A 16-year-old boy presents with a 3-month history of progressive pain and swelling around the knee. Radiographs show a metaphyseal lesion with mixed lytic and sclerotic changes and a sunburst pattern. Which is the most common site of origin for osteosarcoma in this patient?

Explanation

## Most Common Site of Osteosarcoma **Key Point:** The distal femur is the single most common site of osteosarcoma, accounting for approximately 35–40% of all cases. ### Anatomical Distribution Osteosarcoma arises in the metaphyseal regions of long bones, particularly around the knee joint. The distribution follows a predictable pattern: | Site | Frequency | Notes | | --- | --- | --- | | Distal femur | 35–40% | Most common; around knee | | Proximal tibia | 15–20% | Second most common; around knee | | Proximal humerus | 10–15% | Third most common | | Distal tibia | 5–10% | Least common around knee | | Pelvis | 5–10% | Poorer prognosis | | Spine/jaw | <5% | Rare sites | **High-Yield:** Approximately 50% of all osteosarcomas occur around the knee joint (distal femur + proximal tibia combined). This is the classic presentation in adolescents during growth spurts. ### Clinical Pearl The predilection for the metaphyseal regions of long bones reflects the areas of most rapid bone growth during adolescence. The distal femur is the fastest-growing region in the body during puberty, explaining its highest incidence. ### Mnemonic **"FHTP"** — Femur (distal), Humerus (proximal), Tibia (proximal), Pelvis — in order of decreasing frequency. [cite:Robbins 10e Ch 26]