Cytologic subclassification of atypia of undetermined significance may predict thyroid nodules more likely to be malignant at surgery

Background The atypia of undetermined significance or follicular lesion of undetermined significance (AUS/FLUS) category in the Bethesda System of reporting thyroid fine-needle aspiration (FNA) includes various cytological findings considered to have a low risk of malignancy. It is still unclear if any particular subset of the cytological findings within this category carries a higher risk for malignancy requiring a more aggressive treatment plan. Methods We reviewed 221 AUS/FLUS FNAs performed between January of 2006 and May of 2012. Histopathological data from surgery was available in 101 nodules and characteristics of these nodules were analyzed. We reviewed the initial cytology report and subclassified the nodules into one of four groups: architectural atypia (AA, includes abnormal follicular arrangement but no cellular abnormalities), cellular atypia (CA, atypical cellular findings) with or without AA, Hurthle cells Predominant and Others that included otherwise unspecified AUS categories. The surgical pathology confirmed malignancy rate for each group was calculated. Result Papillary thyroid carcinoma was the most common malignancy identified (26 of 29 or 90%). We found that in the AA only category, 2/21 (10%) nodules were malignant, whereas when CA with or without AA was present, 23/66 (35%) were malignant, significantly higher than the former group (P values of 0.025). Conclusion CA seen on initial AUS/FLUS had a higher malignancy rate compared to AA group. Further research is required to consider subclassification of this category to assign appropriate risk of malignancy for AUS nodules.