(A507) An Unexpected Culprit: The Rare Clinical Puzzle of Parathyroid Carcinoma

Introduction: Parathyroid carcinoma (PC) is a rare head and neck malignancy that presents in < 1% of primary hyperparathyroidism (pHPT) cases. Its clinical presentation often overlaps with more common causes of hypercalcemia, leading to delays in diagnosis and management.

Methods: A 61 year old female with a relevant medical history of osteoporosis presented with right knee pain. Her lab work on admission included a calcium of 17.4 mg/dL, a phosphate of 5.6 mg/dL and a PTH >5,000 pg/dL. Differential diagnosis included pHPT, parathyroid neoplasm and multiple myeloma. She denied any family history of malignancy or personal history of kidney stones/psychiatric issues/constipation. CT of her neck demonstrated a large mass at the right inferior lobe of the thyroid. An x-ray of her skull showed sclerotic and osteolytic bony lesions consistent with hyperparathyroidism. Post-excisional pathology included findings consistent with PC, such as necrosis, capsular invasion, reverse polarization of the cells, and negative parafibromin staining. Complete resection of the mass in this patient was achieved and she is doing well

Results: This case highlights the diagnostic difficulty in identifying parathyroid carcinoma, which can masquerade as more common parathyroid disorders. Symptoms of hypercalcemia, such as bone pain and weakness, are often misinterpreted, delaying accurate diagnosis. Imaging and biochemical markers play a crucial role, but definitive diagnosis relies on histopathological examination following surgical resection. The loss of expression of parafibromin, seen in our patient, often suggests a CDC73 mutated tumor, but mutation analysis is required for confirmation.

Conclusions: The rarity of parathyroid carcinoma and symptomatic overlap with benign conditions underscore the need for surgeons to have a high index of suspicion in patients presenting with severe hypercalcemia.