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 areas, periosteal reaction, and Codman's triangle. Histology confirms osteosarcoma. Which of the following 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 40% of all cases. The area around the knee (distal femur + proximal tibia) together account for >50% of osteosarcomas. ### Anatomical Distribution | Site | Frequency | Notes | | --- | --- | --- | | **Distal femur** | ~40% | Most common single site | | **Proximal tibia** | ~15–20% | Second most common | | **Proximal humerus** | ~10–15% | Third most common | | **Pelvis** | ~5–10% | Worse prognosis | | **Proximal femur** | ~5% | Rare | | **Distal tibia** | ~3–5% | Uncommon | **High-Yield:** The "around the knee" region (distal femur + proximal tibia) is the site in >50% of cases. This is a high-yield fact for NEET PG. ### Why the Distal Femur? 1. **Rapid bone growth:** Osteosarcoma arises in areas of rapid metaphyseal growth during adolescence. 2. **Distal femur grows fastest:** The distal femur has the highest growth velocity, making it the prime site for malignant transformation. 3. **Age correlation:** Peak incidence is 10–25 years, coinciding with the adolescent growth spurt. **Clinical Pearl:** Osteosarcoma occurs at sites of maximum skeletal growth velocity. In children and adolescents, the distal femur and proximal tibia are the fastest-growing bone regions, explaining their predilection. **Mnemonic:** **"FATHP"** — Femur (distal) > Around knee > Tibia (proximal) > Humerus (proximal) > Pelvis — lists sites in rough order of frequency.