A Case of Hypophosphatemia due to Oncogenic Osteomalacia in a Patient with Natural Killer T-Cell Lymphoma

Abstract

<b><i>Introduction:</i></b> Oncogenic osteomalacia (Onc-Ost) is a paraneoplastic phenomenon characterized by hypophosphatemia due to elevated fibroblast growth factor-23 (FGF-23). Onc-Ost has been previously reported in patients with germ line mesenchymal tumors and solid organ malignancies. This is the first report of aggressive natural killer (NK) T-cell lymphoma presenting as Onc-Ost. <b><i>Case Description:</i></b> A 33-year-old Vietnamese female with active hepatitis B and <i>Mycobacterium avium</i> complex, on ongoing therapy with tenofovir disoproxil, azithromycin, and ethambutol, presented with persistent fevers and developed refractory hypophosphatemia. Workup confirmed severe renal phosphate wasting. Tenofovir disoproxil was initially suspected; however, presence of isolated phosphaturia without Fanconi syndrome and persistence of hypophosphatemia despite discontinuation of medication led to clinical suspicion of Onc-Ost. Elevated FGF-23 warranted further workup, leading to a definitive diagnosis of clinically subtle NK T-cell lymphoma. Chemotherapy was initiated; however, patient continued to deteriorate clinically and expired. <b><i>Conclusion:</i></b> Along with commonly reported germ line mesenchymal tumors and solid malignancies, NK T-cell lymphoma can also present as Onc-Ost. Timely detection of associated tumors and subsequent antitumor therapy would likely reverse hypophosphatemia and improve clinical outcomes.