A 16-year-old boy presents with progressive pain and swelling around the knee for 3 months. Radiographs show a mixed lytic-sclerotic lesion with periosteal reaction. What is the most common site of origin for osteosarcoma in this patient?

Explanation

## Epidemiology of Osteosarcoma Sites **Key Point:** Osteosarcoma has a highly predictable anatomical distribution, with approximately 50% of cases occurring around the knee joint. ### Most Common Sites | Site | Frequency | Age Group | Key Feature | |------|-----------|-----------|-------------| | **Distal femur** | ~40% | 10–25 years | Around the knee; metaphyseal region | | **Proximal tibia** | ~15–20% | 10–25 years | Around the knee; metaphyseal region | | **Proximal humerus** | ~10% | 10–25 years | Second most common single bone | | **Distal tibia** | ~5% | 10–25 years | Rare; ankle region | | **Proximal femur** | ~3–5% | 10–25 years | Uncommon; hip region | ### Why Around the Knee? **High-Yield:** The distal femur and proximal tibia together account for **50–55%** of all osteosarcomas. This is because: 1. The knee region has the fastest bone growth during adolescence 2. Osteosarcoma arises in areas of rapid osteoblastic activity 3. The metaphyseal regions of long bones are most susceptible **Clinical Pearl:** In a teenager with a bone tumour around the knee, always think osteosarcoma first. The classic presentation is a tall, rapidly growing adolescent with pain and swelling at the knee. **Mnemonic:** **FEMUR-TIBIA RULE** — Osteosarcoma loves the **FE**mur and **TI**bia (around the knee). Together they account for >50% of cases. ### Metaphyseal Predilection Osteosarcoma arises in the **metaphyseal region** of long bones, particularly in areas of rapid bone turnover during growth spurts. This explains why it is rare in: - Diaphysis (shaft) - Epiphysis (articular end) - Small bones of hands/feet - Flat bones (except pelvis, which is rare)