Presentation of 6 cases with parathyroid cysts and discussion of the literature.

INTRODUCTION: Cystic lesions of the parathyroid glands are uncommon, and rare are those that cause primary hyperparathyroidism. Preoperative diagnosis can be challenging and some of these tumors might be misinterpreted as parathyroid carcinoma. With an expertise of more than 1700 patients operated on primary hyperparathyroidism, we present six cases with cystic degeneration of a parathyroid gland causing primary hyperparathyroidism in five patients. CASE REPORTS: A woman at the age of 67 presented with hypercalcaemic crisis due to persistent primary hyperparathyroidism after an operation four years ago. As cervical exploration was unsuccessful, sternotomy was performed and a cystic adenoma of a parathyroid gland could be resected from the anterior mediastinum. The second patient - a 63-year-old female with severe hypercalcaemic crisis, operated on under suspicion of a parathyroid carcinoma - had a functional cyst of the parathyroid gland with a parathyroid hormone level of 700,000 pg/ml in the aspirated fluid. Third, operation on a 70-year-old woman with a benign euthyreot goiter and the laboratory findings of primary hyperparathyroidism revealed a cystic adenoma adjacent to the thyroid gland, whose aspirate had a parathyroid hormone level of 1,500,000 pg/ml. In the fourth case of a 67-year-old female with an adenoma of the right inferior parathyroid gland localized by ultrasonography, the cystic parathyroid adenoma was operated on by video-assistance. A cystic structure in the upper mediastinum was diagnosed in the fifth patient, a 66-year-old woman. It was suspected to be a thyroid cyst at the left-lower pole of the thyroid gland. After hemithyroidectomy pathological evaluation revealed a large parathyroid cyst. The last case of a 56-year-old male illustrates the extensive preoperative work-up of a patient with primary hyperparathyroidism who was preoperatively diagnosed as having a thyroid cyst. Final histopathological examination exposed multiple gland disease with a parathyroid adenoma as well as a cystic parathyroid gland. DISCUSSION: Cystic adenomas of the parathyroid glands are often misdiagnosed as thyroid cysts or - in the case of extremely elevated parathyroid hormone levels - even as parathyroid carcinoma. The routine preoperative diagnostic tools, such as ultrasonography or (99m)Tc-sestamibi-scintigraphy, cannot clearly distinguish between these entities and might be jeopardized by mediastinal localization, which is not uncommon in parathyroid adenomas with cystic degeneration.