SUN-750 Tumor-Induced Osteomalacia Secondary to Orbital Mesenchymal Tumor with Excessive Production of Fibroblast Growth Factor 23

Abstract Disclosure: E.A. Berg: None. M.A. McConnell : None. Introduction: Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by excessive secretion of fibroblast growth factor 23 (FGF-23), leading to phosphate wasting and impaired bone mineralization. Clinically, patients present with progressive bone pain, recurrent bone fractures and height loss1. We report a 53-year-old female with multiple fragility fractures subsequently diagnosed with TIO from an orbital mesenchymal tumor. Case Presentation: A 53-year-old female presented with five years of diffuse body pain, initially affecting her feet, Achilles tendon, and tailbone. MRIs in 2022 revealed calcaneal and tailbone fractures. She developed progressive pain in her hips, ankles, thumbs, and ribs, leading to severe mobility impairment. In July 2024, imaging showed diffuse osteopenia, multiple insufficiency fractures (bilateral femoral neck, scapular, calcaneal, MCP, MTP, rib), a chronic right proximal fibula fracture, and thoracolumbar levoscoliosis. Laboratory workup showed low serum phosphorus 1.8 mg/dL (2.5-4.5 mg/dL), elevated alkaline phosphatase 269 U/L (37-153 U/L), elevated PTH 92 pg/mL (11-51 pg/mL), and low 24-hour urine phosphate 1.6 g/24h (0.9-1.3 g/24hr). Despite profound hypophosphatemia, FGF-23 levels were inappropriately normal (160 RU/mL, 0-180 RU/mL). Additionally, 1,25 vitamin D was 28.2 (19.9-79.3 pg/mL) and corrected calcium was 8.3 mg/dL (8.6-10.4 mg/dL). She was started on calcium, vitamin D, active vitamin D, and phosphate supplementation. Upon review, a right orbital mass was incidentally found on a February 2024 brain MRI. A July 2024 orbital MRI confirmed a 12 × 9 mm right orbital mass abutting the optic nerve. Biopsy identified a phosphaturic mesenchymal tumor with FGF-23 positivity, confirming the diagnosis of TIO. Given the morbidity associated with surgical excision, the patient underwent 5400 cGy of external beam radiation in 30 fractions. Post-treatment, serum phosphate was 2.6 mg/dL (2.5-4.5 mg/dL) and urinary phosphate was 1.1 g/24h (0.9-1.3 g/24hr). 1,25 vitamin D was 98.1 pg/mL (19.9-79.3 pg/mL). Alkaline phosphatase was 157 U/L (37-153 U/L) and PTH was 70 pg/mL (11-51 pg/mL). FGF-23 was 25 pg/mL (</=59 pg/mL). Phosphorus supplementation was significantly reduced, and active vitamin D was discontinued. Conclusion: TIO is challenging to diagnose due to its rarity and nonspecific symptoms. It typically arises from phosphaturic mesenchymal tumors secreting FGF-23. While surgical resection is preferred, radiation therapy is a viable alternative when excision is not feasible. This case highlights the need for recognizing TIO in patients with unexplained fractures. Close biochemical monitoring is crucial for assessing disease resolution and recurrence. References: 1. Minisola S, Peacock M, Fukumoto S, Cipriani C, Pepe J, Tella SH, Collins MT. Tumour-induced osteomalacia. Nat Rev Dis Primers. 2017 Jul 13;3:17044. Presentation: Sunday, July 13, 2025