## Most Common Site of Osteosarcoma **Key Point:** The distal femoral metaphysis is the single most common site of osteosarcoma, accounting for approximately 40% of all cases. The proximal tibia is the second most common site (15–20%), followed by proximal femur (10–15%). ### Anatomical Distribution | Site | Frequency | Notes | | --- | --- | --- | | **Distal femur** | ~40% | Most common; around the knee | | **Proximal tibia** | ~15–20% | Second most common; around the knee | | **Proximal femur** | ~10–15% | Third most common | | **Distal tibia** | ~5–10% | Less common | | **Pelvis** | ~5–10% | Worse prognosis; delayed diagnosis | | **Jaw (mandible/maxilla)** | ~5% | Secondary peak in older adults | | **Spine** | <5% | Rare | **High-Yield:** Around 50% of all osteosarcomas occur around the knee joint (distal femur + proximal tibia combined). This is the classic "around the knee" presentation that is heavily tested. ### Why Metaphyseal Sites? 1. **Rapid bone growth** — metaphyses are regions of active osteoblastic activity during skeletal growth. 2. **Peak incidence in adolescence** — osteosarcoma occurs during the growth spurt (ages 10–25), when metaphyseal activity is maximal. 3. **Increased cell turnover** — malignant transformation is more likely in areas of high mitotic activity. **Clinical Pearl:** The distal femur is not only the most common site but also the most accessible for early clinical detection (palpable swelling, obvious pain on weight-bearing). Pelvic osteosarcomas, by contrast, are often diagnosed late because they are deep and asymptomatic until advanced. ### Mnemonic: "**FTTP**" (Femur, Tibia, Then Pelvis) - **F**emur (distal) — most common - **T**ibia (proximal) — second - **T**hen pelvis — third (and worst prognosis) - **P**elvis — poor outcome due to late diagnosis **Warning:** Do not confuse osteosarcoma with giant cell tumor of bone (which favors the epiphysis/epimetaphysis of long bones in skeletally mature patients) or chondrosarcoma (which favors the diaphysis and pelvis in older patients).
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