MON-352 When Osteoporosis Does Not Make Sense: Tumor-Induced Osteomalacia

Abstract

Background: Tumor-induced Osteomalacia (TIO) is a rare paraneoplastic disorder that relates to excess FGF-23 secretion from mesenchymal tumors. Typical presentations include weakness, severe bone pain, but more importantly; multiple false or true fractures. Severe Hypophosphatemia with hyperphosphaturia is one of the pathognomonic findings due to FGF-23 action on renal tubules as phosphaturic agent. Multiple imaging modalities usually required as tumor can be in any size and anywhere in the body. Case: 58 yo male with past medical history of multiple fractures: bilateral rib cage, and metatarsal, dextroscoliosis who present to endocrine clinic due to low bone mineral density (BMD). On initial visit, screening bloodwork done was unremarkable except for mild elevation of Alkaline Phosphatase and extremely low phosphorus (1.2). 24 Urine phosphorus ordered which was elevated but it was thought to be nutrition related thus phosphorus replacement was prescribed (4 times per day). On follow up, repeat labs noted again with severe hypophosphatemia with elevated FePhos. DEXA was also performed which noted with T score -4.0 of lumbar spine. During that appointment, he informed also that currently in the process of evaluating right lung mass with biopsy resulted with benign mesenchymal tumor which he was happy to hear. TIO was then suspicious to be the cause of decrease BMD along with history of multiple fractures and height loss. Subsequently, FGF-23 staining was then made to the biopsy sample which was positive thus confirming the diagnosis. Patient then underwent surgical resection of the tumor which post-surgery, patient experienced overall symptoms improvement, resolution of phosphatemia, as well as significant improvement of BMD without additional therapy besides calcium and vitamin D. Discussion: Osteomalacia is a pathological bone disease involving poor mineralization of existing bone during remodeling phrase. It is commonly involved with chronic hypophosphatemia and/or hypocalcemia. Persistent decrease in calcium-phosphate product leads to reduction in bone mineralization activity and bone stiffness with Bowing deformities of the long bone being one of the common findings as well as false fracture.In TIO, FGF-23 is excessively secreted by mesenchymal tumor that is usually difficult to locate due to its small size. When FGF-Receptor is activated, NaPi-2a transcription is reduced in proximal tubule cells which leads to Phosphate excretion and wasting.TIO is a curable disease once tumor is located and surgically removed thus it is important for providers to be aware of it and pursue diagnostic work up when high suspicious. One of the most common reason for misdiagnosis is due to standard metabolic panel on its own does not include phosphorus level which is crucial. TIO has to be distinguish from osteoporosis as Bisphosphonates is not indicated as treatment which is first line for the latter.