Radiology Quiz Case 1: Diagnosis

Parathyroid cysts are rare, often asymptomatic lesions that are frequently discovered incidentally. As a result, they are very low on, or absent from, the list of differential diagnoses of a neck mass. In cases of incidental discovery of the lesion and without associated signs or symptoms of hyperparathyroidism, PCs present a clinical diagnostic problem. Diagnosis is often made intraoperatively or postoperatively in these cases, either by frozen section of the mass or by postresection pathologic analysis. This delay is undesirable given the wide differential diagnosis and range of treatments for neck masses.

Radiologic evaluation is essential for a preoperative diagnosis. In this case, fine-needle aspiration of the cyst contents was performed under ultrasonographic guidance after computed tomograms and magnetic resonance images revealed a cystic lesion in the lower part of the left side of our patient's neck. Ultrasonograms showed a unilocular cystic lesion near the inferior pole of the left thyroid lobe. Fine-needle aspiration produced 10.5 mL of clear fluid. Microscopic examination of the fluid revealed proteinaceous material with a few crystalline particles and tissue macrophages. Based on the needle biopsy findings, the mass was thought to be a PC, a small cystic hygroma, or a cystic thyroid nodule. The aspirate was submitted for parathyroid hormone (PTH) assay, and the results showed that the level of PTH was highly elevated (5590 pg/mL [to convert to nanograms per liter, multiply by 1]), confirming the diagnosis of PC. The patient was then taken to the operating room for excision of the left lower parathyroid gland and associated cyst. The surgical specimen was 2.5 × 2.0 × 1.1 cm and weighed 6.4 g. Representative histopathologic sections are shown in Figure 4.

Parathyroid cysts typically occur in the fourth and fifth decades of life and more often in women than men, with a 3:2 ratio.¹ They can be found anywhere between the jaw and the mediastinum but are most commonly located on the inferior parathyroid glands, where they demonstrate a left-sided predominance.^{1 - 2} Mediastinal PCs have been reported in up to 30% of cases and generally involve the anterior mediastinum in association with normal thymic tissue.³

The pathogenesis of PCs is unknown, but several theories have been proposed. First, the cyst may arise from embryological remnants of the third or fourth branchial clefts. Secretion of colloid material containing PTH into a cell-lined cavity may result in the formation of a cyst over time.^{1 ,4} Second, PCs may represent a degenerated parathyroid adenoma.¹ After formation of a secretory adenoma, involution of a portion of the lesion may result in the formation of a cavity containing proteinaceous material and PTH. Finally, a macroscopic PC may be formed by the coalescence of many microcysts. This theory is attractive given the high prevalence of microcysts in normal individuals at autopsy. Although macroscopic PCs are rare, microcysts are common in the aging parathyroid gland, and the incidence of parathyroid cysts at autopsy has been reported to be as high as 40% to 50%.^{1 ,4 - 5}

Parathyroid cysts rarely manifest systemic disease, and fewer than 10% are associated with hyperparathyroidism and subsequent hypercalcemia.⁵ Cyst fluid often contains elevated levels of PTH, although serum PTH levels are within normal limits in most cases. In 1 review of 37 patients with PCs, the intact PTH level in the cystic fluid ranged from 81 to 2000 pg/mL (mean, 552 pg/mL), although all of these cysts were nonfunctional as demonstrated by normal serum calcium, phosphorus, and intact PTH levels.⁶ In functioning PCs, PTH levels in the cystic fluid can reach several million picograms per milliliter.⁷ Functional PCs have been associated with elevated serum PTH levels and malignant hypercalcemia, producing kidney stones, weakness, confusion, and even brown tumors (osteitis fibrosa cystica) of the maxilla and tibia.^{3 ,8}

Parathyroid cysts commonly measure between 3 and 5 cm in diameter; however, masses as large as 12 cm have been reported. Larger cysts can impinge on local structures and may produce symptoms of dyspnea, dysphagia, chest pain, or hoarseness. Thrombosis and compression of the left innominant vein have been reported.³ Diagnosis can be made on histopathologic examination of the excised lesion, which may demonstrate a characteristic thin, membranous lining of cuboidal or columnar epithelium with parathyroid tissue within the cyst wall.¹

Definitive treatment of a PC is surgical resection of the cyst and associated parathyroid tissue. For cysts occurring in the neck, the approach and removal of the cyst are essentially the same as those of a standard selective parathyroidectomy. Mediastinal lesions, however, may require the assistance of a thoracic surgeon. Some authors have suggested that aspiration alone is adequate treatment of a nonfunctional cyst^{6 ,9} ; however, 1 study found that 3 of 4 patients who were treated with aspiration alone required multiple aspirations.¹⁰ Finally, injection of sclerosing agents such as tetracycline has been suggested, particularly for recurrent cysts that had previously been treated with aspiration alone. This procedure, however, carries the risk of misdirected injection of the sclerosing agent and subsequent injury to normal thyroid tissue or the recurrent laryngeal nerve. The sclerosed cyst may undergo fibrosis, with a poor cosmetic result.¹

In conclusion, PCs are a rare cause of neck masses. Cystic lesions in the neck and mediastinum should be subjected to fine-needle aspiration and determination of PTH level. Although PCs are rarely functional, treatment is indicated to arrest growth of the lesion, to prevent potential mass effect complications such as airway compromise, and to preserve the normal contour of the neck. We believe that surgical resection is the best treatment for PCs.

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