Explanation:
In a Giant Cell Tumor of bone, the true neoplastic cells are the spindle-shaped mononuclear stromal cells. These cells excessively express Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). The RANKL recruits and activates normal host monocytes, causing them to fuse into the reactive, non-neoplastic multinucleated giant cells (osteoclast-like cells) that cause the massive local bone resorption. This is the rationale for using Denosumab (a RANKL inhibitor) in unresectable GCTs. Reference: Miller's Review of Orthopaedics, 8th Edition.