THU465 Significant Improvement In Bone Mineral Density Following Treatment For Tumor Induced Osteomalacia

Disclosure: L.C. McHan: None. M. Augustine: None. Background: Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome characterized by renal phosphate wasting and impaired activation of vitamin D due to excess secretion of FGF23, often from phosphaturic mesenchymal tumors (PMTs). TIO typically presents with months to years of bone pain, muscle weakness, and fractures often leading to functional decline. Given these nonspecific symptoms, diagnosis of TIO can be delayed for years. Removal of the tumor is curative, though it can sometimes be a challenge to localize. For those whose tumors are unable to be localized or removed, medical management with phosphorus and vitamin D is recommended. Patients with TIO have low bone mineral density (BMD) at time of diagnosis. Case Reports: We are presenting two cases of TIO, one managed surgically and one managed medically. Patient 1 is a 45-year-old man with Fabry disease who presented with years of progressive pain, difficulty ambulating, and multiple fractures. Lab workup revealed low serum phosphorus, elevated urinary phosphorus, elevated FGF23, and negative genetic testing for heritable causes of hypophosphatemic osteomalacia. DOTATATE imaging identified a subcutaneous left flank mass, which was excised resulting in improvement in symptoms and resolution of biochemical abnormalities. Additionally, lumbar spine (LS) bone mineral density (BMD) nearly doubled; prior to diagnosis of TIO, LS BMD was 0.601 g/cm(2) and 8 months after surgical excision of the tumor, LS BMD was 1.178 g/cm(2). Patient 2 is a 55-year-old nonverbal man with autism and intellectual disability who presented with months of progressive difficulty ambulating, presumed pain, functional decline, and multiple fractures. Workup revealed low serum phosphorus, elevated urinary phosphorus, and elevated FGF23 concerning for TIO. He was initiated on phosphorus and vitamin D supplementation, but no culprit tumor has been identified despite multiple scans over the course of years. However, the patient has had a significant improvement in functioning with medical management in addition to a remarkable 48% improvement in LS BMD, from 0.855 g/cm(2) prior to diagnosis of TIO and 1.264 g/cm(2) following 3 years of medical management. Clinical Lessons: In our review of the literature, there are few case reports and case series documenting the significant improvements in BMD with treatment of TIO. In our experience, two patients who had months to years of debilitating symptoms exhibited remarkable improvements in symptoms and marked improvement in LS BMD following treatment. This emphasizes the importance of recognizing the diagnosis of TIO, as even when the causative tumor is not identified, treatment typically has a significant impact on patient functioning and quality of life. Presentation: Thursday, June 15, 2023