Introduction

Tumor-induced osteomalacia (TIO) is a rare acquired paraneoplastic syndrome caused by excessive secretion of fibroblast growth factor 23 (FGF-23) by phosphaturic mesenchymal tumors (PMTs). Excess FGF-23 reduces renal phosphate reabsorption and suppresses 1,25-dihydroxyvitamin D synthesis, leading to renal phosphate wasting, hypophosphatemia, and impaired bone mineralization. Patients typically present with progressive bone pain, proximal muscle weakness, fatigue, reduced mobility, and recurrent insufficiency or fragility fractures [1].

Advances in functional imaging, particularly somatostatin receptor-based positron emission tomography/computed tomography (PET/CT), have substantially improved tumor localization. Nevertheless, a proportion of tumors remain occult or arise in anatomically challenging locations that preclude complete surgical resection [2]. Complete surgical excision is the treatment of choice and is curative in most cases; however, recurrence occurs in approximately 7-16% of patients, usually within the first three years after surgery [3,4]. When the tumor is unresectable or cannot be localized, conventional treatment with oral phosphate and active vitamin D analogues is often limited by poor tolerability and long-term complications, including secondary hyperparathyroidism, hypercalciuria, and nephrocalcinosis [5,6].

Burosumab, a fully human monoclonal antibody targeting FGF-23, directly addresses the underlying pathophysiology of TIO by restoring renal phosphate reabsorption and increasing circulating 1,25-dihydroxyvitamin D concentrations [7]. Clinical studies have demonstrated sustained improvements in serum phosphate levels, fracture healing, physical function, pain, and health-related quality of life in adults with unresectable or nonlocalizable TIO, supporting its approval for this indication [8]. However, because TIO is exceptionally rare, evidence regarding the long-term effectiveness of burosumab remains limited, particularly in patients with recurrent unresectable disease following previous surgical resection. We report a case of recurrent unresectable TIO successfully treated with burosumab, resulting in sustained biochemical correction and marked clinical improvement. This case adds to the limited evidence supporting its long-term use in this challenging clinical setting.