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P089: SINONASAL PHOSPHATURIC MESENCHYMAL TUMOR WITH ORBITAL AND SKULL BASE INVASION: A CASE REPORT
Matias Gomez, MD¹; Juan Susacasa²; ¹Servicio de Otorrinolaringología, Clínica Alemana de Santiago, Chile; ²Facultad de Medicina Clínica Alemana de Santiago - Universidad del Desarrollo, Chile

Introduction: Phosphaturic mesenchymal tumor (PMT) is a rare mesenchymal neoplasm characterized by excessive production of fibroblast growth factor 23 (FGF23), leading to oncogenic osteomalacia due to renal phosphate wasting and impaired activation of vitamin D. These tumors most commonly arise in the extremities, followed by the nasal cavity, paranasal sinuses, and skull base, locations of particular relevance in otolaryngology. Clinically, patients often present with bone pain, pathological fractures, and persistent hypophosphatemia. Advanced imaging modalities such as CT, DOTATATE PET-CT, and MRI are crucial for tumor localization, assessment of extension, and surgical planning, while histological demonstration of spindle cells and osteoclast-like giant cells with FGF23 expression confirms the diagnosis. Surgical management is challenging, especially for sinonasal and skull base lesions due to proximity to critical neurovascular structures, requiring specialized approaches and multidisciplinary collaboration. Complete resection remains the standard and curative treatment, typically resulting in rapid normalization of serum phosphate and resolution of osteomalacia.

Case Presentation: A 67-year-old male with a history of osteomalacia and hypertension, on multiple medications, presented with six months of progressive right orbital swelling and visual discomfort. Physical examination revealed right-sided exophthalmos. CT and MRI demonstrated a 9.2 × 5 × 4 cm solid mass involving the right ethmoid cells, nasal cavity, frontal sinus, and contralateral frontoethmoidal region, with bone destruction of the maxillary sinus wall and extensive invasion of the right orbit, causing displacement of the globe and optic nerve. Cranial extension into the anterior cranial fossa was noted without cribriform plate involvement. Nasal endoscopy showed a mass nearly occluding the right nasal cavity, and an endoscopic biopsy was performed.

Discussion: Phosphaturic mesenchymal tumor (PMT) is a rare neoplasm that may arise in the sinonasal region and skull base, often posing a diagnostic challenge due to its atypical presentation. In this case, the patient presented with progressive exophthalmos initially attributed to ophthalmologic pathology, delaying suspicion of an underlying tumor. Imaging revealed an expansive lesion with orbital invasion and skull base extension, underscoring the importance of a multidisciplinary approach. Histopathologic examination is a key pillar as it demonstrates characteristic findings like spindle-shaped cells and osteoclast-like giant cells within a variably calcified matrix, with immunohistochemical overexpression of FGF23. Complete surgical resection remains the standard of care, providing both metabolic cure and clinical resolution of osteomalacia. While endoscopic endonasal approaches are preferred for confined sinonasal lesions, orbital or intracranial extension may necessitate combined surgical techniques to achieve safe oncologic clearance. In conclusion, PMTs should be considered in the differential diagnosis of sinonasal lesions with orbital manifestations, with histopathologic confirmation and complete surgical excision being essential to achieve curative outcomes.

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