Response of aneurysmal bone cyst to denosumab

Study Design. Case report and literature review. Objective. To describe a unique case of large sacral aneurysmal bone cysts (ABCs) treated with denosumab and review the literature on this rare entity. Summary and Background Data. ABCs are expansile osteolytic lesions that typically contain blood-filled spaces separated by fibrous septa. Standard treatment includes surgical resection or curettage and packing; however, for some spinal lesions, the standard approach is not optimal. One therapeutic strategy is to treat spinal ABC with an agent that targets a pathway that is dysregulated in a disease with similar pathophysiology. The bone destruction in both giant cell tumors of bone and ABCs is mediated by RANK ligand (RANKL) produced by the tumor cells. Denosumab, a human monoclonal antibody to RANKL, is effective in the treatment of giant cell tumors of bone. Methods. We report a case of a large sacral ABC that responded to denosumab. A 27-year-old male developed increasing back pain. Imaging revealed a lytic lesion in the sacrum with no clear solid component and regions where the cortex was difficult to identify. ABC was diagnosed on biopsy. Surgery or radiation treatment was expected to be associated with serious morbidity; therefore, denosumab was given using the regimen for giant cell tumors of bone (120mg monthly with a loading dose). Results. The patient's pain gradually resolved after 2 months of treatment. New bone formation with a more clearly defined cortex was evident on computed tomographic scan at 16 weeks and continued to show evidence of improvement at 7 and 12 months. Biopsy at 12 months revealed a hypocellular fibrous stroma with new bone formation and no giant cells. Conclusion. We conclude that denosumab can result in symptomatic and radiological improvement in ABC and may be useful in select cases.