MON-353 Tumor-Induced Osteomalacia from a Hypervascular Thoracic Paraspinal Mass: Challenges in Diagnosis and Management

Background: Tumor-induced osteomalacia (TIO) is a paraneoplastic syndrome driven by ectopic production of fibroblast growth factor 23 (FGF23), resulting in renal phosphate wasting, hypophosphatemia, and bone demineralization. Successful therapy requires complete resection of the tumor, which is often challenging to localize. Clinical Case: A bedridden 37-year-old woman presented to Endocrine Clinic after 9 years of progressive pain and weakness in her back, hips, and extremities. She had previously been diagnosed with a neurodegenerative disease, though an MRI from 5 years ago showed bilateral subacute sacral ala fractures. CT scan on presentation showed generalized osteopenia and numerous subacute to chronic atraumatic fractures (involving ribs, scapula, pubic rami, and right femoral shaft) concerning for osteomalacia. On exam, she had profound lower extremity weakness. Laboratory testing was notable for serum phosphorus 0.6 mg/dL (normal (nl) 2.5-4.9), calcium 8.4 mg/dL (nl 8.5-10.5), PTH 216 pg/mL (nl 13-85), 25-OH vitamin D 26 ng/mL (nl 30-100), alkaline phosphatase 345 U/L (nl 45-130), 1,25-(OH)(2) vitamin D 28 pg/mL (nl 18-72), renal tubular phosphorus reabsorption TmP/GFR 0.53 mg/dL (nl 2.97-4.45), and inappropriately normal FGF23 level 170 RU/mL (nl<180). Phosphate salts, calcitriol, and cinacalcet were initiated. Octreotide scan was negative, but (18)F-FDG PET/CT showed mild hypermetabolic activity in the lower thoracic paraspinal area correlating with a 3.4 cm hypervascular mass near the left 8(th) rib head and two adjacent nodules on MRI. Due to bleeding risk, biopsy was deferred and further confirmatory studies were pursued. Genetic testing for hereditary forms of hypophosphatemia (FGF23, CLCN5, DMP1, ENPP1, FGFR1, PHEX) was negative. (68)Ga-DOTATATE PET/CT was not available, but selective venous sampling for FGF23 revealed a 3:1 ratio between venous drainage of the tumor bed and general circulation. Based on this localization, surgical resection was performed. Pathology was consistent with a phosphaturic mesenchymal tumor, but clear margins could not be confirmed due to intraoperative tumor fragmentation. Three months of medical management with phosphate salts and calcitriol normalized her phosphorus levels (2.5-3.3 mg/dL), and she underwent surgical repair of her femoral fracture. Although MRI revealed only postsurgical changes in the spine, her continued requirement for phosphate supplementation and persistently elevated FGF23 levels (258 RU/mL) are concerning for residual tumor. Conclusion: This case illustrates the challenges of TIO due to a hypervascular mass in a rare paraspinal site localized by selective venous sampling for the first time at our institution. The patient’s continued FGF23 elevation underscores the importance of advancing medical therapies, such as burosumab, in patients with TIO that are not amenable to complete surgical resection.