Incidence of Malignancy in Thyroid Nodules Determined to be Follicular Lesions of Undetermined Significance on Fine‐Needle Aspiration

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.