Morphologic evaluation of the effect of denosumab on giant cell tumors of bone and a new grading scheme

Giant cell tumor (GCT) is a rare, usually benign but locally aggressive neoplasm. Recent studies suggest new approaches in light of the elucidation of molecular pathways in bone. The osteolytic nature of GCT is caused by the receptor for activating nuclear factor-kB ligand (RANKL) associated osteoclasts. Denosumab is a monoclonal antibody that affects GCT through RANKL and it prevents normal and neoplastic osteolysis. The aim of this study is to evaluate the histopathologic alterations due to denosumab treatment and the efficiency of this drug in GCT therapy. Ten patients had been treated with denosumab and were included in the study. Pretreatment biopsies were interpreted as conventional GCTs and posttreatment biopsies of the ten patients' GCTs were classified in accordance with the grading system. Only one patient had tumor remaining after treatment. There is limited data on histopathologic alterations that follow denosumab treatment. The bone pathologist should keep these changes in mind because they mimic different types of bone tumors. Furthermore, there is no widely accepted grading system to evaluate the effect of denosumab in GCT. Our study suggested a scheme that would be helpful to evaluate the efficiency of denosumab treatment in GCT.