Progression of phosphaturic mesenchymal tumor in the femoral bone during treatment with burosumab: A case report

Phosphaturic mesenchymal tumors cause tumor-induced osteomalacia via fibroblast growth factor 23 (FGF23) production. Burosumab, an anti-FGF23 antibody, has recently become available to relieve clinical symptoms and improve hypophosphatemia. The effect of burosumab on tumor growth has not been elucidated. Here, we report the case of a 61-year-old woman with a phosphaturic mesenchymal tumors in the femur treated with burosumab. During treatment with burosumab, tumor progression was observed, although the clinical symptoms and hypophosphatemia improved. Therefore, surgical resection of the tumor was performed, and tumor-induced osteomalacia went into remission. To our knowledge, there have been limited reports about the effect of burosumab on tumor progression, and our data highlights the necessity for periodical assessment of tumor progression during burosumab treatment.