PSAT218 Technetium 99m-Methyl Diphosphonate (99mTc MDP) Radioguided Surgery in Patient With Tumor-Induced Osteomalacia (TIO)

INTRODUCTION: TIO is a rare paraneoplastic syndrome of excess FGF23 secretion by mesenchymal tumors that results in myopathy, pain, weakness and osteomalacic fractures. Tumors are small and often difficult to locate, frequently delaying diagnosis. Surgery is considered definitive therapy, but recurrences remain common due to incomplete tumor resection. We present a novel approach of using Gallium (Ga) 68 DOTATATE PET/CT to identify the culprit tumor preoperatively followed by intraoperative 99mTc MDP radioguided surgical resection. CASE REPORT: A 41-year-old female presented with a four-year history of progressive weakness, fatigue, muscle and bone pain, and multiple subacute rib fractures and bilateral hip fractures with minimal trauma. On exam, she had proximal muscle weakness and walked with an antalgic gait aided by crutches. Laboratory evaluation revealed serum phosphorus of 2.1 mg/dL (2.5-4.5), tubular reabsorption of phosphate (TRP) of 78.9% (>85%), elevated alkaline phosphatase of 209 IU/L (31-125), 1,25 dihydroxy vitamin D [1,25 (OH)(2) D] of 17 pg/ml (18-72) and serum FGF23 610 RU/ml (<180). Serum calcium, parathyroid hormone (PTH), estimated GFR and 25 hydroxy vitamin D were normal. Testing for genetic causes of hypophosphatemia was negative. With biochemical and genetic testing consistent with TIO, functional imaging was performed. Ga 68 DOTATATE PET/CT identified a single, 1.8 cm non-expansile sclerotic focus along the anterior left sixth rib with moderate uptake. She denied any recent trauma to the site. She was referred to thoracic surgery for a left sixth rib resection. Intraoperatively, she received 8.1 mCi Tc99m-MDP intravenously and the margin of the tumor was localized using a gamma probe allowing complete resection. Surgical pathology confirmed a 2.0 cm phosphaturic mesenchymal tumor (PMT)with widely negative margins. Postoperatively, her muscle strength, fatigue and ambulation improved significantly within six weeks and her serum phosphorus, TRP, 1,25 (OH)(2) D normalized. DISCUSSION: Localization of PMTs is difficult because tumors are small and can be located anywhere in the body. Functional imaging using somatostatin analogs such Ga 68 DOTATATE PET/CT exploits the presence of somatostatin receptors expressed by many PMTs. Complete resection of PMTs can be challenging because they tend to be locally infiltrative along bone trabeculae. 99mTc MDP is a radiotracer localized to bone in proportion to osteoblastic activity as seen at sites of bony remodeling or bone metastasis. Accurate tumor localization and complete resection with widely negative margins are imperative for definitive treatment and cure. This case is unique in utilizing Ga 68 DOTATATE PET/CT for localization of the PMT paired with intraoperative 99mTc MDP in conjunction with a gamma camera for identifying tumor margins facilitating complete tumor resection and cure of the patient. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.