Clinical and Prognostic Features of Malignant Thyroid Nodules With Atypia of Undetermined Significance Cytology: Evaluation of Nuclear Atypia‐Based Subgroups

Background The aim of this study was to evaluate the clinical and prognostic characteristics of thyroid nodules that were initially diagnosed as atypia of undetermined significance ( AUS ) and subsequently confirmed as malignant following surgery. The study also aimed to investigate their subclassification based on nuclear atypia. Methods Patients who underwent thyroid surgery and were diagnosed with differentiated thyroid carcinoma between January 2016 and December 2024, and who had undergone a preoperative fine‐needle aspiration biopsy ( FNAB ), were categorized retrospectively into three groups based on their cytological classification: AUS , suspicious for malignancy ( SFM ) and malignant (M). The clinicopathological and prognostic characteristics of these groups were then compared. Patients in the AUS group were also categorized according to the presence of nuclear atypia for further analysis. Results Of the 775 patients, 11.7% were assigned to the AUS group, 35.1% to the SFM group, and 53.2% to the M group. Nodules in the AUS group were significantly smaller in diameter and exhibited fewer invasive features compared to those in the SFM and M groups ( p < 0.05). The M group showed higher rates of advanced TNM staging, lymph node metastasis, and distant metastasis. Patients in the AUS group demonstrated the highest proportion of low‐risk classification (93.4%) and the highest rate of excellent response to therapy (96.7%), with no observed recurrence or progression. No significant differences in invasiveness were found between AUS ‐nuclear atypia ( AUS ‐N) and AUS ‐other atypia ( AUS ‐O) subgroups, both of which exhibited similarly indolent profiles. Conclusion Malignant nodules diagnosed as AUS exhibited an indolent course and an excellent prognosis. Our findings suggest that less aggressive, more individualized treatment strategies should be considered for AUS cases, and emphasize the potential prognostic value of nuclear atypia‐based subclassification.