Phosphaturic mesenchymal tumor without osteomalacia: additional confirmation of the "nonphosphaturic" variant, with emphasis on the roles of FGF23 chromogenic in situ hybridization and FN1-FGFR1 fluorescence in situ hybridization.

Phosphaturic mesenchymal tumor (PMT) is a rare, histologically distinctive neoplasm that classically presents with phosphaturia and tumor-induced osteomalacia (TIO; ie, oncogenic osteomalacia). Both the phosphaturia and the TIO are due to paraneoplastic production of FGF23 (a phosphatonin) by the neoplastic cells, which are genetically characterized by rearrangements of FN1 (most often with FGFR1, and less frequently with FGF1). However, rare cases of PMT present without phosphaturia and TIO (ie, the "nonphosphaturic" variant) and are therefore much more challenging to diagnose. Here, we report the first case of a genetically confirmed, nonphosphaturic PMT, in which the correct diagnosis was established through a combination of careful histologic evaluation, FGF23 chromogenic in situ hybridization, and fluorescence in situ hybridization testing for FN1-FGFR1.