Tumour induced osteomalacia (TIO) is a rare cause of atraumatic fracture caused through urinary phosphate wasting in the setting of excessive fibroblast growth factor-23 (FGF-23) secretion by small mesenchymal tumours. We present a case of TIO in a 46 year old male electrician referred for endocrinology assessment with a 3 month history of stress fracture affecting the left distal tibia.
At presentation, bone mineral density (BMD) was reduced at the left total hip (-2.5 SD), but normal at the lumbar spine (+0.1 SD). Serum corrected calcium was normal (2.18mmol/L; NR: 2.10-2.60) with a marginally reduced phosphate (0.77mmol/L; NR: 0.80-1.50), replete 25-OH vitamin D (58nmol/L), and normal parathyroid hormone (5.6pmol/L; NR: 1.3-7.6). He was commenced on treatment with an oral bisphosphonate, but had a poor BMD response and developed new atraumatic fractures in 3 ribs. Despite transition to intravenous bisphosphonates, there were further incident fractures as well as the development of a progressive and persistent hypophosphataemia ranging from 0.56-0.70mmol/L.
In the setting of hypophosphataemia, FGF-23 was inappropriately elevated (56ng/L; NR: 10-54), raising the possibility of TIO. Bisphosphonate therapy was suspended and calcitriol was commenced with some improvement in serum phosphate levels. A 68Ga-DOTATATE PET-CT identified an octreopeptide avid sclerotic 20x26mm lesion in the left lateral femoral condyle suspicious for mesenchymal tumour and multiple new octreopeptide-avid stress fractures. He was referred for surgical excision and histopathology of the excised condylar lesion confirmed the diagnosis of phosphaturic mesenchymal tumour. Immunohistochemical staining was positive for somatostatin receptor 2A (SSTR2A) – supporting the diagnosis, but negative for FGF-23. Serum phosphate levels normalized after tumour resection.
The pathophysiology and diagnosis of TIO will be reviewed, including the immuno-histochemical features and the utility/pitfalls of 68Ga-DOTATATE PET-CT in diagnosis.