A 35-year-old woman undergoes imaging for a painless bony swelling in the proximal femur. Radiographs reveal a solitary, well-demarcated lytic lesion with a thin sclerotic rim, located in the epiphyseal-metaphyseal region. No cortical breakthrough or soft tissue mass is seen. What is the most common benign bone tumor in this location and age group?

Explanation

## Most Common Benign Bone Tumor in Skeletally Mature Adults **Key Point:** Giant cell tumor (GCT) of bone is the most common benign bone tumor that occurs in the epiphyseal-metaphyseal region of long bones in skeletally mature patients (20–40 years). ### Epidemiology & Location - **Peak incidence:** 20–40 years (after physeal closure) - **Most common sites:** Distal femur (20%), proximal tibia (15%), distal radius (10%), proximal humerus - **Anatomical predilection:** Epiphyseal-metaphyseal region, extending to the articular surface - **Frequency:** Most common benign bone tumor in this age group and location ### Imaging Characteristics | Feature | Giant Cell Tumor | |---------|------------------| | **Pattern** | Lytic (radiolucent) | | **Margins** | Well-demarcated, thin sclerotic rim | | **Location** | Epiphyseal-metaphyseal, extends to articular surface | | **Cortical involvement** | Cortical breakthrough possible | | **Soft tissue mass** | Minimal unless aggressive | | **CT/MRI** | Heterogeneous signal, areas of hemorrhage | ### Pathophysiology - Benign but locally aggressive tumor composed of stromal cells and multinucleated giant cells - Occurs after physeal closure (distinguishes it from other benign tumors) - ~10% recurrence rate if not completely excised - Rare malignant transformation (<5%) **High-Yield:** The classic presentation is an **epiphyseal-metaphyseal lytic lesion in a 20–40 year-old patient after physeal closure**. **Mnemonic:** **GELATINOUS** = Giant cell tumor, Epiphyseal-metaphyseal, Lytic, After physeal closure, Thin sclerotic rim, In young adults, Nodular appearance on MRI, Osteoclast-rich histology, Uses (surgical excision), Sclerotic margin **Clinical Pearl:** GCT is one of the few benign bone tumors that extends to the articular surface; this epiphyseal involvement is a key distinguishing feature from other benign lesions. ### Differential Diagnosis - **Enchondroma:** Metaphyseal or diaphyseal, calcified chondroid matrix, no epiphyseal involvement - **Osteochondroma:** Metaphyseal, cartilage cap, exostotic growth, no epiphyseal involvement - **Osteoid osteoma:** Small (<1 cm), central nidus with surrounding sclerosis, painful