## Distinguishing Osteosarcoma Characteristics **Key Point:** Osteosarcoma is a malignant bone tumor with highly specific anatomical and demographic predilections. Understanding what osteosarcoma is NOT helps differentiate it from other bone lesions. ### Correct Features of Osteosarcoma | Feature | Detail | |---------|--------| | **Location** | Metaphyseal region (not epiphyseal) of long bones | | **Most common sites** | Distal femur (40%), proximal tibia (15–20%), proximal humerus (10%) | | **Age of onset** | Peak incidence 10–25 years; second peak >60 years (post-radiation, Paget disease) | | **Growth pattern** | Rapid, aggressive; can breach cortex and lift periosteum | | **Spread** | Crosses joint space and enters soft tissues; NOT confined to metaphysis | ### Why Option 4 is Incorrect **High-Yield:** Osteosarcoma arises in the **metaphysis**, NOT the epiphysis. Critically, it does **NOT** respect the joint space — it readily crosses into the epiphysis and joint cavity in advanced cases. This is a key distinguishing feature from benign lesions (e.g., osteochondroma) that respect anatomical boundaries. **Clinical Pearl:** The aggressive, boundary-crossing nature of osteosarcoma is one reason early surgical staging (including MRI of the entire bone and soft tissues) is essential before treatment planning. ### Radiological Hallmarks - **Sunburst (radiating spicules)** — tumor new bone formation perpendicular to cortex - **Codman triangle** — periosteal reaction at tumor margin - **Mixed lytic and sclerotic** — aggressive bone destruction with reactive new bone - **Soft tissue mass** — extraosseous extension is common **Mnemonic: MOST** — **M**etaphyseal origin, **O**ccurs in young patients, **S**unburst/Codman pattern, **T**ransforms to soft tissue.
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