A 28-year-old woman presents with a painless swelling over the lateral aspect of the knee for 6 months. Radiographs show an eccentric lytic lesion in the distal femur extending to the subchondral bone. Histopathology reveals numerous multinucleated giant cells admixed with mononuclear stromal cells. What is the most common site of giant cell tumor of bone?

Explanation

## Most Common Site of Giant Cell Tumor **Key Point:** Giant cell tumor (GCT) of bone most commonly arises around the knee joint, with the distal femur and proximal tibia being the two most frequent sites. Together, these account for approximately 50–65% of all GCTs. ### Epidemiology of GCT Sites | Site | Frequency | Key Feature | |------|-----------|-------------| | **Distal femur** | ~25–30% | Eccentric, extends to subchondral bone | | **Proximal tibia** | ~20–25% | Epiphyseal origin, extends to metaphysis | | Distal radius | ~10–15% | Epiphyseal, often volar-sided | | Proximal humerus | ~5–8% | Rare, usually in older patients | | Distal tibia | ~5% | Uncommon | **High-Yield:** The mnemonic **"KNEECAP"** captures the distribution: **K**nee (most common), **N**ear epiphysis (always), **E**ccentric location, **C**ommonly 20–40 years old, **A**fter physeal closure, **P**redominantly around large joints. ### Characteristic Features 1. **Location:** Epiphyseal origin, extending into the metaphysis 2. **Age:** 20–40 years (after physeal closure) 3. **Gender:** Slight female predominance 4. **Presentation:** Painless swelling, joint effusion, limited range of motion **Clinical Pearl:** GCT is one of the few bone tumors that arises in the epiphysis and extends across the joint line into the metaphysis — this is pathognomonic. The distal femur is the single most common site, followed closely by the proximal tibia. **Warning:** Do not confuse GCT with osteosarcoma (metaphyseal, younger patients) or chondroblastoma (smaller, more painful, younger patients <20 years). [cite:Robbins 10e Ch 26]