Osteomalacia-Inducing Tumors of the Brain: A Case Report, Review and a Hypothesis

Osteomalacia-inducing tumors (OIT) are mesenchymal tumors that characteristically secrete fibroblast growth factor 23, resulting in a paraneoplastic syndrome of hypophosphatemic osteomalacia. These tumors are known to occur in soft tissues and bones in various sites. It is very unusual for OITs to occur intracranially, with only 10 reported intracranial cases since their discovery in 1959. The most common intracrainal OITs are phosphaturic mesenchymal tumors and hemangiopericytomas. We report a case of hypophosphatemic osteomalacia caused by a tumor in the right anterior cranial fossa. We also hypothesize, based on our review of the literature, that this entity is underdiagnosed.

A 49-year-old woman had a history of a nonhealing ankle fracture that required repeated surgery over 3 years. She subsequently was found to have severe hypophosphatemia and evidence of osteomalacia together with multiple occult fractures. A diagnosis of tumor-induced osteomalacia was suspected. An elevated serum fibroblast growth factor 23 level confirmed the diagnosis. An octreotide scan that was performed to locate the responsible tumor revealed an area of avid uptake in the right frontal lobe. Magnetic resonance imaging showed a large right anterior fossa extra-axial mass. The patient was referred for surgical intervention and was cured clinically after surgical removal of the tumor. Pathologic examination revealed a phosphaturic mesenchymal OIT. Her phosphate levels returned to normal 3 weeks after surgery.

The diagnosis of OIT should be considered in a case of severe hypophosphatemia and metabolic bone disease that is not explained by any other metabolic or hereditary disease. These tumors can occur intracranially and may be confused with a meningioma or a hemangiopericytoma. Taking OIT into consideration in such cases could lead to a shorter time to diagnosis and management, which in our case took 4 years.