Which histological cell type is the hallmark of giant cell tumor of bone?

Explanation

## Histopathology of Giant Cell Tumor **Key Point:** The diagnostic hallmark of GCT is the presence of **abundant multinucleated osteoclast-like giant cells** mixed with mononuclear stromal cells. ### Microscopic Features ### Cellular Composition | Cell Type | Characteristics | Function | | --- | --- | --- | | Multinucleated giant cells | 20–100+ nuclei, osteoclast-like, abundant cytoplasm | Bone resorption; give tumor its name | | Mononuclear stromal cells | Spindle to oval, moderate atypia | Neoplastic component; produce RANKL | | Hemosiderin-laden macrophages | Brown pigment deposits | Reactive response to hemorrhage | **High-Yield:** The **osteoclast-like giant cells are NOT neoplastic**—they are reactive cells recruited and activated by the neoplastic mononuclear stromal cells via RANKL (receptor activator of nuclear factor kappa-B ligand) signaling. ### Immunohistochemistry Clue - **RANKL-positive mononuclear cells** → confirm the stromal cells are the neoplastic component - Giant cells are **TRAP-positive** (tartrate-resistant acid phosphatase) → osteoclast marker **Mnemonic:** **GCT = Giant cells + Stromal cells** - **G**iant cells (osteoclast-like, reactive) - **C**ells (mononuclear stromal, neoplastic) - **T**umor (benign but locally aggressive) ### Why This Matters The dual-cell nature explains: 1. **Aggressive local behavior** → stromal cells produce RANKL → recruit and activate giant cells → bone resorption 2. **Recurrence risk** → if stromal cells are not completely removed, recurrence occurs 3. **Rare malignant transformation** → the stromal component, not giant cells, undergoes sarcomatous change **Clinical Pearl:** Seeing abundant giant cells alone is NOT diagnostic—you must also see the mononuclear stromal component. Giant cells alone could suggest other conditions (tuberculosis, sarcoidosis).