A 16-year-old boy presents with a painful mass over the proximal tibia. Radiographs reveal a mixed lytic and sclerotic lesion with a sunburst periosteal reaction and Codman's triangle. Which is the most common site of osteosarcoma?

Explanation

## Most Common Site of Osteosarcoma **Key Point:** The distal femur and proximal tibia (around the knee joint) account for approximately 50–60% of all osteosarcomas. These are the most common sites of presentation. ### Anatomical Distribution of Osteosarcoma | Site | Frequency | Clinical Notes | |------|-----------|----------------| | **Distal femur** | ~40% | Most common single site; metaphyseal region | | **Proximal tibia** | ~15–20% | Second most common; metaphyseal region | | **Proximal humerus** | ~10% | Third most common | | **Pelvis** | ~5–10% | Often presents late; poor prognosis | | **Distal tibia/fibula** | ~5% | Least common around knee | | **Spine, ribs, other sites** | ~5–10% | Rare | **High-Yield:** **"Around the knee"** (distal femur + proximal tibia) = **50–60% of osteosarcomas**. This is the single most important epidemiological fact for NEET PG. ### Why the Distal Femur and Proximal Tibia? 1. **Rapid bone growth** — these metaphyseal regions have the highest growth velocity in adolescents 2. **Peak incidence during growth spurt** — osteosarcoma occurs during rapid skeletal growth (ages 10–25 years) 3. **Metaphyseal predilection** — malignant transformation occurs in zones of rapid osteoblastic activity **Mnemonic:** **"KNEE = KNOW"** — Know that the knee (distal femur + proximal tibia) is the most common site of osteosarcoma. ### Imaging Features at the Knee - **Sunburst (radiating) periostitis** — perpendicular spicules of new bone formation - **Codman's triangle** — periosteal reaction where tumor lifts periosteum - **Mixed lytic and sclerotic lesion** — aggressive, wide zone of transition - **Soft tissue mass** — often large at presentation - **Cortical destruction** — indicates aggressive behavior **Clinical Pearl:** The metaphyseal location (not diaphyseal) is crucial. Osteosarcomas arise in the metaphysis where growth is most active, whereas Ewing's sarcoma arises in the diaphysis. **Warning:** Do not confuse the site of osteosarcoma (around the knee in 50–60%) with the site of giant cell tumor (epiphysis of distal femur) or Ewing's sarcoma (diaphysis of femur).