SAT-819 - Mediastinal Parathyroid Carcinoma Discovered after Persistent Hyperparathyroidism Post Parathyroidectomy in A Patient with ESRD on Hemodialysis

Background
Parathyroid carcinoma (PC) is a rare cause of hyperparathyroidism (0.5-5% of primary
hyperparathyroidism). Diagnosis is challenging in ESRD due to chronically elevated PTH,
delaying recognition of malignancy. Extreme PTH elevation (>2000 pg/mL), postoperative
persistence, and ectopic localization should raise concern for PC.
Case Presentation
A 53-year-old man with ESRD on hemodialysis and failed renal transplant presented with severe
hyperparathyroidism (PTH 1800 pg/mL) and symptomatic hypercalcemia. He underwent
subtotal parathyroidectomy (3.5 glands removed), but PTH level increased to 2300 pg/mL.
4D CT revealed a mediastinal mass; FNA confirmed parathyroid tissue. Mediastinal resection
demonstrated 3.7 cm parathyroid carcinoma with multifocal angioinvasion and Ki-67 12.33%,
suggestive of aggressive behavior. Postoperative PTH decreased 83%.
Postoperative course was complicated by hungry bone syndrome requiring IV calcium and
calcitriol supplementation. Genetic testing including CDC73 was negative.
He was discharged on oral calcium and calcitriol. PTH level gradually decreased to 300-400. We
adjusted calcium and calcitriol dose to avoid over suppression of PTH.
Patients case was discussed in a tumor board session and the decision was made to follow PTH
and calcium levels every 3 to 6 months and obtain PET scan if persistent elevation in the PTH
level is noted.

Discussion:
Diagnosis of ectopic parathyroid carcinoma can be challenging as it is a rare pathology which
can be missed in a regular imaging like Sestamibi scan or thyroid ultrasound. In this particular
case persistently high PTH after parathyroidectomy led to obtaining neck 4D-CT scan and tumor
resection. The only way to confirm the diagnosis of parathyroid carcinoma is histologic
evaluation of the tumor. Pleomorphic nuclei, macronuclei, abnormal mitoses and Ki-67 above
5% are suggestive of parathyroid carcinoma. Clinical follow up mainly focuses on measuring
PTH and serum calcium every 3 to 6 months. It is also recommended to perform a whole body
4D CT-scan and neck ultrasound 3-6 months after the surgery to look for metastasis.

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