Thyroid nodules represent one of the most frequently encountered endocrine abnormalities, with a global prevalence ranging from 4% to 7% in the general population, and substantially higher rates detected through high-resolution ultrasonography [1]. While the vast majority of thyroid nodules are benign, the critical challenge lies in accurately identifying the subset that harbors malignancy, necessitating timely and appropriate intervention [2].

Fine Needle Aspiration Cytology (FNAC) has emerged as the first-line diagnostic modality in the evaluation of thyroid lesions, owing to its simplicity, cost-effectiveness, and high diagnostic yield in differentiating benign from malignant nodules [3]. However, cytological reporting was historically marred by inconsistent terminology and subjective interpretations, resulting in diagnostic ambiguities and variable clinical management pathways.

The introduction of The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC) in 2007, with its subsequent revisions, has revolutionized thyroid cytology by providing a standardized, evidence-based framework for cytological interpretation, risk stratification, and clinical correlation [4]. TBSRTC categorizes thyroid cytology into six diagnostic categories, each associated with a defined risk of malignancy and recommended clinical management, thus bridging the gap between cytopathologists and clinicians [5].

Despite its widespread adoption, the reproducibility and diagnostic consistency of TBSRTC are subject to institutional practices and inter-observer variability. Therefore, continual audit of its applicability in diverse clinical settings is essential.

The present study aims to evaluate the spectrum of thyroid lesions using TBSRTC in a tertiary care hospital and to assess inter-observer agreement among cytopathologists, thereby validating its diagnostic utility and reproducibility in routine practice.

Study Design and Setting

This was a retrospective and prospective observational study conducted in the Department of Pathology at SPV Government Medical College, Machilipatnam, over a period of two years, from March 2023 to February 2025. The study was approved by the Institutional Ethics Committee, and informed consent was obtained from all participants for FNAC procedures.

Study Population

A total of 120 patients presenting with clinically palpable thyroid nodules were included in the study. Both male and female patients aged 18 years and above were considered. Patients with recurrent thyroid nodules or those who had undergone prior thyroid surgery were excluded to avoid sampling bias.

Inclusion Criteria

Patients with palpable thyroid nodules who underwent FNAC.

Smears with well-preserved cellular morphology and adequate cellularity as per TBSRTC adequacy criteria.

Exclusion Criteria

Smears with extensive degenerative changes, poor fixation, or obscured morphology rendering cytological interpretation unreliable.

Cases with incomplete clinical or radiological data.

Procedure and Cytological Evaluation

FNAC was performed using a 23-gauge needle under aseptic precautions without aspiration (non-aspiration technique) or with aspiration as required. Multiple smears were prepared and stained with Hematoxylin & Eosin (H&E) and May-Grünwald Giemsa (MGG) stains. Cytopathological assessment was performed independently by two experienced pathologists.

All cases were categorized according to the 2023 edition of The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC) into six diagnostic categories. Cases previously reported before the adoption of TBSRTC were retrospectively reviewed and reclassified.

Statistical Analysis

Data were compiled and analyzed using Microsoft Excel and SPSS software (version 26.0). The frequency and percentage of cases in each TBSRTC category were calculated. Inter-observer reproducibility between the two pathologists was evaluated using Cohen’s kappa statistics. Kappa values were interpreted as per Landis and Koch guidelines: <0.20 (slight), 0.21–0.40 (fair), 0.41–0.60 (moderate), 0.61–0.80 (substantial), and 0.81–1.00 (almost perfect) agreement.

Ethical Considerations

The study received prior approval from the Institutional Ethics Committee of SPV Government Medical College, Machilipatnam (Approval No: GMCM/IEC/14/2025). All participants were informed in detail about the purpose, procedures, potential risks, and benefits of the study in their local language, ensuring comprehension and voluntary participation.

Written informed consent was obtained from all individuals undergoing Fine Needle Aspiration Cytology (FNAC), with assurance of confidentiality and the right to withdraw from the study at any stage without affecting their standard medical care. The collected data was anonymized and securely stored, accessible only to the principal investigators, ensuring the protection of participants' privacy and adherence to data confidentiality protocols. No financial incentives were provided for participation.

A total of 120 patients with thyroid nodules were evaluated using fine needle aspiration cytology (FNAC) and classified according to The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC). The distribution of cases across the six TBSRTC diagnostic categories is depicted in Table 1. The majority of the cases were categorized as Benign (Category II), accounting for 66.7% (n = 80). Category I (Non-diagnostic/Unsatisfactory) comprised 4.2% (n = 5) of cases, while Category III (Atypia of Undetermined Significance/Follicular Lesion of Undetermined Significance – AUS/FLUS) constituted 8.3% (n = 10). Category IV (Follicular Neoplasm/Suspicious for Follicular Neoplasm) and Category V (Suspicious for Malignancy) were observed in 6.7% (n = 8) and 5.8% (n = 7) of cases, respectively. Category VI (Malignant) lesions were identified in 8.3% (n = 10) patients (Table 1).

Table 1: Distribution of Cases According to TBSRTC Categories (n = 120)

Among the malignant cases (Category VI), Papillary Carcinoma was the most frequently diagnosed malignancy, comprising 70% (n = 7) of the cases. Medullary Carcinoma accounted for 20% (n = 2), and Poorly Differentiated Carcinoma was identified in 1 case (10%). No cases of Anaplastic Carcinoma were noted in the present cohort (Table 2).

Table 2: Distribution of Malignant Cases (Category VI, n = 10)

The inter-observer agreement between two pathologists, assessed using Cohen’s kappa statistics, demonstrated almost perfect agreement for Categories I (κ = 0.85), II (κ = 0.90), and VI (κ = 0.92). Substantial agreement was observed for Categories III (κ = 0.75), IV (κ = 0.78), and V (κ = 0.80). These findings highlight a high level of reproducibility and consistency in reporting thyroid cytology using the Bethesda system (Table 3).

Table 3: Inter-Observer Agreement (Cohen’s Kappa Statistics) for TBSRTC Categories

The demographic profile of the study population is summarized in Table 4. The cohort exhibited a female predominance, with 90 females (75%) and 30 males (25%), resulting in a female-to-male ratio of 3:1. The patients' ages ranged from 18 to 70 years, with a mean age of 42 years (Table 4).

Table 4: Demographic Distribution of Study Population (n = 120)

Fine Needle Aspiration Cytology (FNAC) continues to be the cornerstone diagnostic tool for evaluating thyroid nodules, attributed to its minimal invasiveness, diagnostic precision, and economic viability. Historically, variability in cytological interpretation and reporting terminology hindered diagnostic consistency across institutions. The introduction and successive updates of The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC) have established a globally accepted standardized framework, ensuring uniform reporting and facilitating effective communication between cytopathologists and clinicians [7].

In our study, the benign category (Category II) was the most prevalent, encompassing 66.7% of cases, which aligns with the findings reported in various Indian and international studies, where benign nodules constituted approximately 60–70% of cases [6,8]. This high proportion underscores FNAC’s specificity in accurately identifying non-neoplastic thyroid lesions, thereby significantly reducing unwarranted surgical excisions.

The incidence of Non-diagnostic/Unsatisfactory smears (Category I) was 4.2%, remaining within the acceptable benchmark of <10% as advocated by TBSRTC [7]. This low rate reflects strict adherence to specimen adequacy criteria, reinforcing the importance of meticulous smear preparation and cytopathologist vigilance in reducing false-negative interpretations.

The Atypia of Undetermined Significance/Follicular Lesion of Undetermined Significance (AUS/FLUS; Category III) represented 8.3% of cases. Although slightly exceeding the ideal threshold of ≤7%, similar variations have been observed in regional studies [9]. This category’s diagnostic ambiguity remains a persistent global challenge due to its inherently subjective cytological features, necessitating cautious interpretation and potential adjunctive molecular testing.

Follicular Neoplasm (Category IV) and Suspicious for Malignancy (Category V) accounted for 6.7% and 5.8% of cases, respectively, aligning with previously documented prevalence rates of 4–10% for these categories in diverse populations [6,8]. The Malignant category (Category VI) comprised 8.3%, with Papillary Thyroid Carcinoma being the predominant malignancy (70%), echoing global epidemiological patterns where papillary carcinoma remains the most prevalent thyroid malignancy [10].

Inter-observer variability is a critical parameter in evaluating any reporting system’s robustness. In this study, Cohen’s kappa statistics demonstrated almost perfect agreement for Categories I, II, and VI (κ = 0.85–0.92) and substantial agreement for Categories III to V (κ = 0.75–0.80). These results are consistent with previous investigations emphasizing the reproducibility of TBSRTC when interpreted by trained cytopathologists [9,10].

The demographic profile revealed a female predominance (75%) and a mean age of 42 years, paralleling well-established trends that indicate a higher susceptibility of thyroid diseases among middle-aged women [11].

The study’s limitations include a relatively small sample size from a single tertiary care center, which may not reflect broader population demographics. Additionally, the lack of histopathological follow-up in all cases limits definitive correlation of cytological diagnoses. The inter-observer agreement, though statistically robust, was assessed between only two cytopathologists, potentially influencing reproducibility assessments.

The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC) proved to be a reliable and effective standardized reporting framework for thyroid FNAC, facilitating clear communication between cytopathologists and clinicians. In our study, the benign category predominated, significantly reducing unnecessary surgical interventions. The inter-observer agreement was substantial to almost perfect, reinforcing TBSRTC’s reproducibility in routine practice. However, diagnostic challenges persist in indeterminate categories like AUS/FLUS, necessitating cautious interpretation and possible ancillary testing. Despite limitations in sample size and histopathological correlation, the study emphasizes the clinical utility of TBSRTC in streamlining thyroid nodule evaluation and guiding appropriate patient management in a tertiary care setting.

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