Malignancy risk and characteristics of thyroid nodules with two consecutive results of atypia of undetermined significance or follicular lesion of undetermined significance on cytology

Purpose: To evaluate the malignancy risk and characteristics of thyroid nodules with two “Atypia of Undetermined Significance” or “Follicular Lesion of Undetermined Significance” (AUS/FLUS) results, and compare characteristics of malignancies with two AUS/FLUS results to those with one AUS/FLUS result. Methods: Of 441 thyroid nodules with initial AUS/FLUS results, 236 underwent repeat fine-needle-aspiration (FNA), with 58 obtaining repeated AUS/FLUS results. Thyroid Imaging Reporting and Data System (TIRADS) categories were assigned, and clinico-pathological characteristics were compared between benign and malignant nodules and between malignancies with two consecutive AUS/FLUS results and those with one AUS/FLUS result. Results: Thirty-one percent (18/58) of nodules with two AUS/FLUS results and 58.1 % (18/31) of confirmed nodules were malignant. Age, gender, nodule size, ultrasound features and TIRADS categories did not differ between benign and malignant nodules or between malignancies with one and two AUS/FLUS results. Malignancies with two AUS/FLUS results had a higher proportion of a follicular variant of papillary thyroid carcinoma (PTC) (46.7 % vs. 13.6 %, P = 0.009). Conclusions: Thyroid nodules with two AUS/FLUS results had a high malignancy risk of at least 31.0 % and a higher proportion of a follicular variant of PTC. Surgery should be considered regardless of ultrasound features. Key Points: • Thyroid nodules with two consecutive AUS/FLUS results had a high malignancy risk. • Ultrasound features are less useful in nodules with two AUS/FLUS results. • The follicular variant of PTC is more frequent in malignancies with two AUS/FLUS results.