ThinPrep for cytologic evaluation of follicular thyroid lesions: correlation with histologic findings

Fine needle aspiration (FNA) is often the first step in management of a thyroid nodule. Although papillary carcinomas have distinctive features on conventional smears as well as ThinPrep preparations, cytopathologists rely on the architectural arrangement of cells to classify follicular lesions. The accuracy of ThinPrep in this regard has not been determined. We reviewed all thyroidectomy specimens from a 22-mo period that showed a follicular lesion and had one or more preoperative FNA. The architecture of the lesions on histology was classified as predominantly (>70%) macrofollicular (MA), predominantly microfollicular (MI), mixed (MX), or cystic (C). The presence of colloid, Hürthle cell features, cystic change, Hashimoto's thyroiditis, and nonspecific lymphocytic thyroiditis were also recorded. All FNA specimens were processed with the ThinPrep(R) method and were categorized as nondiagnostic, benign (MA or MX), indeterminate (due to suboptimal cellularity), or suspicious (consistent with a microfollicular or Hürthle cell neoplasm) based on cellularity, architectural arrangement of the follicular cells, and the presence or absence of colloid on FNA. Those cytologic specimens in the indeterminate category were subcategorized as suggestive of an MA, MX, or MI lesion if possible. Histocytologic findings were correlated using Fisher's exact test. A total of 95 patients with 115 FNAs were included in the study (mean age: 48 yr; 75 females and 20 males). Seven of the FNAs were nondiagnostic, 23 benign, 69 indeterminate (5, 8, and 30 favor MA, MX, and MI, respectively, 6 cyst contents and 20 not subclassifiable) and 16 suspicious. The cytohistologic correlation for architecture was significant (P = 0.003). The cytohistologic correlation of cystic change, Hürthle cell change, and the presence of colloid (large fragments of colloid and "tissue-paper-like material" on cytology vs. abundant colloid on histology) was highly significant (P < 0.001, < 0.001, and < 0.03, respectively). In conclusion, thyroid FNA with ThinPrep is useful in predicting the architectural pattern of follicular lesions. It is also reliable in predicting cystic change, Hürthle cell change, and the presence of colloid. Watery colloid is probably present as "tissue-paper-like material" in thyroid FNA ThinPrep preparations.