewing Sarcoma | | --- | --- | --- | | **Osteoid/bone production** | ✓ Present (diagnostic) | ✗ Absent | | **Cell morphology** | Pleomorphic, spindle, giant cells | Small round cells, uniform | | **Cytogenetics** | Complex karyotype, TP53 mutations | t(11;22)(q24;q12) — EWSR1-FLI1 | | **Age of onset** | 10–25 years (peak 15–20) | 10–25 years (peak 15–20) | | **Site predilection** | Metaphysis of long bones | Diaphysis/diametaphysis | | **Immunohistochemistry** | Osteocalcin+, alkaline phosphatase+ | CD99+, FLI1+, TLE1+ | **High-Yield:** Osteoid/bone formation by tumor cells = osteosarcoma. This is the pathognomonic finding and the basis of the diagnosis. ### Clinical Pearl While both tumors occur in young patients and show high mitotic rates, only osteosarcoma demonstrates the neoplastic production of mineralized matrix — a reflection of its osteoblastic origin. ### Why This Matters Histological confirmation of osteoid/bone is essential because it defines the tumor type and guides prognosis and treatment (neoadjuvant chemotherapy + surgery is standard for osteosarcoma). 
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