A 16-year-old boy from Delhi 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 spicules) pattern of periosteal reaction and cortical destruction. MRI demonstrates a large soft-tissue mass with heterogeneous signal intensity and areas of necrosis. What is the most likely diagnosis?

Explanation

## Diagnosis: Osteosarcoma ### Clinical Presentation **Key Point:** Osteosarcoma is the most common primary malignant bone tumor in adolescents and young adults, with peak incidence in the second decade of life during periods of rapid skeletal growth. ### Imaging Hallmarks **High-Yield:** The "sunburst" (radiating spicules) pattern of periosteal reaction is pathognomonic for osteosarcoma. This represents aggressive tumor growth breaking through the cortex with periosteal new bone formation along blood vessels. | Feature | Osteosarcoma | Ewing Sarcoma | Giant Cell Tumor | Chondrosarcoma | |---------|--------------|---------------|------------------|----------------| | **Age** | 10–25 years | 10–20 years | 20–40 years | 30–60 years | | **Location** | Metaphysis (distal femur, proximal tibia) | Diaphysis/metadiaphysis | Epiphysis (post-fusion) | Metaphysis/diaphysis | | **Periosteal reaction** | Sunburst, Codman triangle | Onion-skin (lamellated) | None typical | Stippled calcification | | **Cortical destruction** | Aggressive | Permeative | Well-defined | Gradual | | **Soft-tissue mass** | Large, heterogeneous | Smaller relative to bone lesion | Minimal | Variable | ### Why Sunburst Pattern? The radiating spicules represent tumor osteoid/bone formation along Haversian and Volkmann canals as the tumor breaks through the cortex. This is virtually diagnostic of osteosarcoma. **Clinical Pearl:** Osteosarcoma commonly arises in the metaphyseal regions of long bones (distal femur > proximal tibia > proximal humerus) where bone growth is most rapid. The anteromedial distal femur is a classic site. **Mnemonic:** **SOBA** — **S**unburst periosteal reaction = **O**steosarcoma; **B**one-forming; **A**ggressive. ### MRI Findings Heterogeneous signal with necrosis reflects the aggressive, rapidly growing nature of the tumor with areas of tumor necrosis and hemorrhage. **High-Yield:** MRI is essential for surgical planning to assess soft-tissue extension, marrow involvement, and neurovascular proximity—not for diagnosis (which is radiographic + histology).