SUN-760 Finding the Needle in a Haystack: The Hunt for a Phosphaturic Tumor

Abstract Disclosure: L. Bernacet: None. S. Peden: None. D.M. Lindskog: None. G. Spilberg: None. C. Bergwitz: Alexion Pharmaceuticals, Inc., Sanofi. K.L. Insogna: None. K. Mekala: None. Tumor-induced osteomalacia (TIO) is a rare paraneoplastic process characterized by osteomalacia from hypophosphatemia due to renal phosphorus wasting. Mesenchymal phosphaturic tumors tend to be small, slow growing and can occur anywhere in the body, making them often occult, and their localization a challenge. The diagnosis of TIO is frequently delayed given its nonspecific, elusive clinical presentation and the rarity of this condition. A 35-year-old woman was referred for evaluation of osteoporosis complicated by multiple stress fractures. She initially sought care for persistent right ankle pain without a history of antecedent trauma. MRI imaging demonstrated stress fractures involving the posterior calcaneus, distal tibia, proximal medial cuneiform and distal cuboid. She reported significant generalized muscle cramping and weakness with progressive difficulty in ambulation, to the extent that she became wheelchair bound. Spinal imaging revealed T3-T10 vertebral compression fractures. Laboratory testing showed a low serum phosphorus (1.7 mg/dL; nml 2.2-4.5 mg/dL), low 1,25 dihydroxyvitamin D (12 pg/mL; nml 18-72 pg/mL) and elevated FGF23 (130 pg/mL; <59 pg/mL). A calculated TmP/GFR of 1.4 mg/mL was consistent with renal phosphate wasting. Whole-body 68Ga-DOTATATE PET/CT showed multiple compression fractures. Careful scrutiny of the imaging revealed high avidity misregistered to the left lateral toes. A left 4th proximal phalanx mass biopsy confirmed the presence of a mesenchymal phosphaturic tumor with a low Ki-67 proliferation index. Wide excision and amputation of the left 4th toe confirmed complete tumor excision and phosphorus levels returned to normal (4.5 mg/dL). The post-operative course was complicated by hungry bone syndrome causing hypocalcemia and secondary hyperparathyroidism. These changes resolved after supportive therapy with calcium, calcitriol, and vitamin D supplementation. This case highlights the importance of checking serum phosphorus level and evaluating for secondary causes of fragility fractures when presenting symptoms include bone pain and muscle weakness. Tumor resection is curative; however, localization is often challenging on conventional imaging given their small size and variable location. Molecular imaging with Dotatate PET/CT -targeting somatostatin receptors- has been shown to be the most sensitive modality. Scans including the entire body, extending to the extremities of the digits, are critical in localizing these tumors. References: Jan de Beur et al. 2023Rayamajhi et al. 2019El-Maouche et al. 2016 Presentation: Sunday, July 13, 2025