A 28-year-old woman presents with a 6-month history of progressive swelling and mild pain over the lateral aspect of her right knee. On examination, there is a firm, non-tender mass palpable just below the knee joint. X-rays reveal a lytic lesion in the distal femur, eccentrically located, extending to within 1 cm of the articular cartilage. The lesion shows a soap-bubble appearance with a well-defined margin. MRI demonstrates a heterogeneous signal with areas of hemosiderin deposition. A core needle biopsy shows numerous multinucleated giant cells admixed with mononuclear stromal cells and hemosiderin-laden macrophages. What is the most likely diagnosis?

Explanation

## Diagnosis: Giant Cell Tumor of Bone ### Clinical Presentation This case presents the classic features of giant cell tumor (GCT): - **Age**: 20–40 years (peak incidence in 3rd decade) - **Location**: Distal femur, epiphyseal-metaphyseal region, extending to subarticular zone - **Presentation**: Insidious onset with swelling and mild pain ### Radiological Features | Feature | Giant Cell Tumor | Aneurysmal Bone Cyst | Chondroblastoma | |---------|------------------|----------------------|------------------| | **Appearance** | Soap-bubble or honeycomb lytic lesion | Multilocular cystic with fluid levels | Well-defined, sclerotic margin | | **Location** | Epiphyseal-metaphyseal, extends to articular surface | Metaphyseal, eccentric | Epiphyseal, intra-articular | | **Margin** | Well-defined | Ill-defined, expansile | Well-defined, sclerotic | | **Signal on MRI** | Heterogeneous with hemosiderin | Cystic with fluid levels | Homogeneous | ### Histopathology **Key Point:** The hallmark finding is numerous multinucleated giant cells (osteoclast-like) interspersed with mononuclear stromal cells and hemosiderin-laden macrophages. The stromal cells, not the giant cells, are the neoplastic component. ### High-Yield Facts **High-Yield:** GCT is a benign but locally aggressive tumor with: - 50% recurrence rate if curettage alone is performed - 10–15% risk of malignant transformation (usually after radiation) - Pulmonary metastases possible (rare, usually benign) - Treatment: Wide excision or curettage with adjuvants (phenol, liquid nitrogen, or cement) **Clinical Pearl:** The "soap-bubble" or "honeycomb" appearance on X-ray is pathognomonic for GCT, though not all lesions show this pattern. The key distinguishing feature is the epiphyseal location extending to the subarticular zone in a skeletally mature patient. ### Differential Diagnosis Exclusion - **Aneurysmal bone cyst**: Multilocular cystic lesion with fluid levels on MRI; lacks the characteristic stromal cells and giant cells on histology - **Chondroblastoma**: Typically smaller, purely epiphyseal, with sclerotic margins; histology shows chondroid matrix and calcifications - **Osteoclastoma**: This is an obsolete term sometimes used for GCT, but not a separate diagnosis **Mnemonic: GCTB** — **G**iant cells (osteoclast-like), **C**ontains stromal cells (neoplastic), **T**uberous (benign but aggressive), **B**enign with malignant potential