The Clinical Challenge of Indeterminate Cytology
For years, the management of Bethesda III (Atypia of Undetermined Significance/Follicular Lesion of Undetermined Significance) and Bethesda IV (Follicular Neoplasm/Suspicious for Follicular Neoplasm) thyroid nodules has presented a significant diagnostic dilemma. While fine-needle aspiration (FNA) is the gold standard for thyroid nodule evaluation, these indeterminate categories carry a variable risk of malignancy, often leading to diagnostic lobectomies that ultimately reveal benign pathology.
The advent of molecular testing (MT) was intended to refine this risk. Mutations in the RAS gene family (NRAS, HRAS, and KRAS) are among the most common molecular alterations identified in these nodules. Traditionally, the presence of a RAS mutation was viewed as a strong indicator for surgical intervention. However, emerging evidence suggests that isolated RAS mutations are frequently associated with indolent lesions, such as Non-invasive Follicular Thyroid Neoplasm with Papillary-like Nuclear Features (NIFTP) or even benign follicular adenomas. This study, published in Thyroid, critically evaluates whether routine resection of all RAS-positive nodules constitutes overtreatment and explores the safety of active surveillance.
Study Design: Evaluating Real-World Management Strategies
This retrospective analysis utilized an institutional prospective thyroid nodule database from a single healthcare region, following the implementation of reflexive ThyroSPECTM molecular testing for all Bethesda III and IV nodules starting in July 2020. The researchers identified patients with isolated RAS mutations between July 2020 and September 2024.
To ensure a clean cohort, the study excluded patients with co-mutations (such as BRAF or TERT), those without recent ultrasound data, and those with concomitant proven malignancies or high-risk contralateral nodules. The final cohort consisted of 203 patients: 175 with Bethesda III and 28 with Bethesda IV cytology. These patients were categorized based on their management strategy: surgery (n=126) or active surveillance (n=77). The primary endpoints included final pathological outcomes for the surgical group and nodule stability (ultrasound growth) for the surveillance group.
Surgical Outcomes: High Rates of Benignity and Low-Risk Disease
Among the 126 patients who underwent surgery, the results highlighted the potential for overtreatment. The final pathology revealed malignancy in only 34% (n=43) of cases. Of these malignant cases, the vast majority were low-risk; only 19% (8 patients) were classified as American Thyroid Association (ATA) intermediate-high or high risk.
Perhaps more striking is the composition of the remaining surgical group: 45% (n=57) were entirely benign lesions, and 21% (n=26) were classified as neoplasms of uncertain or very low malignant potential (such as NIFTP). These data suggest that if surgery were mandated for every RAS-positive nodule, nearly two-thirds of patients would undergo a procedure for a non-malignant or clinically insignificant lesion.
Surgical patients tended to be younger (median age 45 vs. 53) and presented with larger nodules (median diameter 2.9 cm vs. 2.5 cm) compared to the surveillance group. This suggests that clinical factors—rather than molecular status alone—currently drive the decision for surgery in many practices.
Active Surveillance: A Durable Strategy for Selected Patients
One of the most significant contributions of this study is the data regarding the 77 patients who opted for active surveillance. With a median follow-up of 24 months, the outcomes were remarkably stable.
For the 48 surveillance patients with at least one year of follow-up and subsequent imaging, only 6% (3/48) exhibited nodule growth (defined as a >50% increase in volume). The cumulative incidence of growth at two years was a mere 2%. Furthermore, while seven patients eventually crossed over from surveillance to surgery due to patient preference or minor growth, only two of those patients were found to have malignancy, both of which were ATA low-intermediate risk.
These findings suggest that for patients who do not wish to undergo immediate surgery, a period of observation is not only feasible but carries a very low risk of missing an aggressive cancer that would progress during the surveillance window.
Predictors of Malignancy: The Role of Nuclear Atypia
The researchers also sought to identify clinical or cytological features that might help clinicians decide between surgery and surveillance. A key finding was that nuclear atypia was significantly more frequent in malignant resected nodules compared to benign resected nodules (70% vs. 40%, p = 0.007). This suggests that while RAS status provides the genomic context, the traditional cytological assessment of nuclear features remains a vital component of risk stratification.
Clinical Implications: Avoiding the Overtreatment Trap
The management of thyroid nodules is shifting from a ‘detect and resect’ model to a more nuanced, personalized risk-assessment framework. This study reinforces that an isolated RAS mutation is not a definitive mandate for surgery.
For clinicians, the takeaway is clear:
1. Risk Stratification is Essential: Consider nodule size, patient age, and the specific degree of cytological atypia alongside molecular results.
2. Shared Decision-Making: Patients should be informed that while a RAS mutation increases the risk of malignancy compared to mutation-negative nodules, the absolute risk of an aggressive cancer remains low.
3. Safety of Surveillance: For Bethesda III nodules with isolated RAS mutations and no high-risk ultrasound features, active surveillance with serial ultrasounds (at 6-12 month intervals) appears to be a safe alternative to immediate lobectomy.
Expert Commentary and Limitations
While this study provides strong evidence for the safety of surveillance, some limitations must be considered. As a retrospective analysis of a single region’s registry, there may be selection bias in which patients were offered surveillance versus surgery. Furthermore, the median follow-up of 24 months, while encouraging, is relatively short for thyroid cancer, which can have a natural history spanning decades.
The study also emphasizes that ‘isolated’ RAS mutations are the focus. The presence of ‘second hits’ or co-mutations (like TERT promoter or TP53) significantly changes the prognostic landscape, usually necessitating more aggressive surgical management. Therefore, the depth of the molecular testing panel used is a critical factor in the safety of the surveillance approach.
Conclusion
Isolated RAS mutations in Bethesda III and IV thyroid nodules represent a heterogeneous group of lesions, many of which are benign or indolent. Routine surgical resection for all such nodules likely results in significant overtreatment. This study demonstrates that active surveillance is a viable and safe management strategy for appropriately selected patients, characterized by low rates of nodule growth and a high likelihood of avoiding unnecessary surgery. As the field moves forward, long-term prospective data will be essential to further refine the criteria for surveillance and ensure the continued safety of this conservative approach.
References
1. Wu J, Yeo C, Qi J, et al. Outcomes of Patients with RAS-Positive Bethesda III and IV Cytology Thyroid Nodules. Thyroid. 2026. doi:10.1089/thy.2024.0562.
2. Haugen BR, et al. 2015 American Thyroid Association Management Guidelines for Adult Patients with Thyroid Nodules and Differentiated Thyroid Cancer. Thyroid. 2016;26(1):1-133.
3. Nikiforov YE, et al. Impact of the Multi-Gene ThyroSeq Next-Generation Sequencing Assay on Cancer Diagnosis in Thyroid Nodules with Indeterminate Cytology. Cancer. 2014;120(23):3639-46.