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

Abstract

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.

Keywords: fibroblast growth factor 23; hypophosphatemia; oncogenic osteomalacia; ossifying fibromyxoid tumor; paranasal sinus tumor.

Figure 1. Whole Body FDG-PET/CT

Evidence of a small lesion in the left frontal sinus with intense FDG uptake compatible with the recurrence of a bone lesion FDG-PET/CT: 18F-fluorodeoxyglucose positron emission tomography-computed tomography

Figure 2. MRI T2-Weighted with Contrast

Evidence of a 7 mm mass protruding in the left frontal sinus MRI: magnetic resonance imaging

Figure 3. Endoscopic Frontal Sinus Surgery

Endoscopic appearance of the smooth non-ulcerated lesion originating from the posterior wall of the frontal sinus

Figure 4. Histopathology Report

Histopathologic images showing the cell neoplasm constituted by small, round, and oval elements, with mild nuclear pleomorphism and scant cytoplasm immersed in the stroma with abundant collagen, a chondroid pattern in some areas, and osteoid formation.