Treating osteoporosis: a near miss in an unusual case of FGF-23 mediated bone loss.

Oncogenic osteomalacia is a potentially curable condition caused by phosphaturic mesenchymal tumours. We present the case of 54-year-old woman who presented with bony pains and muscle weakness, and was erroneously treated with bisphosphonates elsewhere on the basis of Dual Energy X-ray Absorptiometry (bone density) scan that showed 'severe osteoporosis'. Further evaluation revealed the presence of hypophosphatemia, with a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid avid lesion involving left mandible on PET scan. Surgical excision of lesion led to improvement in her symptoms with normalisation of phosphorus, and histopathology was diagnostic of mesenchymal phosphaturic tumour. A diagnosis of oncogenic osteomalacia was made. A repeat bone mineral density assessment done a year later showed dramatic improvement. A meticulous examination and evaluation is warranted for any patient presenting with bony pains and muscle weakness. The detection of hypophosphatemia in an elderly individual should prompt the consideration of oncogenic osteomalacia, as localisation and excision of tumour usually results in cure.