P49 An unusual cause of transient migratory bone oedema: fibroblast growth factor-23 secreting a mesenchymal tumour
Authors: Fisher Kerry , Melath Sunil , Patel Sanjeev 
Published: April 2020

Abstract

Background
Fibroblast growth factor-23 (FGF-23) is a phosphate regulator primarily expressed by osteocytes. Excess FGF-23 leads to decreased hydroxylation of 25-hydroxyvitamin D and poor renal phosphate reabsorption. This leads to hypophosphatemia and represents a rare cause of osteomalacia, resulting in bone oedema and stress fractures.

Methods
A 57-year-old man with known skin psoriasis presented with a two-year history of left foot and ankle pain. On examination, he had chronic dactylitis of the big toe and extra-articular features included skin psoriasis on the scalp, elbows, knees, and nail pitting. Serum inflammatory markers were normal, and autoimmune screens for RF, anti-CCP and HLA-B27 were negative. Bilateral foot and ankle X-rays showed no bony abnormality. This was diagnosed as likely psoriatic arthritis and MRI showed talar bone oedema, felt to be related to inflammatory arthritis. Symptoms settled well on non-steroidal anti-inflammatories. However, he presented with focal distal tibial swelling and pain, two months later. There was no history of trauma, Repeat MRI2 showed proximal migration of bone oedema with stress fractures of the left posterior talus and distal tibial metaphysis. Bloods tests showed low phosphate, elevated PTH, normal adjusted calcium, raised ALP and low 25-OH vitamin D. DEXA scan confirmed osteoporosis of the hip. The patient was commenced on bisphosphonate with 25-hydroxyvitamin D and phosphate supplementation. Despite this, the patient continued to have migratory joint pain affecting the ankle, hip and sacroiliac joints. Methotrexate was started for psoriasis and whilst his skin improved, his pain remained and further MRI showed left talar and right neck of femur insufficiency fractures.

Results
Although taking vitamin D3 supplementation, he remained hypophosphateamic 0.5mmol/L. Myeloma screen and PET FDG were normal. However, he was noted to have an increased fractional urinary phosphate excretion indicating poor renal phosphate reabsorption. One possible cause of this is elevated FGF-23, which was confirmed with FGF-23 assay (include levels and normal range). The patient underwent PET Ga-DOTATE imaging, utilising a tracer specific for somatostatin receptors found on neuroendocrine tumours. This showed a T9 pedicle lesion and a CT-guided biopsy confirmed a mesenchymal tumour as the cause of FGF-23 secretion, resulting in TBMO and insufficiency fractures. The patient has become asymptomatic on calcitriol and phosphate supplementation. He is now being considered for radiofrequency ablation therapy.

Conclusion
This case illustrates the need for a thorough investigation of symptomatic, treatment-refractory hypophosphataemia. Although mild hypophosphatemia could indicate adult-onset rickets, rarer causes such as FGF-23 secreting tumours should be considered. These tumours are notoriously difficult to locate; Ga-DOTATE PET is probably superior to other imaging modalities including FDG-PET in isolating mesenchymal tumours.

Disclosures
K. Fisher None. S. Melath None. S. Patel None.