Endocrine Abstracts

Background: Oncogenic osteomalacia is a paraneoplastic syndrome that occurs in the context of an FGF23 secreting tumour. We describe a case of a 42 year old gentleman who presented with this rare cause of hypophosphataemia.

Clinical Case: This gentleman presented with a history of multiple fragility fractures, and generalised bone and muscular pain. His biochemistry showed: serum phosphate 0.52 mmol/l (reference range 0.80-1.50 mmol/l), adjusted calcium 2.17 mmol/l (2.20-2.60 mmol/l), 25OH Vitamin D 79 nmol/l (>50 nmol/l), 1,25diOH Vitamin D 88 pmol/l (20-120 pmol/l) and an inappropriately elevated urinary fractional excretion of phosphate. Genetic testing for hypophosphatemic rickets showed no pathogenic variants detected in DMP1, ENPP1, FGF23, PHEX and SLC34A3. FGF23 levels were elevated at 111RU/ml (<100RU/ml). Technetium-99m-hydrazinonicotinamide-Tyr3-octreotide scintigraphy was performed and this showed somatostatin receptor-rich sclerotic areas within the right sacral ala and left femoral head. Whole body PET FDG CT showed several FDG-avid lesions corresponding to fractures within the right sacral ala, posterior iliac, ribs and the T1/T2 spinous processes. Magnetic resonance imaging showed a 9 mm lesion within the left femoral head which was indeterminate. As no definitive surgical target was identified, he was treated with oral phosphate replacement alongside alfacalcidol. His symptoms and biochemistry have improved: phosphate 0.76 mmol/l, adjusted calcium 2.26 mmol/l.

Discussion: Oncogenic osteomalacia is characterised by the presence of renal phosphate wasting due to elevated FGF23, which in turn results in hypophosphataemia and osteomalacia. Symptoms are non-specific and include bone pain, muscle aches, fractures and weakness. These lesions often express somatostatin receptors and may be detected by octreotide scintigraphy. If a causative lesion is identified, surgical removal can lead to biochemical cure. If surgical intervention is not possible as in this case, then patients can be managed medically with either phosphate supplementation and alfacalcidol, or with burosumab which specifically targets FGF23.