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

Explanation

## Anatomical Location of Osteosarcoma **Key Point:** Osteosarcoma arises in the metaphyseal region of long bones, most commonly around the knee joint (distal femur and proximal tibia account for ~50% of cases). ### Why the Metaphysis? The metaphysis is the site of maximum bone growth and remodeling, particularly during periods of rapid skeletal growth in adolescents and young adults. This high cellular turnover and rapid osteoblastic activity predispose the metaphyseal region to malignant transformation. ### Common Sites of Osteosarcoma | Site | Frequency | Notes | | --- | --- | --- | | Distal femur | ~40% | Most common single site | | Proximal tibia | ~15% | Around the knee (combined ~55%) | | Proximal humerus | ~10% | Second most common bone | | Pelvis | ~5-10% | Poorer prognosis | | Proximal femur | ~5% | Less common than distal | | Spine, jaw | <5% | Rare sites | **High-Yield:** The metaphyseal location is a key distinguishing feature—osteosarcoma does NOT typically arise in the diaphysis (unlike Ewing sarcoma, which favors the diaphysis). **Clinical Pearl:** The "around the knee" location (distal femur + proximal tibia) is so characteristic that it is often the first anatomical clue in a clinical case.