Refractory Hypercalcemia Secondary to Metastatic Parathyroid Carcinoma Treated With Immunotherapy.

Parathyroid carcinoma (PC) is a rare endocrine malignancy and an uncommon cause of primary hyperparathyroidism. Metastatic disease confers a guarded prognosis with limited systemic treatment options available. We describe a case of a 64-year-old woman with primary hyperparathyroidism secondary to PC. Despite initial surgical resection, the patient relapsed within 6 months with widespread cerebral and skeletal metastatic disease. She developed worsening parathyroid hormone-mediated hypercalcemia that was refractory to escalating doses of cinacalcet and antiresorptive therapy. Molecular genomics identified high tumor mutation burden within the malignant tissue and single-agent nivolumab immunotherapy was administered. After one dose, there was resolution of her refractory hypercalcemia and primary hyperparathyroidism. The patient has tolerated ongoing treatment with 3 weekly cycles of nivolumab. She remains in biochemical remission as of June 2024, which is now 12 months after commencement of nivolumab.