Fibroblast Growth Factor-23 Secreting Tumour Induced Osteomalacia

Abstract

Tumour Induced Osteomalacia (TIO) caused by a Fibroblast-Growth- Factor-23 (FGF-23) secreting tumour is a rare paraneoplastic disorder. Patients often present with non-specific complaints of pain and stiffness however findings of hypophosphataemia and inappropriately suppressed 1,25 dihidroxyvitamin D are almost pathognemonic of this condition. We present a previously healthy 51-year-old man who presented with an 18 month history of body aches, hypophosphataemia, and an incidental finding of atraumatic foot fractures on x-ray. Technetium bone scan identified diffuse skeletal pathologic fractures. He was further found to have persistent hypophosphataemia, suppressed 1-25-dihidroxyvitamin-D, renal phosphate wasting with reduced Tubular Reabsorption of Phosphate (TRP), and an FGF-23 level more than twice the upper limit of normal. Fluorodeoxyglucose (FDG) Positron Emission Tomography (PET), Gallium-68 Dotatate PET and magnetic resonance imaging further identified and characterised a 40 mm FDG and Dotatate-avid lesion in the right third rib. Histopathological examination of a biopsy specimen revealed phosphaturic mesenchymal tumour with positive FGF-23 immunohistochemical staining arising in a background consistent with osteomalacia. After tumour resection, the patient’s hypophosphataemia, reduced TRP and elevated FGF- 23 normalised and he made an excellent recovery with rehabilitation. This case report illustrates the importance of a diagnosis of hypophosphataemia in a patient with musculo-skeletal aches and alerts clinicians of the differential diagnosis of TIO.