O-28 HYPOPHOSPHATEMIC ONCOGENIC OSTEOMALACIA: A CASE REPORT

Abstract Introduction Oncogenic osteomalacia is a rare paraneoplastic syndrome characterized by persistent hypophosphatemia. It can cause prolonged bone and muscle pain, muscle weakness, walking disturbances, and fragility fractures in adults. Various mesenchymal tumors such as hemangiopericytoma, osteosarcoma, and giant cell tumor can cause oncogenic osteomalacia. The mesenchymal tumors causing the syndrome are difficult to locate due to their small size, slow growth, and frequent occurrence in various hidden anatomical areas such as long bones, distal extremities, nasopharynx, sinuses, and groin. We summarize our case, who was followed up for hypophosphatemic osteomalacia for a long time and later found to have a mesenchymal tumor in the sphenoid bone. The case presentation followed at the Department of Endocrinology and Metabolism Diseases at Eskişehir Osmangazi University between 2013 and 2024 is summarized. Clinical Case A 34-year-old male patient presented in 2013 with an atypical femoral neck fracture following low trauma. The patient had widespread bone pain. Severe levels of hypophosphatemia and phosphaturia were detected in laboratory tests, and the patient was evaluated as having hypophosphatemic rickets. Genetic testing was conducted, and no significant mutation was detected. With a preliminary diagnosis of oncogenic osteomalacia, CT and PET scans were performed, but no tumor focus was detected. FGF-23 levels could not be evaluated as it was not studied in our hospital. The patient was followed up with phosphate Sandoz and vitamin D supplements. It was learned that his symptoms did not decrease during follow-ups, and his tolerance to phosphate Sandoz was low due to diarrhea, but no new fractures were observed. No tumor focus was found during the patient's 11-year follow-up. In the patient's 2023 presentation, physical examination showed a decrease in height, ongoing complaints, poor medication compliance, and severe levels of hypophosphatemia. Repeat imaging was performed to find the tumor focus. Brain MRI reported as “There is a nodular lesion approximately 25x20 mm in size with homogeneous contrast enhancement in the right temporobasal area.” Temporal Bone CT assessed as “Soft tissue mass creating lysis-expansion in the squamous part of the right temporal bone, extending to the mastoid cells and tegmen mastoideum and causing lysis in the tegmen.” Ga-68 DOTATATE PET/CT: reported as “A focal hyperactive appearance measuring 23 x 19 mm is observed in the right mastoid bone region (SUV Max: 55.71).” Evaluated in a council by Neurosurgery, ENT, Endocrinology, an operation was planned due to the mass in the temporal bone. A week after the operation, the patient's phosphate level increased to 2.5 mg/dl. The pathology result was consistent with a phosphaturic mesenchymal tumor. The patient's follow-up and treatment continue.Figure 1:Tumor lesion with low to moderate cellularity, spindle cell proliferation, and prominent vascularity (H&E, x200) Table 1:the patient's preoperative and postoperative laboratory data