Burosumab treatment of tumour-induced osteomalacia from a rib lesion

Tumour-induced osteomalacia (TIO) is a rare cause of acquired hypophosphataemia most often due to paraneoplastic overproduction of fibroblast growth factor-23 (FGF-23). A woman in her 60s presented with an 18 month history of insidious back, knee and ankle pain, and progressive weakness. Biochemistry revealed low serum phosphate and a bone scan showed diffuse bony lesions, initially suspected to be metastatic sarcoma given her history of uterine sarcoma. However, a positron emission tomography scan identified a solitary destructive lytic lesion in the right posterior second rib. Endocrine workup showed low serum phosphate, high fractional excretion of phosphate, normal 25-hydroxyvitamin D and elevated FGF-23 levels. A biopsy revealed spindle cell neoplasm, which later stained positive for FGF receptor 1, consistent with renal phosphate wasting secondary to TIO. The patient was started on burosumab, a human monoclonal antibody against FGF-23, which normalised her serum phosphate and led to marked clinical improvement. She is now awaiting curative surgery for the primary rib lesion.