Large non-functioning parathyroid cysts: our institutional experience of a rare entity and a possible pitfall in thyroid cytology.

OBJECTIVE: Large non-functioning parathyroid cysts represent a rare entity with a benign clinical course. They may be misdiagnosed as thyroid cystic neoplasms on fine needle aspiration cytology (FNAC), resulting in inappropriate surgical treatment. We evaluated our institutional experience in the diagnosis of large parathyroid cystic lesions underlining all the differential diagnoses and pitfalls. METHODS: In the period between 1998 and 2012, we reported the cytology of eight large (>2.5 cm) parathyroid cystic lesions (all female patients) with histological control. The aspirations were performed with a 25-gauge needle with ultrasonographic guidance. The aspirated material was processed with liquid-based cytology (LBC). All the patients had normal serum parathyroid hormone (PTH) and calcium. RESULTS: The cytological samples showed a fluid watery component without colloid and few or absent epithelial cells. The resulting negativity for thyroglobulin and positivity for PTH, carried out on the cystic fluids, suggested parathyroid lesions rather than either thyroid cystic lesions (including follicular thyroid neoplasm) or cystic malignant lesions. All the patients underwent surgery without complications. CONCLUSIONS: To the best of our knowledge, this is one of the largest series with cytohistological evaluation of large parathyroid cysts. The incidence of large parathyroid cysts remains controversial as most patients are asymptomatic. FNAC may be performed with conclusive results in the majority of cystic cases. The detection of PTH and calcium on the cystic liquid is likely to achieve a correct cytological diagnosis, allowing adequate treatment and ruling out a more frequent thyroid lesion.