SAT201 A Rare Case Of A Mediastinal Atypical Parathyroid Adenoma Resulting In Severe Hypercalcemia And Brown Tumor

Disclosure: O.A. Aluko: None. D. Franey: None. R. Quiros: None. W. Burfeind: None. R.E. Amori: None. Introduction: Primary hyperparathyroidism is most often from parathyroid adenomas located usually in close association to the thyroid gland. Ectopic parathyroid glands result in significant variability in the position of the gland, however rarely in the mediastinum. We report a case of a patient with severe hypercalcemia from a large, ectopic mediastinal parathyroid adenoma. Case Report: A 53-year old male with a past medical history of hypercalcemia and nephrolithiasis was sent to the ER for evaluation of hypercalcemia after he was found to have a corrected calcium of 12.5 mg/dl (n8.4 - 10.2 mg/dl) on routine labs. Repeat blood work at presentation confirmed hypercalcemia with a corrected calcium 11.9 mg/dl and elevated ALP of 211 U/l (n 43 - 122 U/L), elevated initial PTH of799 pg/ml (n 18.4 - 80.1 pg/ml), Vit D 32.7 ng/ml (n 30 - 100 ng/ml), and low phosphorus 2.4 mg/dl (n2.7 - 4.5 mg/dl). Calcium slightly improved to 11.7 mg/dl after IV hydration. Chest CT showed a large mediastinal mass compressing trachea and esophagus, suspicious for parathyroid origin, with a smaller left sided lesion suspicious for parathyroid adenoma. CT also showed multiple bone lesions likely brown tumors. He underwent complete robotic resection of the mediastinal mass. Immediately prior to surgery, PTH was 1002.8 pg/ml, and fell to 131.1 pg/ml and 44.1 pg/ml following removal of the mediastinal mass. Adherence of the mass to surrounding structures made resection challenging, resulting in a right vocal cord paralysis. After surgery, he was empirically started on IV calcium and calcitriol for high clinical concern for hungry bone syndrome, both later discontinued as serum calcium remained within normal limits. Pathologic findings reported as highly suspicious, however not definitive for parathyroid carcinoma with no evidence of perineural or lymphovascular invasion. Given rarity, he was referred to a tertiary cancer center where he was diagnosed with Atypical parathyroid Adenoma (APA). He underwent a right pubic biopsy with pathology diagnosis of brown tumor of hyperparathyroidism. He started on cinacalcet 30 mg daily after surgery. There is a future plan for resection of the smaller left sided parathyroid adenoma. Conclusion: This case highlights two rare entities; a mediastinal ectopic parathyroid and atypical parathyroid adenoma. Atypical Parathyroid Adenoma is a not-well defined entity which could mimic a parathyroid carcinoma as seen in our case. The location of the mass in the superior mediastinum is unusual, making it more difficult to diagnose, and could increase risk of complications during resection. Delayed diagnosis of these entities can lead to severe hypercalcemia and skeletal complications infrequently seen in common primary hyperparathyroidism cases. Presentation: Saturday, June 17, 2023