Prolonged generalized osteomalacia associated with a sinonasal cavity phosphaturic mesenchymal tumor: A case report

Key Clinical Message Phosphaturic mesenchymal tumor (PMT) is a rare disorder primarily affecting the extremities. It is notable for its correlation with hypophosphatemic osteomalacia and high FGF23 serum levels, which results in renal phosphate wasting and clinical symptoms associated with low serum phosphorus. We presented a patient with a 5‐year history of progressive osteomalacia who recently experienced a major pathological bone fracture. Laboratory findings showed a persistent low serum phosphate, normal calcium, elevated alkaline phosphatase activity, high parathyroid hormone levels, and increased renal excretion of phosphate. According to ultrasonography and nuclear imaging, there was no evidence of parathyroid adenoma. During further diagnostic assessment, a sinonasal cavity tumor was found and resected. Histologically, the tumor was composed of bland spindle cell proliferation in the background of a calcified matrix with foci of osteoid formation, hemangiopericytoma‐like (HPC‐like) vasculature, and osteoclast‐like giant cells. Tumor cells showed variable positivity for SMA, but CD34, S100, CD99, Melan‐A, p63, and desmin were all nonreactive. Regarding the clinical context, histological and immunohistological findings, a final diagnosis of tumor‐induced osteomalacia (TIO) secondary to a PMT was made. After surgery, laboratory results returned to normal, clinical symptoms disappeared, and the patient did not experience a recurrence during a six‐month follow‐up