Distinguishing parathyroid and thyroid lesions on ultrasound-guided fine-needle aspiration: A correlation of clinical data, ancillary studies, and molecular analysis.

BACKGROUND: Differentiating parathyroid and thyroid lesions can be challenging because of considerable morphologic overlap and anatomic proximity. Therefore, the authors sought to identify characteristic morphologic patterns and useful adjunct tests to distinguish these 2 entities. METHODS: A search was conducted in the study institution database for clinically indeterminate thyroid nodules from 2000 through 2016 with an emphasis on confirmed parathyroid nodules. Pathology reports, slides, ancillary studies, molecular analysis, and clinical and radiologic data were retrieved. RESULTS: A total of 143 cases of clinically indeterminate thyroid nodules were identified; 34 of these were confirmed parathyroid nodules. Three cytologic patterns were identified: 1) oncocytic cell pattern (9 cases; 26%); 2) follicular lesion of undetermined significance-like/papillary-like pattern (14 cases; 41%); and 3) nonspecific endocrine cell clusters (11 cases; 32%). Bare oval nuclei (100%), nuclear overlap (88%), crowded sheets (88%), and intracytoplasmic vacuoles (62%) were observed. Ten cases (29%) demonstrated positive immunostaining for parathyroid hormone (PTH), 7 cases (21%) demonstrated a positive PTH assay, and 9 cases (26%) had PTH detected by ThyroSeq v.2. The remaining 8 cases were morphologically either indeterminate or suggestive of parathyroid origin. The cytologic diagnosis was confirmed clinically (20 cases) or surgically (14 cases). Based on cytology alone, 8 cases initially were diagnosed as thyroid tissue and amended to parathyroid lesion after ancillary studies were performed, including 5 cases based on ThyroSeq v.2 results alone. CONCLUSIONS: Lesions with follicular lesion of undetermined significance-like or oncocytic features are prone to misdiagnosis. The current study identified distinct cytologic patterns in parathyroid lesions suggestive of parathyroid origin, which, together with PTH immunostains or assay, molecular studies, or sestamibi scans, aid in distinguishing parathyroid from thyroid lesions. Cancer Cytopathol 2017;125:674-82. © 2017 American Cancer Society.