Reclassifying uncertainty: Molecular advances in the evaluation of thyroid nodules

Dear Editor,

Fine-needle aspiration (FNA) cytology is widely accepted as the first-line diagnostic tool for thyroid nodules due to its simplicity, cost-effectiveness, and minimal invasiveness.^[1] However, its limitations, especially in cases with indeterminate cytologic features, have long posed challenges in clinical decision-making.^[1,2] These indeterminate results may lead to diagnostic uncertainty, often resulting in either overtreatment (e.g., surgery for benign disease) or undertreatment of malignancy.^[2]

Over the past decade, the integration of molecular diagnostics into thyroid cytopathology has transformed the management of these nodules.^[3] Molecular testing enhances morphologic interpretation, bridging the diagnostic gap, particularly in indeterminate categories.^[3,4] Cytopathologists are now transitioning from purely morphologic assessments to a more integrated, molecularly informed approach.

Approximately 10–30% of thyroid FNA specimens are classified as indeterminate according to the Bethesda System, particularly Category III atypia of undetermined significance and Category IV follicular neoplasm.^[1] These reflect cellular atypia insufficient for a definitive benign or malignant diagnosis. The uncertainty often leads to diagnostic lobectomy, which may later prove unnecessary, especially when final histopathology reveals benign disease. Such procedures carry risks and may require lifelong thyroid hormone therapy, adding to patient stress and healthcare burden.

Molecular testing has become an important adjunct in these scenarios, helping refine malignancy risk and enabling individualized decision-making.^[3] Although certain mutations (e.g., telomerase reverse transcriptase [TERT] promoter, turner protein 53 [TP53]) may have prognostic implications, the primary role of molecular diagnostics in cytology remains diagnostic, clarifying indeterminate cases and informing surgical planning.^[5]

Advancements in molecular profiling have clarified the genetic architecture of thyroid tumors [Table 1].^[4] Broadly, thyroid neoplasms can be grouped into two molecular categories:

• BRAF V600E-like tumors (e.g., classical papillary thyroid carcinoma [PTC]): These activate the mitogen-activated protein kinase pathway, exhibit infiltrative growth, are more likely to metastasize to lymph nodes, and show reduced radioactive iodine avidity.^[4,6]

• RAS-like tumors (e.g., follicular thyroid carcinoma, noninvasive follicular neoplasm with papillary like nuclear features (NIFTP), and follicular variant PTC): These display follicular architecture and typically behave indolently.^[4]

High-risk mutations such as TERT promoter, TP53, and rearranged during transfection/PTC rearrangements are implicated in tumor dedifferentiation and are associated with poor outcomes, particularly in poorly differentiated and anaplastic thyroid carcinomas.^[7]

The main value of molecular diagnostics in thyroid FNA lies in its ability to reclassify indeterminate lesions into actionable categories.^[3,5] Several commercial platforms – such as ThyroSeq v3 and Afirma genomic sequencing classifier – have been validated for use on FNA material.^[3,8] These platforms employ deoxyribonucleic acid/ribonucleic acid-based methods to detect point mutations, gene fusions, and expression profiles. High negative predictive values (>90%) have been demonstrated in Bethesda III/IV categories, enabling clinicians to avoid unnecessary surgeries.^[8] Conversely, detection of high-risk mutations (e.g., BRAF V600E or TERT promoter) supports more aggressive surgical approaches.

From a cytopathologist’s standpoint, pre-analytical factors are critical – adequate cellularity, proper fixation, and prompt processing are necessary to ensure valid molecular results. Many platforms are optimized to work on residual material, even after smears have been made.^[3,9]

One of the most impactful developments in the field has been the reclassification of the non-invasive follicular variant of papillary thyroid carcinoma (NIFTP). Previously considered carcinoma, NIFTP is now viewed as a borderline neoplasm due to its indolent behavior and non-invasive nature.^[10] Molecular testing played a pivotal role in this shift, showing that NIFTPs are predominantly RAS-mutant, lacking aggressive markers like BRAF V600E. Although NIFTP may cytologically fall into Bethesda IV, molecular findings can help guide more conservative management.^[3,10]

To conclude, molecular diagnostics have transformed thyroid cytopathology from a purely morphologic field into one rooted in precision medicine. Cytopathologists now play a key role in merging traditional cytology with molecular insights to improve diagnostic accuracy and guide personalized care. Hence, by adopting molecular tools, optimizing specimen handling, and fostering multidisciplinary collaboration, cytology laboratories can significantly enhance early and accurate thyroid cancer diagnosis. The microscope of today must not only visualize cells but also decode the genetic narratives within them.