Intracranial Phosphaturic Mesenchymal Tumor Mimicking Meningioma and Causing Tumor-Induced Osteomalacia: A Case Report

A 32-year-old woman presented with a two-year history of progressive bone pain, proximal muscle weakness, worsening ambulation, and bilateral hip insufficiency fractures without preceding trauma. Laboratory evaluation revealed severe hypophosphatemia (1.1 mg/dL), elevated bone-specific alkaline phosphatase, progressive secondary hyperparathyroidism, and inappropriately elevated circulating fibroblast growth factor 23 (FGF-23), with fasting paired serum and urine studies confirming renal phosphate wasting (tubular maximum phosphate reabsorption per glomerular filtration rate (TmP/GFR) 1.88 mg/dL; fractional excretion of phosphate 14.6%), consistent with tumor-induced osteomalacia (TIO). Functional imaging with 68Ga-DOTATATE positron emission tomography/computed tomography (PET/CT) demonstrated intense tracer uptake in a small anterior cranial lesion. Brain magnetic resonance imaging (MRI) revealed a dural-based olfactory groove lesion interpreted radiographically as a meningioma. Selective venous sampling demonstrated a cranial-to-peripheral gradient in intact FGF-23 consistent with drainage from the anterior cranial fossa. Surgical resection produced postoperative normalization of FGF-23 and rapid correction of hypophosphatemia. Histology showed meningioma-like architecture with whorls and psammoma bodies, but expanded molecular testing did not identify canonical meningioma-associated alterations, and DNA methylation profiling classified the lesion as a mesenchymal neoplasm not elsewhere classified and fusion negative. FGF-23 in situ hybridization was positive, and the final surgical pathology diagnosis was mesenchymal tumor favoring phosphaturic mesenchymal tumor over meningioma. This case illustrates how a mesenchymal tumor arising in the meninges can mimic meningioma radiographically and histologically while producing the endocrine phenotype of TIO.