Parathyroid Cancer

Excerpt

Parathyroid carcinoma is a rare, malignant neoplasm originating from the parathyroid gland. The normal weight and size of the parathyroid glands vary. The average weight is about 60 mg. The average dimensions of each gland are 5 mm in length by 3 mm in width and 1 mm in thickness. The color of normal parathyroid glands is a yellow-brown color. There are generally 4 parathyroid glands, 2 superior and 2 inferior glands; they are located on the posterior and lateral surface of the thyroid gland. The location of the glands may vary based on the embryological descent during development. The inferior parathyroids and the thymus both develop from the third branchial pouch, whereas the parafollicular C cells and the superior parathyroids develop from the fourth branchial pouch. The superior parathyroid glands are frequently found near the cricothyroid junction, just superior to the intersection of the recurrent laryngeal nerve and the inferior thyroid artery. The superior parathyroids are closely associated with the posterior capsule of the superior thyroid pole. The inferior parathyroids have a more variable location, and greater than 50% are located at the inferior thyroid pole, but they can be located in the anterior mediastinum along the thyrothymic ligament. Rarely there is the presence of a supernumerary gland. The association of the parathyroid glands to the recurrent laryngeal nerve is an important anatomical relationship. The superior parathyroids are deep to the recurrent laryngeal nerve, and the inferior parathyroids are superficial.

In most cases, each parathyroid gland's arterial supply is derived from the inferior thyroid artery, which is a branch of the thyrocervical trunk. In 20% of cases, it may derive its blood supply from the superior thyroid artery, which is a branch from the external carotid artery. There is generally rich anastomosis between the parathyroids, larynx, pharynx, esophagus, and trachea. The venous drainage parallels the arterial vessels and drains into the internal jugular. The lymphatics of the parathyroid drain into the deep cervical and pretracheal lymph nodes, similar to the lymphatic drainage of the thyroid gland.

The major functional cells of the parathyroid are the chief cells, which contain many cytoplasmic secretory granules. These are responsible for producing parathyroid hormone. The second cell type that makes up the parathyroid parenchyma is the oxyphil cell. Their function is unknown, but they tend to be rich in mitochondria. Parathyroid glands with high concentrations of oxyphil cells may be more prone to hyperfunction.

As with all malignancy, parathyroid carcinoma is a result of the uncontrolled or unregulated growth of the parathyroid cells, and most parathyroid carcinomas are functional - meaning they secrete parathyroid hormone (PTH). Parathyroid carcinoma can occur in any of the parathyroids and does not appear to have a predilection for either the superior or inferior glands (or supernumerary glands), though the rarity of the disease precludes any definitive conclusions. The hallmark characteristic is a very high serum PTH level, often into the thousands, which is rare in benign hyperparathyroidism.