A 28-year-old woman presents with a 6-month history of progressive swelling and mild pain over the distal femur just above the knee. On examination, there is a palpable bony swelling with mild warmth and restricted knee flexion. X-ray shows a lytic lesion in the distal femur extending to the articular surface, with a soap-bubble appearance. The lesion is eccentrically located and measures 4 cm × 3 cm. MRI reveals a heterogeneous mass with areas of hemorrhage and hemosiderin deposition. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Giant Cell Tumor of Bone ### Key Diagnostic Features **Key Point:** Giant cell tumor (GCT) is a benign but locally aggressive tumor with a characteristic presentation in skeletally mature patients, typically occurring around the knee joint. ### Clinical Presentation Match | Feature | This Case | GCT Typical | | --- | --- | --- | | Age at presentation | 28 years | 20–40 years (post-fusion) | | Location | Distal femur, epiphyseal | Distal femur/proximal tibia (50–65%) | | Extent to articular surface | Yes, extends to joint | Characteristic—epiphyseal extension | | Lesion pattern | Lytic, soap-bubble | Lytic with soap-bubble/honeycomb | | Eccentricity | Eccentric | Typically eccentric | | Size | 4 × 3 cm | Variable, often 2–8 cm | | Associated symptoms | Swelling, mild pain, restricted ROM | Common; swelling and functional loss | ### Imaging Hallmarks **High-Yield:** The **soap-bubble or honeycomb appearance** on X-ray, combined with **epiphyseal location extending to the articular surface** in a young adult, is pathognomonic for GCT. **Clinical Pearl:** Unlike osteosarcoma (which is metaphyseal and occurs in younger patients with aggressive pain), GCT is epiphyseal, eccentrically located, and presents with slower progression. ### Histopathology (Confirmatory) 1. **Multinucleated giant cells** (osteoclast-like) — numerous and prominent 2. **Stromal mononuclear cells** — spindle-shaped, fibroblast-like 3. **Hemosiderin deposition** — from repeated hemorrhage and necrosis 4. **Mitotic activity** — variable; high mitotic rate does NOT indicate malignancy in GCT **Mnemonic: GIGA** — **G**iant cells, **I**nterstitial hemosiderin, **G**rowth in epiphysis, **A**ggressive but benign. ### Differential Considerations - ~~Osteosarcoma~~ — metaphyseal, younger age (10–20), more aggressive pain, sclerotic margins - ~~Chondroblastoma~~ — smaller, more painful, younger age (< 25), central location - ~~Aneurysmal bone cyst~~ — metaphyseal or metadiaphyseal, rapid growth, younger age ### Management Principles **Key Point:** Treatment is surgical—curettage with or without adjuvants (phenol, liquid nitrogen, bone cement) to reduce recurrence risk (20–50% without adjuvants). [cite:Robbins and Cotran 10e Ch 26]