A 28-year-old woman presents to the orthopedic clinic with a 6-month history of progressive pain and swelling over the right knee. On examination, there is localized warmth and mild effusion. Plain radiographs reveal a lytic lesion in the distal femur, eccentrically located in the metaphyseal-epiphyseal region, extending to within 1 cm of the articular surface. The lesion shows a thin sclerotic margin with a "soap bubble" appearance. MRI confirms a well-circumscribed mass with mixed signal intensity and hemosiderin deposits. What is the most likely diagnosis?

Explanation

## Diagnosis: Giant Cell Tumor of Bone ### Clinical Presentation **Key Point:** Giant cell tumor (GCT) is a benign but locally aggressive tumor that typically presents in skeletally mature patients (20–40 years) with pain and swelling over the affected joint. ### Radiological Features — Pathognomonic Pattern | Feature | GCT | Chondroblastoma | ABC | |---------|-----|-----------------|-----| | **Location** | Epiphyseal-metaphyseal, eccentric | Epiphyseal, central | Metaphyseal | | **Age** | 20–40 years (post-fusion) | 10–20 years (pre-fusion) | Any age | | **Margin** | Thin sclerotic rim | Well-defined, sclerotic | Ill-defined | | **Pattern** | Soap bubble / honeycomb | Stippled calcification | Fluid levels | | **Extension** | To subarticular cortex | Intra-articular rare | Eccentric, metaphyseal | **High-Yield:** The **epiphyseal-metaphyseal location extending to within 1 cm of the articular surface** in a 28-year-old is virtually diagnostic of GCT. The "soap bubble" lytic pattern is classic. ### Histopathology (Confirmatory) - Numerous multinucleated giant cells (osteoclast-like) - Mononuclear stromal cells with hemosiderin - Absence of a fibrous capsule → locally aggressive ### Clinical Pearl **Key Point:** GCT is one of the few benign tumors that can cross the articular surface and cause intra-articular extension, leading to joint involvement and functional impairment. ### Management Principles 1. **Surgical excision** — curettage with adjuvant (phenol, cryotherapy, or cement) 2. **Recurrence rate** — 10–65% depending on technique; adjuvants reduce this 3. **Malignant transformation** — rare (<5%), but possible; monitor long-term **Mnemonic:** **GCTBONE** — Giant Cell Tumor: **B**enign, **O**stoclast-like cells, **N**ear epiphysis, **E**ccentric location. ### Why This Patient? - Age 28 (post-skeletal maturity) - Distal femur (common site: distal femur > proximal tibia > distal radius) - Epiphyseal-metaphyseal, eccentric - Soap bubble appearance - Hemosiderin on MRI (from prior microhemorrhage) [cite:Robbins 10e Ch 28]