Sternal Phosphaturic Mesenchymal Tumor-Induced Osteomalacia, Diagnosis and Management: A Case Report

Background: Tumor induced osteomalacia (TIO) is a rare paraneoplastic disorder in which overproduction of fibroblast growth factor-23 (FGF-23) by mesenchymal tumor results in decreased renal phosphorus reabsorption and low to inappropriately normal 1,25-dihydroxyvitamin D, leading to hypophosphatemia and osteomalacia. Patients often present with bone pain, fractures, muscle weakness, and progressive decline in mobility. Due to the nonspecific nature of presenting symptoms of TIO diagnosis is often delayed. Clinical Case: A 55-year-old male presented with complaints of chest pain, shortness of breath, and generalized weakness following a ground level fall. Patient also reported a 10-year history of osteoarthritis with chronic back pain and 1-year history of generalized weakness, resulting in significant decline in functional status. On work-up, the initial CT scan of chest revealed multiple fractures including ribs, manubrium, scapula, and pubic rami. Subsequent biochemical evaluation was remarkable for hypophosphatemia to low of 1.3 mg/dL (2.4 - 5.0 mg/dL), low of 1,25-dihydroxyvitamin D of 13.1 pg/ml (19.9 - 79.3 pg/mL), reduced tubular phosphate reabsorption rate of 28% (normal > 80%) ratifying for renal phosphate wasting, normal iPTH level, and elevated serum FGF-23 level of 460 (normal < 180). Then, localization imaging for TIO was performed. After PET/CT scan showing increased uptake at the sternal area suggestive of lytic metastasis, subsequent CT angiogram of the chest identified mottled, irregular, mildly expansile appearance of the sternal manubrium. Sternal biopsy revealed phosphaturic mesenchymal tumor with positive FGF 23 mRNA expression. Surgical resection was delayed due to poor functional status and concurrent discovery of an EBV-positive nasopharyngeal carcinoma. Prior to surgery patient was treated with phosphorus and calcitriol supplements. Post-operatively serum phosphorus and FGF-23 levels were normalized. Patient also improved clinically. Patients treatment course was complicated by secondary hyperparathyroidism; however, this improved following surgery. Conclusion: Diagnosis of TIO can be delayed due to its nonspecific symptoms. Thus, in patients with chronic bone pain, muscle weakness, and atraumatic fractures, TIO should be kept on the differential and these patients should undergo thorough biochemical and imaging evaluation. Tumor localization could be challenging. Patients should be managed with supplements of active vitamin D and phosphorus with goal to normalize phosphorus level to prevent further bone demineralization prior to surgery. However, surgical intervention remains the mainstay of management as this is curative of TIO.