A 16-year-old boy presents with a 3-month history of progressive pain and swelling over the distal femur, just above the knee. On examination, there is a firm, fixed mass with overlying skin warmth. Plain radiographs show a mixed lytic and sclerotic lesion at the distal femoral metaphysis with cortical destruction and a wide zone of transition. There is a prominent sunburst pattern of periosteal reaction. MRI reveals a large soft tissue mass with heterogeneous signal intensity and areas of necrosis. Which is the most likely diagnosis?

Explanation

## Clinical Presentation and Imaging Correlation **Key Point:** Osteosarcoma is the most common primary malignant bone tumor in adolescents and young adults, typically arising in the metaphyseal region of long bones around the knee joint. ### Characteristic Imaging Features | Feature | Osteosarcoma | Ewing Sarcoma | Giant Cell Tumor | Chondrosarcoma | |---------|--------------|---------------|------------------|----------------| | **Age** | 10–25 years | 10–30 years | 20–40 years | 30–60 years | | **Location** | Metaphysis (around knee) | Diaphysis/metadiaphysis | Epiphysis (post-fusion) | Metaphysis/diaphysis | | **Lytic vs Sclerotic** | Mixed (sunburst) | Purely lytic | Lytic | Sclerotic/chondroid | | **Periosteal Reaction** | Sunburst, Codman triangle | Onion-skin (lamellated) | Minimal | Minimal | | **Soft Tissue Mass** | Large, heterogeneous | Large, homogeneous | Small | Variable | | **Zone of Transition** | Wide | Wide | Narrow | Narrow | **High-Yield:** The **sunburst (radiating spicules)** and **Codman triangle** periosteal reactions are pathognomonic for osteosarcoma. These reflect aggressive tumor growth breaking through the cortex and lifting the periosteum. ### Pathophysiology Osteosarcoma arises from malignant osteoblasts that produce osteoid and bone. The mixed lytic-sclerotic pattern reflects: 1. Tumor necrosis and destruction (lytic component) 2. Reactive bone formation by tumor cells (sclerotic component) 3. Periosteal new bone formation (sunburst pattern) **Clinical Pearl:** Osteosarcoma commonly occurs at sites of rapid bone growth — the distal femur (around the knee) is the most frequent site (40% of cases), followed by proximal tibia and proximal humerus. **Mnemonic: LOST** — **L**ocation (metaphysis around knee), **O**steoid production (mixed density), **S**unburst periosteal reaction, **T**eenagers/young adults. ### Why This Case Fits Osteosarcoma - Age 16 (peak incidence 10–25 years) - Distal femur metaphysis (classic location) - Mixed lytic-sclerotic lesion - Sunburst periosteal reaction - Wide zone of transition - Large heterogeneous soft tissue mass with necrosis [cite:Robbins 10e Ch 26]