Parathyroid cytology: avoiding diagnostic pitfalls.

Background: Interpretation of parathyroid fine-needle aspirates (FNA) remains problematic not only because this type of specimen is rare but also because the pertinent literature is very limited. We systematically reviewed parathyroid FNAs in our files and sought to delineate additional diagnostic criteria.

Design: Review of all thyroid and parathyroid aspirates from January 1990 to June 1998 disclosed 12 parathyroid lesions. The final diagnoses included four parathyroid adenomas, one intrathyroidal hyperplastic parathyroid, one intrathyroidal parathyroid adenoma, one atypical parathyroid adenoma (all confirmed by histologic screening or immunocytochemistry), and five parathyroid cysts (all confirmed by immunoassay). Papanicolaou and Diff Quik-stained smears of the parathyroid FNAs were reviewed. The cytologic features were compared and contrasted with those of thyroid FNAs to establish criteria for differential diagnoses.

Results: The FNAs of the five parathyroid cysts yielded virtually acellular fluid with a characteristic water-clear appearance and markedly elevated levels of parathyroid hormone. The remaining seven aspirates consisted of moderately cellular smears that showed an admixture of architectural features. Common patterns included cohesive three-dimensional groups, disorganized sheets, papillary fragments, microfollicles, and a single case showing lymphoidlike smears. Although the cells were generally small and round to oval, all cases demonstrated mild to moderate anisokaryosis. The nuclei were hyperchromatic E with coarsely granular chromatin reminiscent of that of small lymphocytes. Occasional nucleoli were noted. Although the cytoplasm was usually pale blue and finely granular with ill-defined borders, two cases showed well-delineated cytoplasmic membranes. Less common findings included cytoplasmic granulation, vacuolization, and rare oxyphilic cells. Naked nuclei were noted in the background of all of the aspirates to varying degrees. Other background findings included the presence of colloidlike material, macrophages, and lymphocytes. One interesting finding that to date has not been reported is the presence of nuclear overlapping (100%) and nuclear molding (71%), which is an uncommon finding in thyroid aspirates.

Conclusions: FNAs of the parathyroid can be easily confused with that of the thyroid, not only because of the clinical similarity between these two types of lesions but also because of the overlap in cytomorphologic features of the aspirated cells. Although no one single cytomorphologic feature is diagnostic, a combination of cytologic parameters noted earlier should raise the possibility of a parathyroid lesion. Aspirates of parathyroid cysts show acellular water-clear fluid with elevated parathyroid hormone measurements.