Oncogenic Osteomalacia with Elevated Fibroblast Growth Factor 23: A Rare Case of Paranasal Sinus Tumor Onset

Tumor-induced osteomalacia, also known as oncogenic osteomalacia, is a rare, acquired paraneoplastic disease characterized by hypophosphatemia and renal phosphate wasting. We report on the case of a 52-year-old-man admitted to our hospital for bone and muscular pains and difficulty in walking. He underwent a computed tomography (CT) scan of the legs that documented fractures in the right tibia, femur, and fifth metatarsus. Laboratory findings showed hypophosphatemia and elevated levels of parathyroid hormone (PTH). The first diagnosis was osteomalacia, treated with calcium and vitamin D, without any benefit. So he underwent a whole body CT scan, showing a small expansive lesion occupying the left frontal sinus. Furthermore, we found high serum levels of fibroblast growth factor 23 (FGF23) using the enzyme-linked immune sorbent assay (ELISA) assay. The patient underwent endoscopic surgical resection of the frontal tumor with complete clinical remission and the histopathological diagnosis of an ossifying fibromyxoid tumor. This is a rare case of oncogenic osteomalacia due to a paranasal sinus tumor. The main symptoms are not associated with nasal sinus involvement but with over-expressed FGF23. To conclude, physicians should never underestimate the chance of paraneoplastic syndrome in the head and neck district, even if such an occurrence is uncommon in this location. The clinical symptoms may be aspecific and not related to nose problems, making the differential diagnosis very difficult.