A 28-year-old woman presents with a 6-month history of progressive left knee pain and mild swelling. She reports no history of trauma. On examination, there is mild effusion and restricted range of motion. X-ray shows a lytic lesion in the distal femur extending to the articular surface, with a thin sclerotic rim. MRI reveals a heterogeneous mass with areas of hemorrhage and hemosiderin deposition. The lesion is eccentrically located in the metaphyseal-diaphyseal region. What is the most likely diagnosis?

Explanation

## Diagnosis: Giant Cell Tumor of Bone ### Clinical Presentation **Key Point:** Giant cell tumor (GCT) classically presents in skeletally mature patients (20–40 years) with pain and swelling around the knee joint. ### Radiological Features | Feature | GCT | Osteosarcoma | Chondroblastoma | ABC | | --- | --- | --- | --- | --- | | **Location** | Epiphyseal-metaphyseal, extends to articular surface | Metaphyseal, rarely extends to joint | Epiphyseal | Metaphyseal | | **Age** | 20–40 years | 10–25 years | 10–20 years | 10–30 years | | **Appearance** | Lytic with sclerotic rim | Mixed lytic-sclerotic, aggressive | Lytic, well-defined | Lytic, multilocular | | **Margin** | Well-defined | Ill-defined, sunburst | Well-defined | Well-defined | **High-Yield:** The **eccentric location extending to the articular surface in a skeletally mature patient** is pathognomonic for GCT. The thin sclerotic rim and lytic appearance confirm the diagnosis. ### Pathology **Key Point:** GCT is composed of uniform mononuclear stromal cells with abundant multinucleated giant cells (osteoclast-like). Despite benign histology, it is locally aggressive and has recurrence potential (10–65% depending on treatment). ### MRI Findings - Heterogeneous signal with areas of hemorrhage and hemosiderin (brown tumor appearance) - Fluid-fluid levels may be present - Soft tissue extension indicates aggressive behavior ### Management 1. Intralesional curettage with adjuvants (phenol, liquid nitrogen, or hydrogen peroxide) 2. Bone grafting or cement packing 3. Wide excision reserved for recurrent or aggressive lesions 4. Denosumab (RANKL inhibitor) for unresectable or metastatic disease **Clinical Pearl:** GCT has a propensity for **pulmonary metastases** (1–5%), which may appear years after primary treatment. Chest imaging surveillance is recommended. **Warning:** Do NOT confuse GCT with chondroblastoma (which is epiphyseal and occurs in younger patients) or ABC (which is typically metaphyseal and multilocular).