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Tumor-induced osteomalacia (TIO) is a very rare paraneoplastic syndrome most commonly caused by either soft tissue tumors or bone mesenchymal tumors capable of producing Fibroblast Growth Factor-23 (FGF23) and possibly other phosphatonins.1 Patients with TIO typically present with osteomalacia-related symptoms including weakness, gait disturbances, pain, and fractures. Persistent hypophosphatemia due to renal phosphate wasting, inappropriately normal or low 1,25 (OH)2D and high or inappropriately normal FGF23 are the biochemical diagnostic hallmarks.1 These severe hormonal disturbances and progressive physical deterioration and disability leading to a lack of mechanical stimulus, converge to generate an extremely harmful combination for bone microstructure. Thus, TIO can have a catastrophic impact on the patient’s quality of life.2

As the typical tumor causing TIO, for which the nomenclature of phosphaturic mesenchymal tumor (PMT) is now well established1, is of small size and grows slowly, clinical presentation based solely on local symptoms is rare.3, 4 PMTs have been associated with FN1-FGFR1 and FN1-FGF1 fusion genes in about half of the cases and can be identified by different imaging methods, with 68Ga DOTATATE-based technology showing high sensitivity.1 Radical excision of these tumors is followed by a rapid drop in FGF23 levels, a more gradual increase in 1,25(OH)2D and an improvement of symptoms.1 When the causative tumor is either undetectable or unresectable or recurs repeatedly, medical treatment with oral phosphate and active vitamin D is necessary.1 Recently, burosumab, a fully human monoclonal antibody that binds to and inhibits the activity of FGF23, has been approved for the treatment of unresolved TIO.1

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