Background Fine-needle aspiration (FNA) for thyroid nodules is the most important method for determining a diagnosis. The system for reporting results is based on a cytopathologic classification that stratifies the risk of malignancy. Methods We retrospectively studied 197 patients who underwent FNA for diagnostic evaluation of a thyroid nodule and had their results reported as a follicular lesion of undetermined significance (FLUS) using the Bethesda classification system. The objective of the study was to analyze the incidence and histopathologic types of malignancy in these cases. Results The final histopathologic breakdown is as follows: 65 cases (32.9%) of follicular adenoma, 81 cases (41.1%) of microfollicular adenomatoid nodule, 19 cases (9.6%) of microfollicular adenomatoid nodule on the background of thyroiditis, 17 cases (8.6%) of follicular carcinoma, 9 cases (4.6%) of follicular variant papillary carcinoma, and 6 cases (3.1%) of classic papillary carcinoma, for a 16.2% incidence of malignancy. Beyond these diagnoses in the FNA-biopsied nodules, we observed 29 cases (14.7%) of incidental ipsilateral papillary thyroid microcarcinoma (PTM) and 13 cases (6.6%) of incidental contralateral thyroid lobe PTM. Conclusions This study observed a 16.2% incidence of thyroid cancer in the nodule designated FLUS compared to the 5 to 15% rate reported by the Bethesda FNA classification. The overall incidence of incidental PTM in the thyroid gland was 21.3%. These data support considering surgical intervention for at least diagnostic purposes in a patient with the FNAB diagnosis of FLUS.
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