Bethesda system for reporting thyroid cytopathology

Thyroid nodules, prevalent in up to 50% of adults, have become increasingly detectable due to advancements in imaging technologies. These nodules are often benign, and thyroid fine-needle aspiration (FNA) has established itself as a first-line triaging tool post-ultrasound examination. The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC), first introduced in 2010, offers a standardized, category-based reporting framework. This system, widely adopted globally, includes six categories with defined diagnostic criteria, estimated risks of malignancy (ROM), and management guidelines.

The system's first major update in 2017 introduced the histologic category of noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP), affecting ROM assessments, particularly for indeterminate categories. The second edition incorporated this new entity and introduced molecular testing to refine management of indeterminate lesions. In 2023, the third edition focuses on simplifying diagnostic categories, updating terminologies in line with the latest WHO Classification of Thyroid Neoplasms, and refining ROM based on new data. This review summarizes these updates, discusses potential diagnostic pitfalls, and examines the role of molecular testing.

Evolution of TBSRTC

The introduction of the NIFTP category in 2017 significantly impacted ROM calculations, especially in indeterminate categories such as atypia of undetermined significance (AUS), suspicious for follicular neoplasm (SFN), and suspicious for malignancy (SM). These categories previously assumed higher malignancy risks for follicular-patterned lesions with papillary carcinoma-like nuclear features. The 2017 edition adjusted ROM and added notes to acknowledge that certain SFN and SM cases might represent NIFTP. This edition also included molecular testing to enhance management decisions for indeterminate lesions.

Since then, new prospective studies have provided updated ROM data, and molecular testing has continued to evolve. The American Thyroid Association (ATA) and WHO have also updated their guidelines and classifications. Consequently, the 2023 edition of TBSRTC reflects these developments, emphasizing simplified diagnostic categories and the application of updated histologic terminologies.

The Reporting System

The third edition of TBSRTC maintains the structure of six diagnostic categories:

1. Non-diagnostic: This category encompasses specimens that fail to meet adequacy criteria, such as those with poorly preserved or obscured follicular cells, cyst fluid with few cells, or samples with significant artifacts. The ROM for non-diagnostic samples is approximately 13%, with a management recommendation to repeat FNA with ultrasound guidance.
2. Benign: This category includes follicular nodular disease (FND) and lymphocytic thyroiditis (LT). FND comprises various benign thyroid lesions, characterized by colloid and cellularity, with benign follicular cells arranged in flat sheets or small tissue fragments. LT, primarily chronic lymphocytic thyroiditis (Hashimoto's thyroiditis), requires clinical and serologic correlation for accurate subtyping. The ROM for benign lesions is around 4%, and management typically involves clinical and sonographic follow-up.
3. Atypia of Undetermined Significance (AUS): This category covers cases with cellular atypia that does not fit neatly into benign or malignant categories. Diagnostic criteria include focal nuclear atypia and histiocytoid cells. The ROM for AUS is about 22%, with management options including repeat FNA, molecular testing, or diagnostic lobectomy.
4. Follicular Neoplasm (FN): Simplified in the third edition, this category includes diagnoses with architectural patterns such as microfollicular, crowded three-dimensional, and trabecular patterns, with cytologic findings indicating follicular neoplasm. The ROM for FN is approximately 30%, and management typically involves molecular testing or diagnostic lobectomy.
5. Suspicious for Malignancy (SM): This category includes cases with cytologic features highly suggestive of malignancy but not definitive. The ROM for SM is around 74%, with management involving molecular testing or lobectomy.
6. Malignant: This category includes cases with cytologic evidence of malignancy, such as papillary thyroid carcinoma (PTC). The ROM for malignant diagnoses is nearly 100%, and management involves lobectomy or near-total thyroidectomy.

Conclusions

The third edition of TBSRTC represents a significant update, reflecting the latest advancements in thyroid cytopathology and molecular testing. By simplifying diagnostic categories, updating terminologies, and refining ROM estimates, this edition aims to enhance diagnostic accuracy and clinical management of thyroid nodules. As molecular testing continues to evolve, TBSRTC remains a crucial tool for pathologists and clinicians in the diagnosis and management of thyroid lesions.

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https://www.xiahepublishing.com/2771-165X/JCTP-2023-00005

The study was recently published in the Journal of Clinical and Translational Pathology.

Journal of Clinical and Translational Pathology (JCTP) is the official scientific journal of the Chinese American Pathologists Association (CAPA). It publishes high quality peer-reviewed original research, reviews, perspectives, commentaries, and letters that are pertinent to clinical and translational pathology, including but not limited to anatomic pathology and clinical pathology. Basic scientific research on pathogenesis of diseases as well as application of pathology-related diagnostic techniques or methodologies also fit the scope of the JCTP.

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