MON-502 It's Not Always Osteoporosis: FGF-23 Induced Osteomalacia

It’s not always osteoporosis: FGF-23 induced osteomalacia. Introduction Osteomalacia is a bone disorder that is caused by many conditions which could be inherited or acquired, associated with frequent fractures and pain. Tumor induced osteomalacia (TIO) is caused by excess FGF-23 most commonly by mesenchymal tumors, which leads to phosphaturia and inactivation of 1,25 (OH)(2)D(3), in turn causing chronic pain and/or severe muscle weakness. The diagnoses for these patients is typically delayed by years and many times labelled as osteoporosis. Case Presentation This is a 83 yo African-American male with history of prostate cancer with prostatectomy in 2002 and osteoporosis diagnosed in August 2016 at which time he was started on alendronate, cholecalciferol, and calcium. Since 2013 he had an elevated alkaline phosphatase, and given this oncology was consulted in November 2016 due to history of prostate cancer and metastatic work up. A bone scan performed in November 2016 showed increase uptake in the ribs concerning for metastatic disease, which were not seen in CT of the chest, and PET scan was negative for malignancy. In January 2018, on follow up clinic visit, he was reporting severe bilateral lower extremity weakness and pain causing difficulty walking, progressively getting worse to the point of requiring a wheelchair for about 6-12 months, due to this he was admitted to the hospital for evaluation in February 2018. His phosphorus level was 0.7, and 1,25 (OH)(2)D(3) was also low, suggestive of tumor induced osteomalacia. An FGF-23 level was sent, which was elevated at 456 RU/ml (44-215). On clinic follow up, full physical exam revealed a left chest mass that was 3-4 cm, ovoid, non-tender, mobile and indurated. Patient then stated it had been present for 2-3 years. He underwent excision of the mass in March 2018 which was consistent with osteoclast-like giant cell. Follow up FGF 23 level was 44 (44-215). After the surgery the Patient’s pain resolved, he regained his muscle strength and was able to ambulate without assistance. Conclusion TIO is a rare paraneoplastic syndrome which requires a good level of suspicion. Many patient are treated as osteoporosis erroneously and the actual cause of the disease goes unnoticed for many years. Should be suspected in patients with phosphaturia, low 1,25 (OH)(2)D(3), and clinical symptoms of muscle weakness and pain. Many times finding this tumors can be challenging and detailed physical exam is important to make the diagnosis. References1. Florenzano P, Gafni RI, Collins MT. Tumor-induced osteomalacia. Bone Rep. 2017 Sep 20;7:90-97. doi: 10.1016/j.bonr.2017.09.002. eCollection 2017 Dec. 2. Fukumoto S. Physiological regulation and disorders of phosphate metabolism--pivotal role of fibroblast growth factor 23. Intern Med. 2008;47(5):337-43. Epub 2008 Mar 3.