Phosphaturic Mesenchymal Tumor of Tibia: A Challenging Diagnosis Rendered on Percutaneous Core Needle Biopsy

Abstract Phosphaturic mesenchymal tumors produce excessive fibroblast growth factor 23 (FGF-23) leading to hypophosphatemia, phosphaturia, and osteomalacia. A 50-year-old male presented with pain and swelling over the anterior and medial aspect of the right leg. Imaging showed a lytic lesion in the right proximal tibia suggesting a possibility of metastasis. Though characteristic grungy calcification was not seen, a diagnosis of phosphaturic mesenchymal tumor was suggested on core biopsy. Subsequent positron emission tomography-computed tomography revealed additional fractures involving multiple sites. Further investigations showed elevated serum levels of FGF-23 and hypophosphatemia. Following excision of the tumor, phosphate levels were restored and his symptoms relieved. A diagnosis of phosphaturic mesenchymal tumors should suspected in all cases of hypophosphatemic osteomalacia. The morphology overlaps with other low-grade mesenchymal neoplasm and diagnostic difficulty enhances especially in absence of grungy calcification, as in this case. Restoration of the biochemical parameters to normal levels provide supportive evidence in establishing the diagnosis.