Malignancy in giant cell tumor of bone: analysis of an open-label phase 2 study of denosumab

BackgroundGiant cell tumor of bone (GCTB) is a rare osteoclastogenic stromal tumor. GCTB can rarely undergo malignant transformation. This post hoc analysis evaluated and classified malignancies in patients with GCTB who received denosumab.MethodsThis analysis was conducted on patients with pathologically confirmed GCTB and measurable active disease treated with denosumab 120mg subcutaneously once every 4weeks, with loading doses on study days 8 and 15, as part of a phase 2, open-label, multicenter study. We identified potential cases of malignancy related to GCTB through an independent multidisciplinary review or medical history, associated imaging or histopathologic reports, and disease course. The findings were summarized and no statistical analysis was performed.ResultsTwenty of five hundred twenty-six patients (3.8%) who received at least one dose of denosumab were misdiagnosed with GCTB that was later discovered to be malignancies: five primary malignant GCTB, five secondary malignant GCTB, four sarcomatous transformations, and six patients with other malignancies (giant cell-rich osteosarcoma, undifferentiated pleomorphic sarcoma, spindle cell sarcoma, osteogenic sarcoma, phosphaturic mesenchymal tumor of mixed connective tissue type, and fibrosarcoma/malignant fibrous histiocytoma). Many malignancies were present before denosumab was initiated (8 definitive cases, 7 likely cases), excluding potential involvement of denosumab in these cases. Signs associated with potential misdiagnoses of GCTB included poor mineralization with denosumab treatment, rapid relapse in pain, or a failure of the typical dramatic improvement in pain normally observed with denosumab.ConclusionsAlthough rare, GCTB can undergo malignant transformation, and rates in this study were consistent with previous reports. Signs of poor mineralization or lack of response to denosumab treatment may warrant close monitoring.Trial registrationclinicaltrials.gov, (NCT00680992). Registered May 20, 2008.

Automated summary

GCTB is a rare bone tumor that can rarely transform into malignancies. A post hoc analysis found that 3.8% of patients misdiagnosed with GCTB were actually malignancies, including primary and secondary malignant GCTB, sarcomatous transformations, and other types of malignancies. Signs of poor mineralization or lack of response to denosumab treatment may indicate potential misdiagnoses and warrant close monitoring.