Introduction
Tumour-induced osteomalacia (TIO) is a rare paraneoplastic syndrome characterised by renal phosphate wasting due to excess fibroblast growth factor-23 (FGF23) production from a phosphaturic mesenchymal tumour (PMT). While surgery is potentially curative, localisation is often challenging.
Case report
This case describes a 53-year-old lady who had recurrent fragility fractures in the spine, hip and pelvis, associated with lower limb pain and weakness developing over 2 years. Biochemistry revealed hypophosphataemia, hyperphosphaturia, inappropriately suppressed 1,25(OH)2D and raised FGF23. Localisation for the PMT was unsuccessful, despite multiple investigations including 68-Gallium-DOTANOC PET-CT, bilateral lower limb MRI for non-specific inguinal lymph nodes and various ultrasonographic evaluation of soft tissue lesions (including a benign breast tumour which was biopsied). Selective venous sampling was not performed due to uncertain utility and lack of local expertise. She was treated medically with oral phosphate, cholecalciferol and calcitriol, with a view to perform interval surveillance DOTA-peptide scan. Phosphate level was kept in the low-normal range and alkaline phosphatase reduced from baseline with an aim for normalisation as a marker of disease activity. Development of secondary hyperparathyroidism improved with uptitration of calcitriol. There was no hypercalciuria on routine monitoring of urinary calcium excretion. Symptoms of generalised body pain and weakness, as well as her bone mineral density have improved over 21 months with medical therapy.
Conclusion
This is a challenging case of TIO which has failed to localise despite best efforts. One must consider pursuing FGF23-independent and dependent causes of osteomalacia with hyperphosphaturia when patients present with severe frailty and hypophosphatemia as substantial morbidity results from delayed diagnosis and treatment. TIO-related PMTs can unfortunately be difficult to localise, even with a combination of functional and anatomical imaging. With medical therapy, bone mineralisation and symptoms can improve significantly. Patients need to be monitored for complications of long-term phosphate and calcitriol replacement.