Thyroid nodules are common, affecting up to 25% of the general population, with an even higher prevalence in women (36.5%) and older adults (44.7%) [1,2]. Advances in diagnostic technologies have increased thyroid nodule detection rates, contributing to the global rise in thyroid cancer prevalence [3]. With a marked increase in incidence over recent decades, thyroid cancer is currently the most common cancer in Korea, with 68.8 cases per 100,000 population according to the National Cancer Information Center [4,5]. However, the mortality rate of thyroid cancer has remained stable, suggesting potential overdiagnosis and overtreatment of thyroid nodules and cancers [5,6]. In 2020, Korea exhibited the highest incidence-to-mortality rate ratio among 185 countries, underscoring that thyroid cancer overdiagnosis remains a critical health concern in Korea [7].

Excessive diagnosis and subsequent treatment of thyroid nodules might offer minimal therapeutic benefit compared to active surveillance, and could even cause harm, economic burdens, and psychological distress for patients. These issues include surgical complications, long-term follow-up necessities, and anxiety from diagnosis, treatment, and remission stages [8-10]. To ensure optimal management of thyroid nodules, several research groups and societies have recommended risk stratification systems and size criteria for fine-needle aspiration (FNA) [11-16]. Adhering to these guidelines is crucial for minimizing overdiagnosis and overtreatment in clinical practice. Nevertheless, a substantial number of thyroid nodules at tertiary hospitals undergo biopsy despite not meeting Korean Thyroid Imaging Reporting and Data System (K-TIRADS) size thresholds, indicating that biopsy guidelines are loosely implemented in Korea [16,17].

Although ultrasound (US)-guided FNA is the standard diagnostic method for thyroid nodules due to its cost-effectiveness and safety, inconclusive cytology results remain relatively frequent [11,18,19]. As K-TIRADS aims to improve the efficiency and cost-effectiveness of thyroid nodule management, reducing inconclusive biopsy rates would significantly improve diagnostic accuracy and decrease patient burdens. However, little is known about the specific association between adherence to K-TIRADS guidelines and inconclusive biopsy outcomes.

Definitions of "inconclusive biopsy results" vary among studies, encompassing either category III alone, categories I and III, or categories I, III, and IV [20-23]. Despite this variability, investigating inconclusive biopsy results remains essential, especially given interobserver variability among cytopathologists in Bethesda category III and the intrinsic characteristics of nodules, such as low cellularity, cystic changes, or architectural atypia, which may influence cytological outcomes [20,24]. In this study, Bethesda categories I and III were categorized as inconclusive, as the Bethesda system specifically recommends repeat FNA for managing these categories [22,23]. To examine the clinical relevance of adhering to K-TIRADS biopsy criteria, this study investigated factors associated with inconclusive biopsy results and evaluated whether strict adherence to K-TIRADS biopsy guidelines could decrease their occurrence.

Since repeated biopsies and surveillance due to inconclusive results can cause increased anxiety and financial burdens for patients, reducing unnecessary biopsy procedures is essential in the management of thyroid nodules [9,17]. In this study, inconclusive biopsy results were defined as Bethesda category I (nondiagnostic) and category III (AUS), while conclusive biopsy results included Bethesda categories II (benign), IV (FN/SFN), V (suspicious for malignancy), and VI (malignant). This study aimed to evaluate the relationship between adherence to existing FNA guidelines and inconclusive biopsy outcomes to highlight the importance of strict guideline implementation. It also examined various clinical factors that could influence biopsy results. Nodules not indicated for FNA, operators with less than 1 year of experience, nodule sizes ranging from 0 to 5 mm, older patient age, and K-TIRADS category 4 were all identified as independent risk factors for inconclusive biopsy results.

Of the 2,440 nodules analyzed in this study, 900 (36.9%) yielded inconclusive biopsy results, and 1,543 (63.2%) did not satisfy the established FNA indications. Nodules not indicated for biopsy significantly increased the odds of inconclusive results (P<0.001). Furthermore, nodules sized between 0 and 5 mm emerged as independent predictors of inconclusive results, whereas those sized 6 to 10 mm did not show a significant difference compared to nodules of 11 mm or larger. These findings are consistent with previous studies reporting high rates of inconclusive biopsy results in smaller nodules, especially subcentimeter nodules [26,27]. Therefore, strict adherence to nodule size-based biopsy guidelines is effective in reducing inconclusive biopsy rates, particularly for micronodules, except in special circumstances where FNA may provide direct clinical benefit, such as nodules closely adjacent to critical structures like the trachea or esophagus.

When analyzed by US characteristics, K-TIRADS category 4 nodules showed a high rate of inconclusive biopsy results (64.1%), significantly higher than nodules categorized as K-TIRADS 3 or 5. This finding aligns with previous studies demonstrating that TIRADS 4 nodules tend to have the highest inconclusive biopsy rates compared to categories 3 or 5 [28]. Despite variations in biopsy thresholds for TIRADS 4 nodules (intermediate-risk nodules) across different guidelines—15 mm for American College of Radiology Thyroid Imaging Reporting and Data System (ACR-TIRADS) and uropean Thyroid Imaging Reporting and Data System (EU-TIRADS), 10–15 mm for revised K-TIRADS, and 10 mm for American Thyroid Association (ATA) guidelines—reported malignancy rates remain similar among ACR-TIRADS (12.7%-29.8%), EU-TIRADS (6.5%-17%), revised K-TIRADS (13.8%-29.6%), and ATA guidelines (17%-25.3%) [11-13,16,29-31]. Compared with the fixed 10 mm threshold of the 2016 K-TIRADS, the 2021 revision recommends biopsy for K-TIRADS 4 nodules measuring 10-15 mm, allowing clinicians to make final decisions based on US features and other risk factors. This revision significantly reduced unnecessary biopsies while maintaining high sensitivity [16]. In this study, despite employing a fixed 15 mm biopsy threshold for K-TIRADS 4 nodules rather than a size range, these nodules still comprised the highest proportion of inconclusive biopsy results. Additionally, among nodules not indicated for biopsy, K-TIRADS 4 nodules accounted for the largest proportion (595/1,543, 38.6%). This significant association between K-TIRADS categories and biopsy indications (P<0.001) may have further contributed to the high inconclusive rate for K-TIRADS 4 nodules, reinforcing the need to maintain relatively larger size thresholds for biopsy decisions. A similar pattern was noted with other TIRADS classifications: applying a 15 mm threshold from ACR-TIRADS to intermediate-risk nodules defined by ATA guidelines improved diagnostic accuracy, specificity, and reduced unnecessary biopsies compared to using the original 10 mm threshold [29].

Within K-TIRADS 4 nodules specifically, suspicious US features significantly reduced the odds of inconclusive biopsy results (odds ratio, 0.631; 95% CI, 0.460 to 0.866; P=0.004). Consistent with previous studies associating suspicious US features with higher malignancy rates in K-TIRADS 4 nodules, the present findings emphasize the relevance of integrating US features into biopsy decision-making [32]. Although the 2021 K-TIRADS guidelines recommend biopsy decisions for K-TIRADS 4 nodules sized between 1.0 and 1.5 cm based on additional US findings, this study applied a uniform 1.5 cm threshold for K-TIRADS 4 nodules due to the retrospective nature of data collection, precluding individualized threshold determination [16]. However, supplementary analyses demonstrating lower inconclusive rates among K-TIRADS 4 nodules with suspicious features provide additional support for considering US characteristics when setting biopsy thresholds, especially in intermediate-risk nodules.

Although nodules that underwent two or more needle passes tended to be smaller in size and categorized as K-TIRADS 4, multiple needle passes were not significantly associated with inconclusive biopsy results. Operators typically performed multiple needle passes when previous aspirates visually appeared insufficient for cytological evaluation. Prior studies present conflicting views regarding the relationship between the number of needle passes and inconclusive biopsy outcomes; some studies indicate no significant impact on cytological adequacy, while others suggest that multiple needle passes may reduce inadequate samples [33,34]. In this study, performing multiple needle passes did not lower the rate of inconclusive biopsy results. Thus, multiple passes may not necessarily improve cytological adequacy for nodules inherently prone to inconclusive results, such as micronodules or K-TIRADS category 4 nodules. Additionally, biopsies conducted by operators with less than 1 year of experience showed higher rates of inconclusive results and more frequent use of multiple needle passes. This aligns with previous research demonstrating that variability in operator experience and technique directly influences biopsy outcomes [33,35]. Methods that provide assistance or guidance to less experienced physicians, such as the presence of an on-site cytopathologist (though not currently implemented at the authors’ affiliated institution), may help increase sample adequacy [34,35]. Further research is needed to precisely identify factors associated with inconclusive biopsy results, thus refining biopsy indications based on US characteristics and minimizing unnecessary repeated FNAs.

Several limitations of this study must be acknowledged. First, the study employed a retrospective design conducted at a single institution, raising the possibility of selection bias. Second, the rate of inconclusive biopsy results in the study sample (36.9%) was relatively high compared to previous reports (ranging from 5% to 34.7%) [27,36]. Notably, 63.2% of nodules in this study were classified as not indicated for FNA, which likely contributed to the high rate of inconclusive results, alongside several other influencing factors. As the authors’ institution is a tertiary medical center, staging US prior to surgery frequently prompts biopsy of nodules that do not strictly meet FNA guidelines, to determine surgical extent. Additionally, repeat FNAs are often requested by primary care clinics following inconclusive initial results, or patients independently seek tertiary centers for further evaluation. Such circumstances likely influenced the frequency of inconclusive outcomes observed in this study. Third, interobserver variability in thyroid US interpretation might limit the reproducibility of these findings in other institutions. Although interobserver agreement is typically adequate, particularly when standardized US classification systems are used, interobserver consistency across different centers remains lower, and variability persists among examiners in US reporting and classification [37,38]. Nevertheless, given the statistical significance of these findings, the identified factors related to inconclusive biopsy results should meaningfully assist Korean institutions in applying the K-TIRADS guidelines.

In conclusion, performing FNA on nodules that do not meet the K-TIRADS biopsy criteria was associated with higher inconclusive biopsy results, along with other factors such as nodule size of 0-5 mm, operators with less than 1 year of experience, older patient age, and K-TIRADS category 4. Among K-TIRADS 4 nodules, suspicious US characteristics were associated with malignancy and lower inconclusive biopsy rates, supporting the K-TIRADS biopsy guidelines. Therefore, strict adherence to the K-TIRADS biopsy guidelines is essential for reducing inconclusive biopsy outcomes. Additionally, providing education and guidance for less experienced operators and conducting continued research aimed at refining biopsy indications based on nodule size and US characteristics may further improve clinical outcomes.