Parathyroid carcinoma is a rare malignancy of the parathyroid glands. These tumors usually secrete parathyroid hormone, thereby producing hyperparathyroidism, which is usually severe. Parathyroid carcinoma may be suspected, but it usually cannot be confirmed prior to operation. This complicates the treatment strategy. 
Parathyroid carcinoma causes hyperparathyroidism (for a more complete discussion see Hyperparathyroidism) in all but a few rare cases of nonfunctional parathyroid carcinoma. Hyperparathyroidism is caused by overproduction of parathyroid hormone. In most cases, hyperparathyroidism is caused by a single benign adenoma (approximately 85%). The vast majority of the remaining cases are caused by parathyroid hyperplasia or multiple adenomas. In less than 1% of cases of hyperparathyroidism, the etiology is parathyroid carcinoma.
The hyperparathyroidism seen in parathyroid carcinoma is usually severe, with high serum calcium levels, severe bone disease, and renal stones. Rarely, parathyroid cancer can be nonfunctional, that is, it may cause a tumor but not produce parathyroid hormone and the sequelae of hyperparathyroidism. Overproduction of authentic parathyroid hormone from nonparathyroid sources is extremely rare but has been reported. Modern assays for parathyroid hormone measure the intact, authentic molecule and do not detect similar molecules produced by other tumors. Therefore, an elevated parathyroid hormone level is virtually always parathyroid in origin.
Untreated, parathyroid carcinoma leads to severe symptoms of hyperparathyroidism and may cause renal stones, nephrocalcinosis, pathologic fractures, and neuropsychiatric symptoms. The diagnosis is often not made prior to parathyroidectomy. This results in inadequate tumor excision and a propensity to local recurrence. Regional lymph node and distant metastases are usually not present at initial diagnosis. Recurrences may be treated by local excision or ablative treatments such as radiofrequency ablation. Death is usually caused by medically refractory hypercalcemia and seldom tumor burden alone.
A large European study also demonstrated the rarity of parathyroid carcinoma with an estimated incidence of 2 cases per 10,000,000 persons/year. 
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A significant number of patients continue to die of recurrent disease many years after the initial treatment.
Permanent cure is achieved in the minority of patients.
No known racial predilection exists.
Parathyroid cancer usually occurs in patients older than 30 years. After that, no predominant age association has been noted.
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