Background
To improve risk stratification, the 2023 TBSRTC divides Atypia of Undetermined Significance (AUS) nodules into nuclear atypia (AUS-N) and other (AUS-O). While studies have reported different malignancy risk according to the nature of atypia, little is known about the molecular profile between AUS-N and AUS-O thyroid nodules. This study aims to investigate molecular findings and histological outcomes between AUS-N and AUS-O thyroid nodules across a large, real-world cohort of molecularly tested thyroid nodules.
Design
Thyroid fine needle aspiration (FNA) samples diagnosed as AUS with subsequent Afirma GSC testing were analyzed and compared between the AUS-N and AUS-O subcategories. Results of AUS samples with surgical resections from University of Michigan (UM) were also analyzed.
Results
Of 5345 AUS nodules that had central cytopathology review, 920 were AUS-N and 4425 AUS-O. AUS-N were GSC-suspicious(S) at a significantly higher rate than AUS-O (58% vs 28%, p<0.001). Among these GSC-S samples, AUS-N were significantly smaller than AUS-O (2.5 vs 2.7 cm, p<0.001). AUS-N nodules were significantly more enriched with BRAFp.V600E mutations compared to AUS-O nodules (12.1% vs. 1.2%) as well as NTRK3 and ALK fusions. AUS-N nodules were significantly more BRAF-like by the BRAF-RAS (BRS) score and had significantly higher extracellular signal-regulated kinase (ERK) and Follicular Mesenchymal Transition (FMT) expression (Figure 1). Histological results from 201 separate surgically treated AUS cases from UM showed a 34% malignancy rate in AUS-N nodules as compared to 15% in the AUS-O group. Notably, supporting the molecular findings of increased BRAFp.V600E mutation enrichment, a significantly higher proportion of PTCs was observed in AUS-N nodules compared to AUS-O group among all carcinoma cases (34% vs. 15%, p=0.02) (Table 1).
Table 1: Histopathological Diagnosis of GSC suspicious nodules
|
AUS-N |
AUS-O |
Total |
| Malignant histopathology |
28 |
18 |
46 |
| PTC |
18 |
10 |
38 |
| PTC (follicular variant) |
7 |
2 |
9 |
| Follicular carcinoma |
1 |
4 |
5 |
| Oncocytic carcinoma |
0 |
2 |
2 |
| Medullary carcinoma |
1 |
|
1 |
| Lymphoma |
1 |
|
1 |
| Benign histopathology |
54 |
101 |
155 |
| Follicular nodular disease |
30 |
61 |
91 |
| Follicular adenoma |
16 |
28 |
44 |
| Oncocytic adenoma |
2 |
7 |
9 |
| Hashimoto thyroiditis |
3 |
2 |
5 |
| NIFTP* |
3 |
3 |
6 |
| Rate of Malignancy |
28/82=34% |
18/119=15% |
p=0.002* |
Link to Figure 1 Here
Conclusions
AUS-N thyroid nodules show higher rates of PTC with BRAFp.V600E, distinct molecular profiles, and overall increased malignancy risk compared to AUS-O nodules. These findings support AUS subclassification and molecular testing to enhance risk stratification and potentially guide personalized management of AUS thyroid nodules.
Conference Materials
Afirma Thyroid
Afirma GSC and mRNA Expression Features in Bethesda III Thyroid Nodules
Jin X, et al. USCAP. 2026. DOI: 10.1016/j.labinv.2025.104589