Abstract
Objective: presenting a case of phosphaturic mesenchymal tumor-induced osteomalacia (TIO).

Case presentation: A 50-year-old woman arrived with severe widespread muscle weakness, weight loss, wheelchair reliance, gradual functional deterioration, and several spontaneous fractures involving both hips and the right arm. Investigations in the lab revealed severe hypophosphatemia with renal phosphate wasting, high alkaline phosphatase, normal calcium, low 1,25-dihydroxyvitamin D, normal 25-hydroxyvitamin D, and significantly elevated FGF23 (1,029; normal <110). Octreotide and CT scans were negative at first. Multiple fractures and osteopenia were identified by bone mineral density. Oral phosphate and calcitriol supplementation were used to treat the patient, resulting in a partial biochemical improvement. Repeat functional imaging using octreotide scan with SPECT-CT identified an avid subcutaneous lesion in the left upper medial thigh (Krenning score II). Following surgical resection, histology revealed a phosphaturic mesenchymal tumor with basophilic matrix, hemangiopericytomatous vasculature, multinucleated large cells, bland spindle and stellate cells, low mitotic activity (1-2/10 HPF), and focal degenerative atypia. Following surgery, the patient showed notable clinical recovery, quick phosphate level normalization, and notable functional improvement.

Conclusion: This case demonstrates that patients with unexplained hypophosphatemia and fractures should be suspected of TIO. Tumor localization requires careful biochemical evaluation in conjunction with sophisticated functional imaging. Rapid biochemical correction and significant clinical recovery are the outcomes of early detection and surgical excision.

Key words: Tumor-induced osteomalacia, Phosphaturic mesenchymal tumor, Fibroblast growth factor23, Hypophosphatemia, Octreotide Scan, Case report.