A Rare Case of Oncologic Osteomalacia

Introduction: Oncogenic osteomalacia is a paraneoplastic syndrome characterized by hypophosphatemia due to FGF-23 secreting tumors. FGF 23 acts on the proximal tubules to reduce α hydroxylation of Vitamin D2 and reduce phosphate reabsorption. The secreting tumors must be identified swiftly as the treatment lies in complete excision of the tumor. Case Description: A 52 year old male with Stage IV prostatic adenocarcinoma with known metastasis to ribs, humerus and pelvis who presented with severe fatigue and diffuse myalgias. Laboratory evaluation revealed serum Phosphate 0.5 mg/dl ( 2.7-4.6 mg/dL), Corrected serum calcium 7.2 (8.6-10.3 mg/dl), Alkaline phosphatase 765 ( 40-116 U/L), PTH 55 ( 10-65 pg/mL). Further evaluation revealed FePhos 62% with FGF 23 level of 3481 pg/ml (< 59 pg/ml). The patient was treated with IV Calcitriol along with judicious phosphate repletion however, the patient remained hypophosphatemic to < 2 mg/dL. Discussion: Oncogenic osteomalacia is generally associated with mesenchymal tumors found in bone and soft tissue. It is rarely associated with malignant or metastatic disease. In the systematic review completed by Bosman et al of 895 cases of TIO, 10% of cases were reported malignant per histology. In this review, 98% of patients were noted to have hypophosphatemia and low Vitamin D levels, and FGF 23 levels were found to be related to tumor size (r = 0.344, P < 0.001). In 2020, the FDA approved the use of Burosumab for hypophosphatemic hyperphosphaturic TIO in patients whose lesions are not amenable to excision. Novel therapies such as FGF Receptor tyrosine kinase inhibitor Infigratinib are underway.