Fibroblast growth factor 23-induced hypophosphatemia in a malignant phosphaturic mesenchymal tumor: presentation of a rare case.

INTRODUCTION

Tumor-induced osteomalacia is a rare paraneoplastic disorder caused by excess fibroblast growth factor 23 (FGF23), most often produced by phosphaturic mesenchymal tumors. Delayed diagnosis may result in severe metabolic and skeletal complications.

CASE PRESENTATION: We report the case of a 32-year-old woman with a three-year history of progressive weakness and a rapidly enlarging thoracic mass. Imaging revealed an 18 × 13 cm highly vascularized thoracic lesion with multiple lytic bone metastases. Laboratory evaluation showed severe hypophosphatemia (0.9 mg/dL), renal phosphate wasting (fractional excretion of phosphate 24.5%), elevated intact parathyroid hormone, low vitamin D levels, and markedly increased serum FGF23 (7926 kRU/L). Histopathological examination and immunohistochemistry demonstrated a malignant phosphaturic mesenchymal tumor with positivity for CD56, SATB2, and SSTR2A.

DISCUSSION: This case highlights the aggressive clinical behavior that malignant phosphaturic mesenchymal tumors may exhibit, including extensive local invasion, metastatic disease, and profound metabolic derangements. The diagnosis of tumor-induced osteomalacia requires a high index of suspicion and an integrated approach combining biochemical evaluation, functional imaging, and detailed pathological assessment.

CONCLUSIONS

Tumor-induced osteomalacia should be considered in patients with persistent hypophosphatemia and phosphate wasting. Early recognition is essential, as complete surgical resection remains the only curative option. In advanced or unresectable disease, management is challenging and may be limited by tumor burden and access to targeted therapies, including anti-FGF23 agents, which offer biochemical and symptomatic benefit in selected cases.