Identification of parathyroid tissue in thyroid fine-needle aspiration: A combined approach using cytology, immunohistochemical, and molecular methods.

OBJECTIVES: Parathyroid (PT) lesions can be difficult to recognize in thyroid fine needle aspirations (FNAs), and when not identified correctly, PT cells may be mistaken for potentially abnormal thyroid cells. We therefore studied the utility of combining cytology, immunohistochemistry, and a molecular classifier to identify PT cells in thyroid FNAs.

METHODS: Thyroid FNAs were received in CytoLyt, and were evaluated initially using The Bethesda System for Reporting Thyroid Cytology (TBSRTC). The PT molecular classifier was performed along with the Afirma Gene Expression Classifier (GEC) on samples with indeterminate cytology. Immunohistochemistry (IHC) for PT was performed on all samples using Cellient cell block sections. Clinical and ultrasound information was collected, when available.

RESULTS: PT tissue was identified in 60 thyroid FNAs. Forty-seven (47) samples had cytologic features that were suggestive of PT cells, and were subsequently confirmed with IHC. Thirteen (13) samples were not recognized as PT, and were considered to be either Bethesda III or IV indeterminate thyroid nodules; a PT gene expression signature was subsequently detected by the GEC. These samples were also confirmed as PT by IHC. Clinical and ultrasound features were suggestive of a PT lesion in only a third of cases.

CONCLUSIONS: Cytologic features, coupled with IHC, can identify intrathyroidal PT cells in the majority of CytoLyt samples. However, a significant minority (22%) of these FNAs may be misclassified as indeterminate by TBSRTC criteria, and molecular detection of the PT tissue can be helpful to potentially avoid an additional biopsy or diagnostic surgery. Diagn. Cytopathol. 2017;45:526-532. © 2017 Wiley Periodicals, Inc.