Oncogenic osteomalacia caused by an occult paranasal sinus tumor

Oncogenic osteomalacia (OOM) is a rare bone disease characterized by inadequate bone mineralization and is caused by a humoral factor mainly produced by benign mesenchymal tumors. We report a case of OOM caused by an occult phosphaturic mesenchymal tumor in the paranasal sinus. The causative tumor was small and localized in the ethmoid sinus, and the patient did not exhibit any nasal symptoms. 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) depicted the location of the occult tumor, and systemic venous sampling followed by assessments of the samples' fibroblast growth factor 23 (FGF23) concentrations confirmed that the tumor secreted FGF23. The tumor was resected via an external surgical approach, resulting in the complete relief of the patient's symptoms. The combination of FDG-PET and systemic venous sampling to assess serum FGF23 levels is useful for identifying small asymptomatic OOM-associated tumors. Such tumors are rare, but a significant proportion of them develop in the head and neck region, and complete resection is the most effective treatment. It is important that ENT surgeons are aware of the characteristics of OOM.