Oncogenic ostemalacia is a rare paraneoplastic syndrome characterised by renal phosphate wasting secondary to secretion of FGF-23 from mesenchymal tumours. Localisation of the tumour is wanted, as resection can lead to complete clinical and biochemical cure. We present a case of a 57 year old woman with a background of Vitamin D deficiency and secondary hyperparathyroidism, who presented with severe generalised aches and pain, worst in her ribs and thighs, such that she could no longer manage stairs. After Vitamin D repletion orally (100.3 nmol/l), the biochemistry still revealed a low phosphate (0.56 mmol/l), elevated alkaline phosphatase (195Units/l), and elevated parathyroid hormone (7.3 pmol/l). She remained symptomatic. Further investigation identified inappropriately high urinary phosphate excretion (10.89 mmol/l with fractional excretion 45.38%). FGF-23 levels were high at 213units/ml with an inappropriately normal 1,25(OH)2 Vitamin D of 29 pmol/l. Retinol binding protein level was normal. This picture was suggestive of oncogenic osteomalacia. Treatment with Sandoz phosphate and alfacalcidol was started. A diagnostic Dotatate PET scan was performed, identifying multiple tracer avid bone lesions at sites typical for insufficiency fractures, but failed to localise any specific lesions. Subsequent targeted history taking identified a small left sided lesion on the patients oral mucosa developing over the past year, with the patient revealing daily betel nut usage. Excision biopsy was performed, which showed features consistent with a giant cell fibroma. On further review of the original Dotatate PET scan, subtle uptake corresponding to this location was noted. The patient improved biochemically, with fasting serum phosphate normalising off phosphate treatment post-operatively, but requires a repeat wide excision to ensure there is no recrudescence. Our case highlights the importance of proper work-up of hypophosphataemia. Localising FGF-23 producing mesenchymal tumours can be difficult, and a good history and examination are crucial.
14 Nov 2022 - 16 Nov 2022