Two Cases of Tumor-induced Osteomalacia Resulting in Surgical Resection during Burosumab Therapy.

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome that results from tumors that secrete fibroblast growth factor 23 (FGF23). This leads to chronic hypophosphatemia. Burosumab, an anti-FGF23 antibody, is an effective treatment when surgery is not possible; however, it complicates FGF23 measurements and postoperative monitoring. We describe the first case report of TIO in which the responsible tumors were discovered during burosumab therapy and successfully resected. Despite tumor removal, the serum FGF23 levels remained elevated. In the postoperative monitoring of patients treated with burosumab, physicians should focus on the serum and urine levels of phosphate rather than FGF23.