A case of phosphaturic mesenchymal tumor: A rare entity

Abstract Phosphaturic mesenchymal tumor is a rare neoplasm of soft tissue and bone that produces tumor-induced osteomalacia. These tumors are almost always benign and are typically associated with over expression of ‘fibroblast growth factor 23 (FGF23)’ that inhibits renal tubular phosphate re-absorption causing low serum phosphate, causing osteoporosis. This tumor usually occurs in middle-aged adults with slight female preponderance. Symptoms may range from chronic bone pain and pathological fractures to progressive muscle weakness. In this case report we aim to highlight the diagnostic modalities and utility of Immunohistochemistry in a case of Phosphaturic mesenchymal tumor (PMT). A 32 years old female presented with progressive nasal obstruction, discolored rhinorrhea, occasional nasal bleed from right nostril. Nasal endoscopy revealed a large, smooth, mucosa-covered mass filling the naso-pharynx. Serum calcium was within normal range whereas serum phosphorus and serum 1,25 dihydroxy vitamin D levels were reduced and raised Serum FGF-23 level. Histopathology revealed a tumor composed of sheets of oval to spindle cells with fine chromatin and inconspicuous nucleoli, interspersed by thin walled gaping vascular channels. On Immunohistochemistry, the tumor cells show positivity for vimentin and negative for SMA, CD34 and STAT6. Ki67 labeling index is 5%. Our final diagnosis was Phosphaturic mesenchymal tumor. PMTs are rare diseases with confusing clinical manifestations that can make them difficult to diagnose. Immunohistochemistry is an important adjunct to other diagnostic methods i.e. clinical manifestations, biochemical examinations, and imaging characteristics to help accurately diagnose PMTs. The histologic differential diagnosis of this tumor includes solitary fibrous tumor, mesenchymal chondrosarcoma, and paraganglioma. However, the immunophenotype of PMT (positive for FGF23 mRNA and negative for STAT6) is helpful in confirmation of diagnosis.