THU463 Etiology Of Hypophosphatemia In A Patient With Co-existing Mesenchymal Tumor And Fibrous Dysplasia

Disclosure: M. Patil: None. S. Wangsiricharoen: None. A. Lazar: None. T.A. Guise: None. Tumor induced osteomalacia (TIO) is a rare paraneoplastic phenomenon. Fibroblast growth factor-23 (FGF23) secretion from phosphaturic mesenchymal tumor (PMT) is implicated as the cause for hypophosphatemia and osteomalacia. Other rare conditions associated with dysregulated FGF23 production include fibrous dysplasia (FD). Coexistence of two unusual conditions which could increase FGF23 concentrations is rare. We report a case of PMT causing TIO in a patient with evidence of FD. A 79-year-old male was evaluated for worsening rib and right flank pain of more than one year duration. He also complained of generalized muscle weakness. His total alkaline phosphatase (ALP) levels were noted to be increased for many years. Computed tomography (CT) showed scattered sclerosis within ribs, and lytic lesions with cortical bone destruction in right iliac bone (4.3 x1.5 cm) and L4 vertebra. Biopsy of the right iliac lesion revealed findings compatible with PMT. Positron Emission Tomography (PET) showed fluorodeoxyglucose (FDG) avid lesion of the right anterior iliac wing, diffuse osteomalacia, insufficiency fractures of the ribs, as well as FDG avid lesions in left supra-acetabular area and in L4 vertebral body concerning for metastatic disease. Laboratory evaluation revealed ALP 306 (40-129 U/L), bone specific ALP 99 (0-20 mcg/L), phosphorus 1.5 (2.5-4.5 mg/dL), 25-(OH)-vitamin D 27 (30- 100 ng/mL), 24 hour urinary phosphorus 0.6 (0.4-1.3 g/24 hr), PTH 51.3 (15-65 pg/mL), FGF23 303 (<180 RU/mL), Calcium 10.3 (8.4-10.2 mg/dL), albumin 4.4 (3.5- 5.2 gm/dL), C-terminal telopeptide of type 1 collagen (CTX) 1611 (10-854 pg/mL). CT-guided cryoablation of the right iliac lesion was performed. Also, biopsy and cryoablation of the left supra-acetabular lesion was performed; pathology showed fibrous dysplasia (FD). Mutation analysis confirmed a mutation in exon 8 of GNAS gene. Post ablation, FGF23 dropped to 102 RU/mL and phosphorus increased to 3.0 mg/dL. The patient had significant improvement in bone pain and weakness. The patient with classic features of TIO improved with simultaneous cryoablation of both the PMT as well as the FD lesions. The unusual feature of the case is co-existence of PMT and FD. Malignant transformation of FD lesions is a rare complication. We confirmed that the PMT did not have GNAS mutation and is unlikely to arise from FD. And FGF23 in situ hybridization was strongly positive in the PMT and negative in the FD, ruling out FD as a contributor to patient’s hypophosphatemia. We plan to perform DOTATATE scan to differentiate PMT from FD in the L4 lesion. This is a unique case of coexistence of two separate conditions, both of which could secrete intact FGF23. Our studies suggest that PMT is the only etiology of hypophosphatemia and TIO. Presentation: Thursday, June 15, 2023