Subclassification of "atypia of undetermined significance" in thyroid fine-needle aspirates.

To identify the subtypes of atypia of undetermined significance (AUS) that confers a different magnitude for the risk of malignancy (RM), thyroid fine-needle aspiration (FNA) cases carrying a diagnosis of "atypical follicular cells" or "follicular lesion" with surgical pathology followup were included in this study. The direct smears of the aspirates were rereviewed and subclassified into four subgroups based on cytomorphology: AUS cannot exclude follicular neoplasm (AUS-FN), AUS cannot exclude Hürthle cell neoplasm (AUS-HCN), AUS cannot exclude papillary carcinoma (AUS-PTC) and AUS, not otherwise specified (AUS-NOS). Based on the followup histopathologic findings, RM not including papillary microcarcinoma (PMC), RM including PMC and the risk of neoplasm (RN) were calculated for each of the four AUS subgroups. A total of 138 AUS cases were subclassified into AUS-NOS (48), AUS-PTC (41), AUS-FN (32), and AUS-HCN (17). RM not including PMC was 32% for AUS-PTC (P < 0.001), 25% for AUS-FN, 8% for AUS-NOS, 0% for AUS-HCN, and 18% for all AUS cases. RM including PMC was 54% for AUS-PTC (P < 0.001), 34% for AUS-FN, 19% for AUS-NOS, 18% for AUS-HCN, and 33% for all AUS cases. RN was 63% for AUS-PTC (P = 0.05), 81% for AUS-FN (P < 0.01), AUS-HCN 53%, AUS-NOS 44% and 59% for all cases. In our study, subclassification enabled us to further divide AUS cases into high- and low-risk groups. The high-risk group includes AUS-PTC with a significantly higher risk of malignancy and AUS-FN with a significantly higher risks of neoplasm. AUS-HCN and AUS-NOS subgroups demonstrate a lower risk of malignancy of <10%.