Development of high-grade osteosarcoma in a patient with recurrent giant cell tumor of the ischium while receiving treatment with denosumab.

Malignant transformation of giant cell tumor of bone (GCTB) without radiotherapy exposure is exceptionally rare, occurring in less than 1% of GCTBs. The safety and efficacy of denosumab in patients with GCTB was recently reported. We herein report a case of a benign recurrent GCTB with an H3F3A mutation that underwent secondary malignant transformation during treatment with denosumab. A 29-year-old woman underwent curettage of a GCTB of the left ischium in 2005. Ten years after the first surgery, the GCTB recurred locally. We started treatment with denosumab. During the first 5 months of treatment, we observed a demarcated area of osteosclerosis in the recurrent lesion, and the patient's clinical condition improved. At 6 months, however, the patient developed pain, and a rapidly growing mass was detected by computed tomography. An incisional biopsy was performed. Histologic analysis showed a high-grade osteosarcoma. The patient developed lung metastases and died soon after beginning chemotherapy. The mechanism of sarcomatous transformation of GCTB during denosumab therapy is unclear. These findings suggest that the scientific community should be aware of the possible malignant transformation of GCTB during denosumab treatment.