SAT210 A Case Of Recurrent Metastatic Parathyroid Cancer Complicated By Multiple Brown Tumors

Disclosure: A. Elmaaz: None. E. Bustamante: None. M. Batra: None. Introduction: Parathyroid cancer is a rare malignancy with about 25 % incidence of metastasis. Brown tumors are benign bone lesions caused by excessive bone turnover which are rarely associated with parathyroid carcinoma. Here, we present a case of recurrent metastatic parathyroid cancer complicated by multiple brown tumors. Case Presentation: A 63-year-old male presented with back pain, generalized weakness, and significant weight loss. Laboratory findings showed hypercalcemia 14.4 mg/dl, Alkaline phosphatase 253 U/L, and phosphorus 2.3 mg/dl. He was admitted to the hospital for hypercalcemia workup. MRI spine showed a heterogeneous mass arising from the left thyroid lobe and conglomerate lymph node, along with extensive metastatic sclerotic lytic lesions throughout the spine. MRI brain revealed multiple metastatic lesions in the skull base, facial bones, and the right angle of the mandible. Further workup revealed normal PSA, normal serum levels of free lambda and kappa light chains, and normal urine and serum protein electrophoresis. PTH levels, 2435.1 (8.7-77.1) pg/ml, and PTH-rP levels <1.1 pmol/l were noted. Ultrasound-guided FNA of the thyroid mass showed solid growth of malignant parathyroid cells strongly stained for PTH. Biopsy of the body of the T12 vertebra and mandibular lesions showed giant cell lesions of the bone with fibrous dysplasia, consistent with brown tumors. IV hydration, calcitonin, cinacalcet, and IV zoledronic acid started, followed by left hemithyroidectomy and left inferior parathyroidectomy. Surgical pathology was positive for parathyroid carcinoma invading the thyroid gland. His calcium and PTH levels post-surgery significantly improved. A few months later, the patient presented again with hypercalcemia and high levels of PTH. Sestamibi parathyroid scan showed no evidence of parathyroid adenoma. Because of enlarging left neck mass, confirmed by MRI neck, he then underwent excision of the left subcutaneous cervical lesion involving the left sternomastoid muscle. Surgical pathology was consistent with parathyroid carcinoma involving soft tissue. His calcium and PTH levels improved on subsequent follow-up visits. Conclusion: Parathyroid carcinoma is not a common cause of hypercalcemia. Though a rare entity, parathyroid cancer should be considered in the differentials of hypercalcemia and extremely high levels of PTH. Metastasis either locally or remotely should not be overlooked. Brown tumors could mimic bone metastasis and giant cell tumors; therefore, the clinical presentation is important for the diagnosis. Presentation: Saturday, June 17, 2023