Which histological cell type is the primary neoplastic component of giant cell tumor of bone?

Explanation

## Histology of Giant Cell Tumor **Key Point:** Giant cell tumor of bone is a neoplasm of *mononuclear stromal cells* (spindle cells), NOT the osteoclast-like giant cells. The giant cells are reactive and derived from monocyte-macrophage precursors, not the neoplastic component. ### Histological Composition | Component | Origin | Role | | --- | --- | --- | | **Mononuclear stromal cells** | Neoplastic | Proliferating spindle cells; true tumor cells | | **Osteoclast-like giant cells** | Reactive/inflammatory | Derived from monocyte-macrophage lineage; respond to RANKL from stromal cells | | **Hemosiderin-laden macrophages** | Reactive | Accumulate from hemorrhage and necrosis | **High-Yield:** The mononuclear stromal cells express high levels of *RANKL (receptor activator of nuclear factor kappa-B ligand)*, which recruits and activates osteoclast precursors to form the characteristic multinucleated giant cells. This RANKL-driven mechanism is the basis for denosumab therapy (anti-RANKL monoclonal antibody). ### Molecular Basis 1. **Neoplastic cells:** Mononuclear spindle cells (express RANKL) 2. **Giant cells:** Fusion of osteoclast precursors (reactive, not neoplastic) 3. **Result:** Characteristic histology with numerous multinucleated giant cells mimicking osteoclasts **Clinical Pearl:** Denosumab (anti-RANKL) can shrink GCT by blocking osteoclast recruitment, providing a non-surgical option for unresectable or recurrent tumors. This confirms that RANKL-producing stromal cells, not giant cells, are the therapeutic target. **Mnemonic:** **RANK**L = **R**eceptor **A**ctivator of **N**uclear factor **K**appa-B Ligand — produced by the neoplastic stromal cells to recruit giant cells.