Recurrent Tumor-Induced Osteomalacia After Surgical Resection: A Case Report Demonstrating Successful Treatment With Burosumab.

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome in which a mesenchymal tumor secretes fibroblast growth factor 23 (FGF-23), causing renal phosphate wasting and osteomalacia. A 62-year-old woman was referred to our hospital with a two-year history of progressive muscle weakness and generalized bone pain. The patient was nonambulatory and required a wheelchair for mobility. Laboratory evaluation revealed hypophosphatemia and low serum 1,25-dihydroxyvitamin D levels. Further investigation of the hypophosphatemia demonstrated an elevated fractional excretion of phosphate (FeP) and increased serum FGF-23 levels. Genetic testing was negative. A 68Ga-DOTANOC PET/CT scan identified an infiltrative cervical lesion with increased somatostatin receptor expression. The tumor was surgically resected, after which the patient recovered functional ambulation, and serum phosphate normalized. One year later, the patient reported recurrence of musculoskeletal symptoms. Laboratory findings showed hypophosphatemia with an elevated FeP, raising suspicion of tumor recurrence. A repeat 68Ga-DOTANOC PET/CT scan demonstrated increased somatostatin receptor expression in a right parietal calvarial lesion that was not amenable to surgical resection or radiotherapy. Initiation of burosumab therapy led to the normalization of serum phosphate levels and complete resolution of symptoms. TIO is a rare disorder that is frequently misdiagnosed as osteoporosis, resulting in substantial diagnostic delay. A systematic evaluation of hypophosphatemia is essential for timely diagnosis and appropriate management. This case supports burosumab as an effective therapeutic option for achieving biochemical and clinical improvement in patients with unresectable TIO.