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

Explanation

## Diagnosis: Osteosarcoma **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 (distal femur and proximal tibia account for ~50% of cases). ### Classic Radiological Features | Feature | Osteosarcoma | Ewing Sarcoma | GCT | Chondrosarcoma | |---------|--------------|---------------|-----|----------------| | **Age** | 10–25 years | 10–20 years | 20–40 years | 40–60 years | | **Location** | Metaphysis (around knee) | Diaphysis/metadiaphysis | Epiphysis (post-fusion) | Metaphysis/diaphysis | | **Periosteal reaction** | Sunburst, Codman angle | Onion-skin (lamellated) | None | Stippled calcification | | **Matrix** | Osteoid/bone formation | Permeative, no matrix | Soap-bubble lysis | Chondroid matrix | | **Soft tissue mass** | Large, heterogeneous | Large, homogeneous | Usually contained | Variable | **High-Yield:** The **"sunburst" periosteal reaction** (radiating spicules of new bone perpendicular to cortex) and **mixed lytic-sclerotic lesion** with cortical destruction are pathognomonic for osteosarcoma. The tumor produces osteoid/bone matrix, which appears as increased density on plain films. **Clinical Pearl:** Osteosarcoma commonly arises at sites of rapid bone growth (around the knee in adolescents). The presence of a large heterogeneous soft-tissue mass with necrosis on MRI indicates high-grade malignancy. **Mnemonic: OSTEOSARCOMA features = "SUNBURST + CODMAN"** - **S**unburst periosteal reaction - **U**nder 25 years old - **N**ear the knee (metaphysis) - **B**one-forming tumor (osteoid) - **U**ncontrolled growth - **R**adiating spicules - **S**oft-tissue mass - **T**umor in adolescents - **C**ortical destruction - **O**steoid matrix - **D**istal femur/proximal tibia - **M**ixed lytic-sclerotic - **A**ggressive periosteal reaction - **N**ecrosis on MRI ### Imaging Algorithm for Bone Tumors ```mermaid flowchart TD A[Bone lesion in adolescent]:::outcome --> B{Location?}:::decision B -->|Metaphysis around knee| C[Sunburst periosteal reaction?]:::decision B -->|Diaphysis/metadiaphysis| D[Onion-skin pattern?]:::decision B -->|Epiphysis post-fusion| E[Soap-bubble lysis?]:::decision C -->|Yes| F[Osteosarcoma]:::action C -->|No| G[Consider other tumors]:::action D -->|Yes| H[Ewing sarcoma]:::action E -->|Yes| I[Giant cell tumor]:::action ``` [cite:Robbins 10e Ch 24]