A 58-year-old man underwent a chest X-ray after a minor elective surgery, and was found to have a 4 cm left pleural-based mass. His past medical history was significant for hypertension, hyperlipidemia, osteomalacia, and obesity. Surgical resection of the mass demonstrated loose arrays of spindled and stellate cells without nuclear atypia. The signing pathologist ordered in-situ hybridization of FGF23 mRNA, which showed overexpression.
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This patient has a rare soft tissue neoplasm called a phosphaturic mesenchymal tumor. The neoplastic cells overexpress and secrete FGF23, a hormone that inhibits phosphate reabsorption by proximal renal tubules. Laboratory workup prior to resection may have shown hypophosphatemia, hyperphosphaturia, and normocalcemia. Many patients present with osteomalacia with recurrent bone fractures and vitamin D deficiency. The tumor-induced osteomalacia often resolves with resection of the tumor. Roughly half of these tumors possess an FN1-FGF1 gene fusion.
Peter Kundert, M.D., Ph.D.
Resident, Anatomic and Clinical Pathology
Jorge Torres-Mora, M.D.
Consultant, Anatomic Pathology
Assistant Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science