A 28-year-old woman presents with a 6-month history of progressive swelling and pain in the right knee. On examination, there is mild effusion and limited range of motion. Plain radiographs show a well-defined lytic lesion in the distal femur extending to the articular surface, with a thin sclerotic rim. MRI reveals a heterogeneous mass with hemosiderin deposits. She is otherwise healthy with no systemic symptoms. What is the most likely diagnosis?

## Diagnosis: Giant Cell Tumor of Bone ### Clinical Presentation **Key Point:** Giant cell tumor (GCT) is a benign but locally aggressive neoplasm that typically presents in young adults (20–40 years) with pain and swelling around the knee joint. ### Radiological Features | Feature | Giant Cell Tumor | Osteosarcoma | Chondroblastoma | ABC | |---------|------------------|--------------|-----------------|-----| | **Age** | 20–40 years | 10–25 years | 10–20 years | 10–30 years | | **Location** | Epiphyseal/metaphyseal, extends to articular surface | Metaphyseal, around knee | Epiphyseal | Metaphyseal | | **Appearance** | Lytic, well-defined, thin sclerotic rim | Mixed lytic/sclerotic, aggressive | Lytic with calcification | Multilocular, fluid levels | | **Hemosiderin** | Present (MRI) | Absent | Absent | May be present | **High-Yield:** GCT characteristically arises in the **epiphysis and extends to the articular surface** — a hallmark distinguishing it from other bone tumors. The thin sclerotic margin indicates a benign lesion with slow growth. ### Pathology & Microscopy **Key Point:** Histologically, GCT shows numerous multinucleated giant cells (osteoclast-like) in a background of mononuclear stromal cells and hemosiderin-laden macrophages [cite:Robbins 10e Ch 26]. ### Treatment Algorithm ```mermaid flowchart TD A[Giant Cell Tumor diagnosed]:::outcome --> B{Surgical accessibility?}:::decision B -->|Yes, resectable| C[Curettage + bone graft/cement]:::action B -->|No, or recurrent| D[Wide excision]:::action C --> E{Recurrence risk?}:::decision E -->|High| F[Adjuvant: denosumab or radiotherapy]:::action E -->|Low| G[Observe]:::action D --> H[Low recurrence]:::outcome ``` **Clinical Pearl:** Denosumab (RANKL inhibitor) is increasingly used as neoadjuvant or adjuvant therapy to reduce recurrence, especially in recurrent or unresectable cases [cite:Harrison 21e Ch 397]. ### Why This Case Fits GCT 1. **Age & sex:** 28-year-old woman (peak incidence 20–40 years) 2. **Location:** Distal femur extending to articular surface (epiphyseal) 3. **Imaging:** Lytic, well-defined, sclerotic rim, hemosiderin on MRI 4. **Benign course:** No systemic symptoms, slow growth over 6 months