A 16-year-old boy presents with progressive left knee pain and swelling for 3 months. Physical examination reveals a firm, non-tender mass over the anterolateral aspect of the distal femur. Plain radiographs show a mixed lytic and sclerotic lesion at the metaphyseal region of the distal femur with a "sunburst" periosteal reaction and cortical destruction. MRI demonstrates a large soft tissue component with heterogeneous signal intensity. What is the most likely diagnosis?

Explanation

## Clinical Presentation & Imaging Features **Key Point:** The combination of age (adolescent), location (metaphyseal region of long bone), and classic imaging findings (sunburst periosteal reaction, mixed lytic-sclerotic lesion, cortical destruction) is pathognomonic for osteosarcoma. ### Imaging Pattern Recognition | Feature | Osteosarcoma | Ewing Sarcoma | Giant Cell Tumor | Chondrosarcoma | |---------|--------------|---------------|------------------|----------------| | **Age** | 10–25 years (peak 15–20) | 10–25 years | 20–40 years | 40–60 years | | **Location** | Metaphyseal (distal femur, proximal tibia) | Diaphyseal/metadiaphyseal | Epiphyseal (post-fusion) | Metaphyseal/diaphyseal | | **Periosteal reaction** | Sunburst, Codman triangle | Onion-skin (lamellated) | None or subtle | Stippled calcification | | **Matrix** | Osteoid/bone (mixed lytic-sclerotic) | Purely lytic | Osteolytic | Chondroid (rings & arcs) | | **Soft tissue** | Large, heterogeneous | Large, homogeneous | Minimal | Minimal to moderate | **High-Yield:** The **sunburst (radiating spicules)** and **Codman triangle** periosteal reactions are hallmarks of osteosarcoma and reflect aggressive tumor growth breaking through cortex with reactive bone formation. ### Pathophysiology Osteosarcoma arises from primitive mesenchymal cells in the metaphyseal growth plate region, where rapid bone turnover creates a permissive environment for malignant transformation. The mixed lytic-sclerotic pattern reflects simultaneous tumor osteolysis and reactive osteogenesis. **Clinical Pearl:** Osteosarcoma is the most common primary malignant bone tumor in children and young adults. It has a predilection for the **distal femur (40%), proximal tibia (15%), and proximal humerus (10%)** — sites of rapid bone growth. ### MRI Findings Heterogeneous signal intensity on T1 and T2 reflects: - Tumor cellularity (intermediate signal) - Necrosis (high T2 signal) - Hemorrhage (variable signal) - Osteoid/bone (low signal) The large soft tissue component indicates extraosseous extension, critical for surgical planning and staging. **Warning:** Do not confuse Ewing sarcoma (onion-skin periosteal reaction, diaphyseal location, homogeneous soft tissue, younger age) with osteosarcoma. Ewing typically presents in the 10–20 age group with fever and elevated inflammatory markers, whereas osteosarcoma is afebrile. ## Why This Is Osteosarcoma **Mnemonic: SOMA** — **S**unburst periosteum, **O**steoid matrix, **M**etaphyseal location, **A**dolescent age. [cite:Robbins 10e Ch 26]