Malignant giant cell tumor of bone: possibilities of diagnosis and monitoring of the process when using the denosumab

Introduction . Giant cell tumor (GCT) of bone is a relatively rare neoplasm. Its malignant transformation in the form of sarcomatous changes occurs in 8–10 % of cases. The literature lacks data on radiological signs and differential diagnostic criteria of primary malignant GCT of bone. Clear guidelines on the density range of the affected area for differential diagnosis are also absent. Aim. To analyze dynamics of tumor density changes in patients with malignant GCT of bone using computed tomography densitometry. Materials and methods . Ten patients (mean age 45.8 ± 4.2 years, range 28–58 years) with malignant GCT of bone were examined. All underwent treatment with denosumab with subsequent surgical intervention. Bone changes were assessed visually. Densitometric analysis was performed in accordance with a method developed by us involving determination of tumor density in Housefield units (HU) and relative tumor density index (k) (ratio between tumor density and density of the unaffected limb at the same level). Results . After 6 injections of denosumab, a clear sclerotic rim appeared at the tumor periphery containing thin lines of ossification. Analysis of densitometric parameters showed that tumor density after denosumab administration increased from 46.3 (median (Мe) 35.0; interquartile range (IQR) 51.4) HU to 88.8 (Мe 84.8; IQR 118.4) HU, while relative tumor density index increased from 0.33 (Me 0.22; IQR 0.45) to 0.68 (Me 0.62; IQR 0.89). Earlier we have published threshold values of these parameters (≥139.5 HU and ≥0.97, respectively) at which probability of residual tumor is lowered (sensitivity 81.8 %, specificity 96.6 %). Taking into account these values, 9 of 10 patients required continuation of therapy. Conclusion . Computed tomography densitometry allows to evaluate changes caused by denosumab therapy in patients with malignant GCT of bone. According to radiological data, progression of this disease has unique features. Taking into account histological types of malignant GCT, longer periods of observation and denosumab administration or changes in treatment tactics at the preoperative stage can be considered, as well as potential use of this drug in other primary bone tumors.