A 16-year-old boy from Mumbai presents with a 3-month history of progressive left knee pain and swelling. On examination, he has a firm, non-tender mass over the anteromedial aspect of the distal femur. Plain radiographs show a metaphyseal lesion with a "sunburst" (radiating spiculated) periosteal reaction and cortical destruction. MRI reveals a large soft tissue mass with mixed signal intensity and areas of necrosis. What is the most likely diagnosis?

Explanation

## Clinical Presentation & Epidemiology **Key Point:** Osteosarcoma is the most common primary malignant bone tumor in children and adolescents, with peak incidence in the second decade of life during periods of rapid skeletal growth. **High-Yield:** The distal femur (around the knee) is the most common site, accounting for ~40% of all osteosarcomas, followed by the proximal tibia and proximal humerus. ## Radiological Features ### Pathognomonic Imaging Findings | Feature | Osteosarcoma | Ewing Sarcoma | Giant Cell Tumor | |---------|--------------|---------------|------------------| | **Location** | Metaphysis (distal femur, proximal tibia) | Diaphysis/metadiaphysis | Epiphysis (post-fusion) | | **Periosteal reaction** | Sunburst (radiating spicules) or Codman triangle | Onion-skin (lamellated) | Absent | | **Cortical involvement** | Cortical destruction with aggressive margins | Permeative, poorly defined | Well-defined, eccentric | | **Soft tissue mass** | Large, mixed signal on MRI | Large, homogeneous | Small, confined | | **Age group** | 10–25 years (peak 15–19) | 10–20 years | 20–40 years | **Clinical Pearl:** The "sunburst" periosteal reaction in this case is virtually pathognomonic for osteosarcoma. This represents radiating spicules of new bone formation perpendicular to the cortex, indicating aggressive tumor growth breaking through the periosteum. ## Mechanism of Imaging Pattern 1. Tumor arises in metaphyseal region (area of rapid bone turnover in adolescents) 2. Aggressive growth destroys cortex from within 3. Periosteum attempts to form new bone (Codman triangle initially) 4. Rapid growth overwhelms periosteal response → sunburst pattern 5. Large soft tissue mass with necrosis and hemorrhage (MRI shows mixed signal) **Mnemonic:** SOMA = **S**unburst, **O**steoid (tumor produces osteoid), **M**etaphysis, **A**dolescents ## Why This Case Fits Osteosarcoma - Age 16 years (peak incidence) - Location: distal femur metaphysis (most common site) - Sunburst periosteal reaction (pathognomonic) - Cortical destruction with aggressive margins - Large soft tissue mass with mixed signal and necrosis [cite:Robbins 10e Ch 26]