MON-LB058 Mediastinal Parathyroid Adenoma Presenting as a Neuroendocrine Tumor

Introduction: Tertiary hyperparathyroidism caused by renal failure can lead to ectopic parathyroid tissue hyperplasia creating a diagnostic challenge. Here we describe a case of mediastinal parathyroid hyperplasia presenting as a neuroendocrine tumor. Case Presentation: A 40-year-old male presented for endocrine workup of a possible mediastinal paraganglioma. At age 22 he was diagnosed with resistant hypertension followed by rapidly progressive renal failure. Hemodialysis was initiated following a failed kidney transplant at age 30. He was treated by nephrology for secondary hyperparathyroidism with calcitriol and cinacalcet and serum calcium was within the normal range. Parathyroid hormone (PTH) was 598 pg/mL (12-65 pg/mL). The patient developed shortness of breath and a right pleural effusion while awaiting a second transplant and a CT of the chest was completed. This revealed a 3.1x4.0x5.0 cm heterogenous mediastinal mass within the superior thorax, medial to the proximal thoracic esophagus, and posterior to the trachea at the level of T2. It did not communicate with the thyroid gland. An FNA of the mass showed cohesive groups of atypical cells with small uniform nuclei and a moderate amount of chromatin arranged in nests and acini. No mitotic figures were identified. Cells were positive for chromogranin and focally for CK7, and negative for CK20, synaptophysin, and TTF-1. Cytology was interpreted as a well differentiated neuroendocrine tumor/typical carcinoid. A Galium-68 DOTATATE scan revealed moderate tracer uptake in the mass only. His endocrine workup was notable only for a mildly elevated chromogranin A with normal serum catecholamines, calcitonin, and serotonin, as well as normal adrenal cortical function. The patient underwent thoracotomy with mediastinal mass resection. He developed severe postoperative hypocalcemia treated with calcium infusion and increased calcitriol doses. Pathology revealed a low-grade neuroendocrine tumor consistent with parathyroid hyperplasia with at least one lymph node positive for involvement. The tumor showed positive staining for PTH, chromogranin, and synaptophysin with Ki67 <1%. Conclusion: This case demonstrates a rare occurrence of mediastinal parathyroid hyperplasia related to tertiary hyperparathyroidism of renal failure. Early calcitriol replacement and PTH level monitoring represents the mainstay of prevention of tertiary hyperparathyroidism. Severe post-op hypocalcemia is a hallmark of disease and more extensive immunostaining of mediastinal masses should be applied in cases suggestive of parathyroid etiology, especially in patients with long-standing renal failure. Despite a low Ki67, lymph node involvement suggests the possibility of parathyroid carcinoma. Follow-up Gallium-68 DOTATATE scan and parathyroid scan are imperative to evaluate for recurrence or residual disease. Unless otherwise noted, all abstracts presented at ENDO are embargoed until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. Abstracts presented at a news conference are embargoed until the date and time of the news conference. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.