Which of the following is the most common site of origin for osteosarcoma in the metaphyseal region of long bones?

Explanation

## Most Common Sites of Osteosarcoma **Key Point:** Osteosarcoma has a highly predilective distribution in the metaphyseal regions of rapidly growing long bones, particularly around the knee joint. ### Site Distribution | Site | Frequency | Reason | | --- | --- | --- | | **Distal femur** | ~40% | Fastest growing metaphysis in adolescents | | **Proximal tibia** | ~20% | Second fastest growing metaphysis | | **Proximal humerus** | ~10% | Metaphyseal involvement less common | | **Pelvis** | ~10% | Extra-skeletal sites, worse prognosis | | **Jaw (mandible/maxilla)** | ~5–10% | Associated with prior radiation | **High-Yield:** The **distal femur and proximal tibia together account for approximately 50–60% of all osteosarcomas**. This is because these regions have the highest rates of bone growth during the adolescent growth spurt, when osteosarcoma typically arises. ### Why the Metaphysis? Osteosarcoma arises in areas of **rapid bone turnover and growth**. The metaphyseal regions of long bones are sites of intense osteoblastic activity during skeletal development, making them vulnerable to malignant transformation of osteoblasts. **Clinical Pearl:** The tumor typically arises in the metaphysis but may extend into the diaphysis and occasionally the epiphysis. It rarely crosses the joint space due to the barrier of the articular cartilage. ### Mnemonic: "FEMUR FIRST" - **F**emur (distal) — 40% - **E**xtremity around knee — 50–60% combined - **M**etaphysis — always metaphyseal origin - **U**pper extremity — less common - **R**apid growth zones — site predilection