An eggshell-like mineralized recurrent lesion in the popliteal region after treatment of giant cell tumor of the bone with denosumab.

We report a case of recurrent giant cell tumor of the bone (GCTB) in which treatment with denosumab gradually enhanced the eggshell-like mineralization at the periphery of the tumor. A 28-year-old male presented with a mass on his left distal femur that had enlarged within the past few months. Before curettage, GCTB of the distal femur was diagnosed based on histological analysis of a biopsy specimen; the tumor consisted of a proliferation of ovoid mononuclear stromal cells with evenly scattered multinucleated osteoclast-like giant cells. The tumor recurred three times after the initial diagnosis; at the time of the third relapse, the patient underwent en bloc resection and reconstruction with a knee joint prosthesis. He was also treated with denosumab postoperatively because some studies have recently shown the benefits of the receptor activator of nuclear factor kappa-B ligand (RANKL) inhibitor denosumab as adjuvant therapy in patients with GCTB. Six months after starting adjuvant treatment with denosumab, radiography revealed a mineralized nodule >2 cm in diameter at the popliteal region; this lesion was considered a soft tissue recurrence of GCTB. Treatment with denosumab was continued for another 1.5 years, and the lesion was resected. Histological examination showed residual mononuclear stromal cells expressing RANKL without multinucleated giant cells surrounded by the peripheral mineralization. The patient was successfully treated by complete resection with the support of adjuvant treatment with denosumab.