Combined molecular and histologic end points inform cancer risk estimates for thyroid nodules classified as atypia of undetermined significance

[Molecular and other ancillary tests proposed by The Bethesda system for reporting thyroid cytopathology 2023]

For the first time the 2023 version of The Bethesda System for Reporting Thyroid Cytology dedicates a whole chapter (chapter 14) to ancillary studies almost exclusively represented by molecular testing. The latest data reported bring some evidence that molecular testing could help to optimize the diagnostic performance of « indeterminate » categories (AUS and NF). Other studies suggest a promising role to guide the management of suspicious of malignancy and malignant categories. Indeed, the recognition of prognostic and predictive biomarkers analyzed on cytological samples, regardless of how it is collected, has progressed thanks to advances in our knowledge of molecular abnormalities of thyroid tumors. The chapter 14 is presented here highlighting the current and emerging roles of « in-house » and commercialized molecular testing as presented by TSBRTC.

Binary subclassification scheme (AUS-Nuclear versus AUS-Other) adequately risk-stratifies thyroid fine needle aspiration specimens classified as Atypia of Undetermined Significance

INTRODUCTION

The Bethesda System for Reporting Thyroid Cytopathology previously described 4 subclasses of atypia within the Atypia of Undetermined Significance (AUS) category: nuclear (AUS-Nuc), architectural (AUS-A), oncocytic (AUS-Onc), and atypia not otherwise specified (AUS-NOS). Accumulating evidence supports a binary AUS subclassification scheme based primarily on the presence of nuclear atypia only. The purpose of this study is to compare the risk stratification of binary versus 4-tier AUS subclassification systems among AUS nodules with molecular and/or histologic follow-up.

MATERIALS AND METHODS

Thyroid aspirates classified as AUS and tested using Afirma (Veracyte, Inc.) between 6/2013 and 7/2021 were included. For resected nodules, histological classification was considered as the final outcome. For unresected nodules, benign Afirma results were considered low-risk outcomes, similar to histologically benign nodules. Suspicious or nondiagnostic Afirma results were considered indeterminate outcomes. The prevalence of outcomes warranting surgery (noninvasive follicular thyroid neoplasm with papillary-like nuclear features [NIFTP] or cancer) was calculated for each AUS subclass.

RESULTS

A total of 559 AUS nodules with Afirma testing were identified. Excluding nodules with indeterminate molecular outcomes, NIFTP/cancer prevalence for AUS-Nuc was 21% (57/266), which was higher than that for AUS-A (6%, 11/188), AUS-Onc (8%, 4/53), and AUS-NOS (0%, 0/9). A binary AUS subclassification scheme based on nuclear atypia showed a significant difference in NIFTP/cancer prevalence (21% versus 6%, P < 0.0001).

CONCLUSIONS

Binary reporting of AUS subclasses based on nuclear atypia distinguishes cases with a higher risk of NIFTP/cancer. There is a low but non-negligible prevalence of NIFTP/cancer in cases without nuclear atypia.

Risk stratification of cytologically indeterminate thyroid nodules with nondiagnostic or benign cytology on repeat FNA: Implications for molecular testing and surveillance

BACKGROUND

Evidence guiding the management of cytologically indeterminate thyroid nodules with nondiagnostic (ND) or benign cytology on repeat fine-needle aspiration (FNA) is limited. This study evaluates the utility of molecular testing and estimates the risk of noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) and cancer among such nodules.

METHODS

This was a retrospective single-institution review of thyroid nodules from adults that were classified as atypia of undetermined significance (AUS) or follicular neoplasm (FN) on initial FNA and underwent repeat FNA for cytology and Afirma testing (June 2013-July 2021). The association between repeat FNA cytology and RNA yield for Afirma was determined. Histologic outcomes were integrated with Afirma results to define end points for each nodule.

RESULTS

A total of 691 AUS and FN nodules underwent repeat FNA and Afirma testing. Diagnostic Afirma results were obtained in 98% of cases overall and in 91% of nodules with ND cytology on repeat FNA. Using combined molecular and histologic end points, the NIFTP and/or cancer prevalence for nodules with ND cytology on repeat FNA was 9% (95% confidence interval [CI], 0.042-0.182), falling between those nodules classified as benign (5%; 95% CI, 0.029-0.094) and those classified as AUS or FN (18%; 95% CI, 0.140-0.218) on repeat FNA, although not reaching statistical significance from either subgroup (p = .38 and .10, respectively).

CONCLUSIONS

AUS and FN nodules that are ND on repeat FNA have low but nonnegligible risk of NIFTP and/or cancer and may benefit from molecular testing, given the low test failure rate in this subgroup. Conversely, AUS and FN nodules reclassified as benign on repeat FNA have a very low risk of NIFTP and/or cancer and are unlikely to benefit from molecular testing.

Molecular Characterization of Thyroid Follicular Lesions in the Era of "Next-Generation" Techniques

It is unequivocally recognized that thyroid nodules are frequently detected in the adult population and mostly characterized by benign lesions (up to 70% of them), with only 5%-15% malignant lesions. The evaluation of thyroid lesions with fine-needle aspiration cytology (FNAC) represents one of the first and most useful diagnostic tools in the definition of their nature. Despite the fact that the majority of thyroid lesions are correctly diagnosed as either benign (70%-75%) or malignant (5%-10%) entities, the remaining nodules (20%-25%) represent the "gray zone" of follicular lesions, which belong to indeterminate categories, according to the different classification systems. This indeterminate group of lesions includes both benign and malignant entities, which cannot be easily discriminate with morphology alone. In these last decades, the increasing role of molecular testings, feasibly performed on cytological material combined with the discoveries of specific genetic alterations in the field of thyroid pathology, has opened the pace to their more accurate and specific contribution on cytology. In fact, in 2015, in the revised management guidelines for patients with thyroid nodules and well-differentiated thyroid cancers (WDTCs), the American Thyroid Association (ATA) confirmed the performance of molecular testing in thyroid indeterminate cytology, and the same performance was addressed in recent update of the management of thyroid nodules in the second edition of the Bethesda system for reporting thyroid cytopathology (TBSRTC). In the current review, we discuss the role of molecular tests for the different thyroid diagnostic categories of the Bethesda system for reporting thyroid cytopathology, mostly focusing our attention on the follicular and indeterminate lesions.