## Anatomical Location of Giant Cell Tumor **Key Point:** Giant cell tumor (GCT) of bone has a highly characteristic location that is one of its defining features. ### Classic Location Pattern Giant cell tumors arise in the **epiphysis and extend into the metaphysis** of long bones, particularly around the knee joint. This epiphyseal origin is a distinguishing feature from other bone tumors. ### Frequency by Site | Location | Frequency | | --- | --- | | Around knee (distal femur + proximal tibia) | 50–65% | | Distal radius | 10–15% | | Sacrum | 3–5% | | Proximal humerus | 2–3% | | Other long bones | Rare | **High-Yield:** The **epiphyseal location extending into metaphysis** in a skeletally mature patient (typically 20–40 years old) around the knee is pathognomonic for GCT. ### Why Epiphyseal? GCT arises from the epiphyseal region in skeletally mature patients (after physeal closure). This contrasts with: - **Osteosarcoma** → metaphyseal, around the knee in adolescents - **Ewing sarcoma** → diaphyseal - **Chondroblastoma** → epiphyseal but in skeletally immature patients **Clinical Pearl:** When you see an epiphyseal lesion crossing the joint space in an adult, think GCT first. 
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