Phosphaturic Mesenchymal Tumor of the Greater Trochanter: A Case Report.

This is the case of a 56-year-old Hispanic male with a history of multiple fractures and electrolyte abnormalities, including hypophosphatemia and phosphaturia. Physical examination, imaging studies, and laboratory workup may have suggested the presence of a phosphaturic mesenchymal tumor (PMT) causing osteomalacia. The patient underwent surgery for en bloc tumor removal, and the histopathological analysis confirmed the presence of neoplastic cells consistent with PMT with minimal immunohistochemical positivity to S100 protein, which is atypical for this type of tumor. This case highlights the challenges in diagnosing PMTs due to their rarity and variable presentation. It emphasizes the importance of considering PMT in the differential diagnosis for unexplained hypophosphatemia and osteomalacia-like symptoms, especially in persistent disease after parathyroidectomy for presumed primary hyperparathyroidism. The atypical immunohistochemical profile observed in this case contributes to the growing body of knowledge about the heterogeneity of PMTs and underscores the need for comprehensive diagnostic approaches in suspected cases.