MON-499 Tumor Induced Osteomalacia: (68)Ga-DOTATATE PET/CT Scan For Localization

Introduction: Tumor Induced Osteomalacia (TIO) is a rare condition and a notoriously difficult diagnosis to make due to lack of awareness and nonspecific symptoms. But it is a surgically curable disease, so it should not be missed. We are reporting a case of TIO in a patient with insufficiency fractures who was initially diagnosed as ‘severe osteoporosis’. Clinical Case: A 60-year-old female presented to the Metabolic Bone Disease Clinic at our institution for recurrent stress fractures of hip and femur. A recent DXA scan showed worsening of BMD at the spine and hip with T-scores of -5.0 and -4.3 respectively. She was taking calcium and vitamin D supplementation. She had previously taken Alendronate and stopped for unclear reasons. She was taking Denosumab at the time of her presentation to our clinic. She was found to have very low phosphorus level of 0.8 mg/dl. Osteomalacia was suspected, so we asked her to stop Denosumab. Phosphorus replacement was started and she remained hypophosphatemic with phosphorus levels <1 mg/dl despite increasing doses of phosphorus supplements. A 24-hour urine phosphorus and fractional excretion of phosphorus was inappropriately elevated. A diagnosis of hyperphosphaturic hypophosphatemia causing osteomalacia was made. A hereditary disorder such as X-linked hypophosphatemia was not considered because of lack of family history and onset at an adult age. FGF-23 was elevated and a diagnosis of TIO was made. We used (68)Ga-DOTATATE PET/CT scan as our initial, and single test for functional imaging/identification of the culprit tumor. The scan showed uptake in the left proximal medial tibia. Anatomical imaging with MRI confirmed the lesion. She underwent surgery for excision of the tumor and fixation of the tibia. Pathology was consistent with phosphaturic mesenchymal tumor. During follow up, phosphorus levels normalized within one month of surgery. She is currently doing well without any symptoms and no new fractures. We plan to repeat DXA scan and perform periodic monitoring of phosphorus level. Conclusion: TIO is a paraneoplastic disorder caused by FGF-23 secreted by mesenchymal tumors. It results in decreased reabsorption of phosphorus from the proximal tubule of kidneys, causing hyperphosphaturia and subsequently hypophosphatemia leading to defective mineralization of bones. A high index of suspicion is needed to diagnose this rare disorder. Any patient with an insufficiency fracture or a new referral for osteoporosis should have phosphorus level checked. If the phosphorus level is low, renal tubular reabsorption of phosphorus should be evaluated by measuring 24-hour urinary excretion of phosphorus. The culprit tumor should be localized using advanced imaging modalities such as (68)Ga-DOTATATE PET/CT scan.