O-25 THE MASQUERADE OF BONES: A CASE REPORT OF BROWN TUMOURS CONFOUNDING DIAGNOSES

Abstract Introduction Brown tumour (osteitis fibrosa cystica) is a rare, benign bone lesion resulting from excess osteoclastic activity in hyperparathyroidism. Although typically found in the pelvis, femur, and ribs, it can affect any bone and radiologically mimic skeletal metastases or multiple myeloma. Clinical Case A 39-year-old woman with no significant history presented to the emergency department with right lower abdominal pain, vomiting, and fever. She was septic, and CT revealed right-sided pyelonephritis with hydronephrosis from an obstructing mid-ureteric stone. Incidental findings included a 21 mm left adrenal nodule, a 15 mm endometrial mass, and multiple lytic bone lesions in the vertebrae, pelvis, and scapulae. She underwent emergency percutaneous nephrostomy and improved with treatment for urosepsis due to hypercalcaemic nephrolithiasis. Her presentation biochemistry as table 1, showed severe hypercalcaemia (corrected calcium 15.4 mg/dL) and markedly elevated parathyroid hormone (PTH 2099.1 pg/mL). She received intravenous zolendronate and calcitonin, with biochemical improvement, and was started on a loading Vitamin D3 course. Parathyroid SPECT-CT showed a 3.5 cm partially cystic mass at the inferior pole of the left thyroid lobe, with tracer uptake confirming localisation. Ultrasound supported these findings. Due to significantly elevated PTH and imaging features, parathyroid carcinoma was also considered as a differential. After exclusion of pheochromocytoma (normal plasma metanephrines), she underwent urgent left inferior parathyroidectomy and hemithyroidectomy. Histology revealed completely excised cystic parathyroid lesion without malignancy features. One week post-operation, she developed paraesthesia with hypocalcaemia (8.0 mg/dL), managed with increase daily oral calcium intake to 2 grams. PTH initially remained elevated (207 pg/mL), attributed to hungry bone syndrome and vitamin D deficiency. At follow-up, she had persistently elevated PTH (296.1 pg/mL) with normocalcaemia. Bone profile revealed severe hypophosphataemia (