A 28-year-old woman presents with a 6-month history of progressive pain and swelling over the right knee. On examination, there is mild effusion and limited range of motion. X-ray reveals a lytic lesion in the distal femur extending to the articular surface with a well-defined margin. MRI shows a heterogeneous lesion with areas of hemosiderin deposition. The lesion is eccentric and located in the epiphyseal-metaphyseal region. What is the most likely diagnosis?

Explanation

## Giant Cell Tumor of Bone: Clinical Recognition ### Key Clinical Features **Key Point:** Giant cell tumor (GCT) is a benign but locally aggressive lesion with a characteristic presentation in skeletally mature patients. ### Diagnostic Criteria | Feature | GCT | Chondroblastoma | ABC | |---------|-----|-----------------|-----| | **Age** | 20–40 years (post-fusion) | <25 years (open physes) | Any age | | **Location** | Epiphyseal-metaphyseal, extends to articular surface | Epiphyseal, intra-articular | Meta-diaphyseal, eccentric | | **Margin** | Well-defined, lytic | Well-defined, sclerotic rim | Expansile, fluid levels | | **Hemosiderin** | Present on MRI | Absent | May be present | | **Growth** | Slow, aggressive locally | Slow | Rapid | ### Why This Case is GCT 1. **Age 28** — post-physeal closure (GCT occurs after skeletal maturity; chondroblastoma occurs before) 2. **Epiphyseal-metaphyseal location extending to articular surface** — pathognomonic for GCT 3. **Hemosiderin deposition on MRI** — indicates repeated microhemorrhages from vascular granulation tissue, characteristic of GCT 4. **Well-defined lytic lesion** — typical radiographic appearance **High-Yield:** GCT = epiphyseal lesion in a 20–40-year-old that reaches the joint surface. Chondroblastoma = epiphyseal lesion in a teenager with open growth plates. ### Pathology Hallmark **Clinical Pearl:** Histologically, GCT contains numerous multinucleated giant cells (osteoclast-like) admixed with mononuclear stromal cells. The giant cells are reactive, not neoplastic — the stromal cells are the neoplastic component. ### Management Implications - Intralesional curettage + bone graft or cement - Adjuvant therapy: denosumab (RANKL inhibitor) for aggressive/recurrent cases - Recurrence rate: 10–65% depending on treatment and grade [cite:Rockwood & Green's Fractures in Adults Ch 33]