Predicting malignancy in thyroid nodules with benign cytology results: The role of Conventional Ultrasound,Shear Wave Elastography and BRAF V600E

Abstract

BACKGROUND:

Ultrasound-guided fine-needle aspiration (US-FNA) is the most accurate method for preoperative diagnosis of thyroid nodules, but how to deal with false negative results?

OBJECTIVE:

This study aimed to find preoperative diagnosis methods including Conventional Ultrasound (CUS), Shear Wave Elastography (SWE) and BRAF V600E testing to differentiate false negative nodules.

METHODS:

Forty-nine nodules in 49 patients with benign FNA results and pathological diagnoses were included. CUS and SWE features were evaluated. BRAF V600E analysis was performed after FNA. Diagnostic performances of three methods were analyzed in predicting malignancy in benign FNA results.

RESULTS:

Twenty-seven of 49 nodules were malignant, and 22 nodules were benign. Hypoechogenicity, taller-than-wider, irregular boundary, microcalcification, SWE max, SWE mean and BRAF V600E mutation were risk factors for malignancy. All 7 malignant nodules with BRAF V600E mutations and 18 of 20 malignant nodules without BRAF V600E mutations have two or more suspicious CUS features. Six of 7 malignant nodules with BRAF V600E mutations and 16 of 20 malignant nodules without BRAF V600E mutations had SWE mean value greater than the cut-off value.

CONCLUSIONS:

CUS, SWE and BRAF V600E were diagnostic tools for malignancy in FNA benign nodules. Further clinical decisions should be considered for nodules with two or more suspicious CUS features and SWE parameters greater than cut-off values whether BRAF V600E is mutational or not.

Keywords

Conventional Ultrasound Shear Wave Elastography fine needle aspiration BRAF V600E

1 Introduction

Fine needle aspiration (FNA) is the most accurate and economical method for diagnosis suspicious thyroid nodules. The Bethesda System for Reporting Thyroid Cytopathology (BSRTC) system was widely used to evaluate FNA results, which determined the next treatment for patients. Further diagnosis or treatments were not required immediately according to the 2015 American Thyroid Association (ATA) guideline if the nodules were FNA benign results based on BSRTC system. However, approximately 25% of thyroid nodules could not be diagnosed by FNA and results of FNA benign cytology results (Bethesda II) still carry the risk of malignancy up to 3% [1 –4]. These nodules will be missed based on current guideline. How to find thyroid cancers from FNA benign nodules is an unsolved problem.

Conventional Ultrasound (CUS) plays an important role in the management process of thyroid nodules, which has a suggestive significance for each Bethesda category of thyroid nodules [5]. Several US features have been proved to be associated with thyroid cancer: solid component, hypoechogenicity, taller-than-wide shape, irregular margins and microcalcifications [6]. The above US features have high sensitivity (87.0%) and specificity (86.5%) for the differential diagnosis of benign and malignant thyroid nodules [7]. Thyroid imaging report and data system (TI-RADS) was established to determine the risk classification based on the malignant characteristics of thyroid nodules [8, 9].

Elastography plays an auxiliary role in the diagnosis of malignant thyroid nodules based on the differences in hardness between tissues. The basic imaging fundamental ability of shear wave elastography (SWE) is to contrast the stiffness between benign and malignancy because the latter supposedly being harder and showed on quantitative or qualitative results. SWE has good sensitivity and specificity for identification of thyroid nodules. It is useful in evaluating the stiffness of thyroid nodules and differentiating between malignant and benign nodules [10, 11]. In recent years, with the increasingly appearance of thyroid nodules with undetermined cytology, SWE, as a supplementary method, had performed relatively high sensitivity when acquiring the cut-off value of 3.3 KPa (E_SD) and reduced repeated FNA for benign [12]. SWE improved the sensitivity and accuracy of diagnosis in thyroid nodules with undetermined cytology when combined with ACR TI-RADS categories [13]. Samir et al reported that SWE can be used to identify malignancy using a lower 22 kPa cut-off value in 35 nodules with FNA indeterminate results [14].

Molecular diagnosis of thyroid nodules had been increasingly developed and especially highlighted by BRAF V600E, which is a typical and specific genetic marker that could enable effective preoperative evaluations [15, 16]. Papillary thyroid carcinoma (PTC) is the most common thyroid cancer and the molecular marker of BRAF V600E mutations can be found in 50–87% PTCs [17]. BRAF V600E mutations had been proved to be useful to predict the higher degree of malignancy and greater possibility of lymph node metastasis of PTC [18, 19]. The combination of FNA and BRAF V600E detection significantly increased the diagnostic sensitivity from 77.3% to 86.7% of PTC compared with FNA alone [20]. A retrospective study also reported that BRAF mutations can predict malignancy in AUS/FLUS thyroid nodules and help to make better surgical decisions [21]. However, molecular marker testing was recommended for cytology undetermined results and not applicable to FNA benign results according to the updated 2015 ATA guideline [4].

To reduce the false negative results of FNA, our study retrospectively analyzed the usefulness of three methods for preoperative diagnosis of thyroid nodules, including CUS, SWE and BRAF V600E test to predict malignancy of thyroid nodules with benign cytology results and improve the current management strategy of thyroid nodules.

2 Materials and methods

2.1 Patients

This retrospective study was authorized by the ethics committee of the Shanghai Tenth Hospital and complied with the Declaration of Helsinki. From December 2016 to February 2018, 1232 consecutive patients with 1232 thyroid nodules were enrolled. The flow chart was shown in Fig. 1. Eligible patients met the following criteria: (1) aged of 18 years or older; (2) CUS and SWE were both performed before FNA; (3) Benign FNA results (Bethesda II); (4) Underwent BRAF V600E test before FNA. The excluded criteria were as follow: (1) FNA results of Bethesda I, III-VI; (2) no pathological result; (3) poor quality of CUS and SWE images. Finally, 534 nodules were benign cytology results. After excluding 467 nodules without surgical removed and 18 nodules with poor quality images, a total of 49 thyroid nodules from 49 patients with benign FNA results and pathological diagnosis were included.

Fig. 1

Process for including nodules and the histopathology examination results.

Fig. 2

Images of a 59-year-old woman with benign FNA results, and then pathologically confirmed as nodular goiter. (a) Image of CUS showed hypoechogenicity feature in the longitudinal plane. (b) SWE max and SWE mean were showed on the elastic image with 21.8 KPa and 12.8 KPa respectively. Images of a 44-year-old woman with benign FNA results, but then pathologically confirmed as MPTC. (c) Image of CUS showed hypoechogenicity, irregular boundary, microcalcification features in the longitudinal plane. (d) SWE max and SWE mean were showed on the elastic image with 61.7 KPa and 53.5 KPa respectively.

Fig. 3

(a) ROC curves showing the diagnostic performances for malignancy of CUS features with hypoechogenicity, taller-than-wider, irregular boundary and microcalcification. (b) ROC curves showing the diagnostic performances alone and combined respectively for malignancy of combined CUS, SWE features and BRAF V600E.

2.2 CUS and SWE examinations

The high-frequency linear transducer (Supersonic imagine, Aix-en-Provence, France; 4–15 MHz transducer) was applied in the CUS and SWE examinations from December 2016 to February 2018. Examinations were performed by four radiologists with 10 years of experience in thyroid CUS and 3 years of experience in thyroid SWE.

For CUS examinations, patients’ necks should be fully exposed. At least six images were saved, including two grayscale images, two color Doppler images and images with recording the size of the target nodule in the longitudinal and transverse planes. Characteristics of target nodules were recorded with following imaging features: component, echogenicity, shape, boundary and calcification. All nodules were classified according to Kwak TI-RADS. Suspicious US features were as following: solid component, hypoechogenicity, marked hypoechogenicity, microlobulated or irregular margins, microcalcifications, and taller-than-wide shape. As the number of suspicious US features increased, the fitted probability and risk of malignancy also increased: TI-RADS category 3 (no suspicious US features), category 4a (one suspicious US feature), category 4b (two suspicious US features), category 4c (three or four suspicious US features), and category 5 (five suspicious US features).

Before switching to SWE, a relaxed patient neck was essential for acquiring successful images. SWE was conducted by clearly visualizing and centering the nodule on a grayscale image. An appropriate sampling frame for elastography was chosen with two-thirds of which was occupied by the target nodule area, and the other one-third was occupied by normal thyroid tissues. The elastography scale set was selected uniformly at 100 KPa. During a breath hold, images were obtained after 3–5 seconds of stable operation with no pressure on the skin. Seven elastography images were obtained in the longitudinal plane. To quantitatively analyze the nodules’ stiffness, the region of interest (ROI) should be covered the nodules as much as possible. Optimal images were selected and the maximum, minimum and mean values of target nodules (SWE max, SWE min, SWE mean) were documented for analysis. The image selection criteria were as follows: (1) elastic graph assessed from a clear grayscale image; (2) clear outline of the target nodule on the elastic graph; (3) elasticity measurements that avoided calcification and cystic components; and (4) nodules of the same depth were surrounded by adequate thyroid tissue.

2.3 FNA and BRAF V600E mutation detection

Based on the Kwak TI-RADS risk assessment, thyroid nodules with TI-RADS 4a and above received FNA. The appropriate path into the target nodule was guided by US. 21G fine needles were used for aspiration. Furthermore, cystic or calcified regions were avoided to improve the success rates of aspirations. Cytology samples were fixed immediately with 95% ethanol. Each thyroid nodule usually required four samples. Three samples were for cytology detection, and one for BRAF V600E testing. A specific kit (QIAGEN QIAamp DNA FFPE Tissue) was used to extract DNA samples, and exon 15 of the V600E gene was detected by the Merinton SMA4000 spectrophotometer (Merinton Inc, Beijing, China). Cytology results were reported with the Bethesda Reporting System for thyroid cell pathology (I, non-diagnostic; II, benign; III, AUS/FLUS; IV, FN/SFN; V, suspected malignancy; VI, malignancy). Results of BRAF V600E detection were divided into two types: mutation or not.

2.4 Statistical analysis

Statistical analysis was performed by SPSS 25.0 software (SPSS Inc, Chicago, IL, United States) and MedCalc. The differences between the mean values and standard deviations (SD) of continuous variables of the two groups were analyzed by independent-sample tests if the variables were normally distributed. The Mann-Whitney U test was used if the continuous variables did not follow a normal distribution. P-value (two-sided) < 0.05 indicated statistically significant differences between two sets of data. The diagnostic efficiency including sensitivity, specificity, PPV, NPV, accuracy, AUC and cut-off values were evaluated by receiver operating characteristic (ROC) curves.

3 Results

3.1 Patient characteristics

A total of 49 thyroid nodules from 49 patients (9 males/40 females; mean age, 53.16±9.95 years; range from 33 to 73 years) with benign cytology results were included in the retrospective study. Basic information were summarized in Table 1. There was no statistically difference in the mean age, sex of patients and the mean diameter and location of nodules between malignant and benign lesions (p > 0.05). Pathology results confirmed nodules as 21 nodular goiters (42.9%), one adenoma (2.0%), 23 MPTCs (46.9%) and 4 PTCs (8.2%).

Table 1
Basic information of the benign and malignant nodules

Characteristic Benign nodules Malignant nodules P value

Number of patients 22 27

Mean age (ys)

Mean±standard deviation 52.0±7.86 50.0±10.50 0.193

Range 33–73 35–70

Patient sex 0.060

Male 7 (77.7%) 15 (37.5%)

Female 2 (22.3%) 25 (62.5%)

Mean diameter (mm)

Mean±standard deviation 11.57±7.63 8.72±7.08 0.181

Range 2.7–33.0 3.5–39.0

Location 0.617

Left 13 (59.1%) 14 (51.9%)

Right 9 (40.9%) 12 (44.4%)

Isthmic 0 (0.0%) 1 (3.7%)

Characteristic	Benign nodules	Malignant nodules	P value
Number of patients	22	27
Mean age (ys)
Mean±standard deviation	52.0±7.86	50.0±10.50	0.193
Range	33–73	35–70
Patient sex			0.060
Male	7 (77.7%)	15 (37.5%)
Female	2 (22.3%)	25 (62.5%)
Mean diameter (mm)
Mean±standard deviation	11.57±7.63	8.72±7.08	0.181
Range	2.7–33.0	3.5–39.0
Location			0.617
Left	13 (59.1%)	14 (51.9%)
Right	9 (40.9%)	12 (44.4%)
Isthmic	0 (0.0%)	1 (3.7%)

3.2 Performances of CUS in predicting malignancy

The comparisons of CUS features between malignant and benign thyroid nodules were summarized in Table 2. There were significant differences in hypoechogenicity (P = 0.035), taller-than-wider (P = 0.013), irregular boundary (P = 0.000) and microcalcification (P = 0.005) between benign and malignant nodules. The combination of above CUS features showed better performance with AUC of 0.892 (95% CI: 0.791, 0.994) in predicting malignancy than using each CUS feature alone. Diagnostic performances of CUS features in thyroid nodules with benign cytology results were showed in Table 3. The sensitivity, specificity, PPV, NPV, and accuracy of combined CUS features were 92.6%, 81.8%, 86.2%, 90.0% and 87.7% respectively. (Table 3).

Table 2
CUS, SWE features and BRAF V600E result of Benign and Malignant Nodules

Characteristic Benign nodules Malignant nodules P-Value

Solid component 0.882

Yes 21 (95.5%) 26 (96.4%)

No 1 (4.5%) 1 (3.8%)

Hypoechogenicity 0.035

Yes 18 (81.8%) 27 (100.0%)

No 4 (18.2%) 0 (0.0%)

Taller-than-wider 0.013

Yes 1 (4.5%) 9 (33.3%)

No 21 (95.5%) 18 (66.7%)

Irregular boundary 0.000

Yes 4 (18.2%) 25 (92.6%)

No 18 (81.8%) 2 (7.4%)

Microcalcification 0.005

Yes 3 (13.6%) 14 (51.9%)

No 19 (86.4%) 13 (48.1%)

SWE max (KPa)^* 24.22±9.52 (11.40–47.00) 34.04±12.77 (15.80–62.30) 0.006

SWE min ((KPa)^* 12.37±8.79 (1.40–22.70) 16.65±11.09 (5.10–42.40) 0.149

SWE mean (KPa)^* 18.26±9.33 (6.80–43.00) 25.56±11.13 (10.20–53.80) 0.013

BRAF V600E (+)^#

Yes 1 (4.5%) 20 (74.1%) 0.000

No 21 (95.5%) 7 (25.9%)

Characteristic	Benign nodules	Malignant nodules	P-Value
Solid component			0.882
Yes	21 (95.5%)	26 (96.4%)
No	1 (4.5%)	1 (3.8%)
Hypoechogenicity			0.035
Yes	18 (81.8%)	27 (100.0%)
No	4 (18.2%)	0 (0.0%)
Taller-than-wider			0.013
Yes	1 (4.5%)	9 (33.3%)
No	21 (95.5%)	18 (66.7%)
Irregular boundary			0.000
Yes	4 (18.2%)	25 (92.6%)
No	18 (81.8%)	2 (7.4%)
Microcalcification			0.005
Yes	3 (13.6%)	14 (51.9%)
No	19 (86.4%)	13 (48.1%)
SWE max (KPa)^*	24.22±9.52 (11.40–47.00)	34.04±12.77 (15.80–62.30)	0.006
SWE min ((KPa)^*	12.37±8.79 (1.40–22.70)	16.65±11.09 (5.10–42.40)	0.149
SWE mean (KPa)^*	18.26±9.33 (6.80–43.00)	25.56±11.13 (10.20–53.80)	0.013
BRAF V600E (+)^#
Yes	1 (4.5%)	20 (74.1%)	0.000
No	21 (95.5%)	7 (25.9%)

^* Data are means±standard deviations, with range in parentheses. Unless otherwise specified, other variables are numbers and percentages in parentheses. Abbreviations: SWE, shear wave elastography.

Table 3

Diagnostic performances of CUS, SWE features and BRAFV600E in thyroid nodules with benign cytology results

US features	Benign (n = 22)	Malignant (n = 27)	Sensitivity	Specificity	PPV	NPV	Accuracy	AUC (95% CI)
Hypoechogenicity	18	27	100%	18.1%	60.0%	100.0%	76.9%	0.591 (0.427,0.755)
Taller-than-wider	1	9	33.3%	95.4%	90.0%	53.8%	61.2%	0.644 (0.490,0.798)
Irregular boundary	4	25	92.5%	81.8%	86.2%	90.0%	87.7%	0.872 (0.760,0.984)
Microcalcification	3	14	51.8%	86.3%	82.3%	59.3%	67.3%	0.691 (0.542,0.840)
CUS features	4	25	92.5%	81.8%	86.2%	90.0%	87.7%	0.892^*# (0.791,0.994)
BRAF V600E (+)	1	20	74.0%	95.4%	95.2%	72.4%	83.6%	0.848^# (0.733,0.962)
SWE max≥32.9 KPa (cut-off)	3	15	55.5%	86.3%	83.3%	61.2%	69.3%	0.751 (0.613,0.889)
SWE mean≥17.1 KPa (cut-off)	7	22	81.4%	68.1%	75.8%	83.3%	75.5%	0.763 (0.620,0.907)
CUS features + SWE max + BRAF V600E (+)	1	26	96.3%	95.5%	96.2%	95.4%	95.9%	0.987^$ (0.963,1)
CUS features + SWE mean + BRAF V600E (+)	3	27	100%	81.8%	90.0%	100%	91.8%	0.971^$ (0.933,1)

^*Compared with the predictive performances in hypoechogenicity, taller-than-wider and microcalcification, P < 0.000. ^$ Compared with the predictive performances in the combination of CUS features, BRAF V600E, SWE max and SWE mean, P < 0.05. ^# means that there was no statistically significant among them. CUS features means combined statistically significant US features including hypoechogenicity, taller-than-wide, irregular boundary and microcalcifications. BRAF V600E (+) means the mutation results of BRAF V600E. Abbreviations: PPV, positive predictive value; NPV, negative predictive value; AUC, area under the ROC curve; 95% CI, 95% confidence interval; SWE, shear wave elastography.

Table 4

Comparisons of characteristics of CUS and SWE in nodules with BRAF V600E mutation or not

	Nodules with BRAF V600E (+) (n = 21)		Nodules without BRAF V600E (+) (n = 28)
characteristic	Benign (n = 1)	Malignant (n = 20)	Benign (n = 21)	Malignant (n = 7)
≥2 suspicious CUS features	0 (0%)	18 (90%)	5 (24%)	7 (100%)
SWE max≥32.9KPa (cut-off)	0 (0%)	10 (50%)	5 (24%)	5 (71%)
SWE mean≥17.1KPa (cut-off)	0 (0%)	16 (80%)	8 (38%)	6 (86%)

BRAF V600E (+) means the mutation results of BRAF V600E. Data are percentage in parentheses.

3.3 Performances of SWE in predicting malignancy

SWE parameters of malignant nodules were higher (SWE max, 34.04±12.77 KPa; SWE min, 16.65±11.09 KPa; SWE mean, 25.56±11.13 KPa) than that of benign nodules (SWE max, 24.22±9.5 2KPa; SWE min, 12.37±8.79 KPa; SWE mean, 18.26±9.33 KPa) (Table 2). Furthermore, significant differences were found in SWE max and SWE mean compared malignant nodules with benign ones (p < 0.05), with the cut-off values of 32.9 KPa and 17.1 KPa. The AUC of SWE max and SWE mean were 0.751 (95% CI: 0.613, 0.889) and 0.763 (95% CI: 0.620, 0.907) respectively (Table 3).

3.4 Performances of BRAF V600E in predicting malignancy

The sensitivity, specificity, PPV, NPV, accuracy of BRAF V600E mutation in the diagnostic performance for malignancy was 74.0%, 95.4%, 95.2%, 72.4% and 83.6% respectively and the AUC of BRAF V600E mutation is 0.848 (95% CI: 0.733, 0.962).

3.5 Comparisons of diagnostic values in predicting malignancy of combination and prediction alone

When the diagnostic methods (combined CUS, SWE max/mean and BRAF V600E) used alone, the diagnostic performance of AUC for malignant nodules was 0.892, 0.751/0.763 and 0.848 respectively, and there was no significant difference between combined and BRAF V600E (P > 0.05). The sensitivity, specificity, PPV, NPV, accuracy and AUC in predicting malignancy was 96.3%, 95.5%, 96.2%, 95.4%, 95.9% and 0.987 when combined CUS features, SWE max and BRAF V600E mutation. The sensitivity, specificity, PPV, NPV, accuracy and AUC in predicting malignancy was 100%, 81.8%, 90.0%, 100%, 91.8% and 0.971 when combined CUS features, SWE mean and BRAF V600E mutation. The AUC of combined methods were superior to diagnosis alone (p < 0.05) (Table 3).

3.6 Comparisons of characteristics of CUS and SWE in nodules with BRAF V600E mutation or not

Among 49 nodules, twenty-one nodules had BRAF V600E mutation results, of which 20 were malignant and 1 was benign. Eighty (90%) of the 20 malignant nodules had 2 or more suspicious CUS features and 16 (80%) nodules had SWE mean greater than the cut-off value. One benign nodule did not possess the above malignant features were surgically proved as nodular goiter. Seven of 28 nodules without BRAF V600E mutation were malignant and all (100%) had 2 suspicious CUS features. Six (86%) of 7 nodules had SWE mean greater than the cut-off value. Other 21 of 28 nodules were benign, of which 5 (24%) nodules had 2 or more suspicious CUS features and 8 (38%) had SWE mean greater than the cut-off value.

4 Discussion

FNA has become the predominant method for preoperative diagnosis of thyroid nodules. After FNA performed, the cytology results were graded according to Bethesda System for Reporting Thyroid Cytopathology, which had good coherence in large groups of patients, with 89% –95% of samples being acceptable; 55% –74% of samples were benign, and 2% –5% of samples were malignant [22]. However, there were still some non-diagnostic or missing cases because of the limitations of cytology diagnosis. The false negative rate for benign thyroid nodules based on cytology is still as high as 5% [23], especially if the nodule had suspicious malignant features. Similarly, twenty seven of 534 cytology benign nodules were pathologically confirmed malignancy in our study and the false negative rate accounted for 5%. For such false negative nodules, the 2015 ATA guidelines recommend no further examination or treatment. Based on the current FNA strategy, these malignant nodules with benign cytology results might be missed without further treatment [4]. How to find out these 3–5% malignant nodules from cytology benign nodules was an unsolved problem.

As for Bethesda III or IV nodules, several studies had been reported that the malignancy of thyroid nodules with AUS/FLUS could be predicted by suspicious US features (irregular margins, taller-than-wide shape, marked hypoechogenicity or microcalcification). Moreover, the number of suspicious US features can predict the malignancy of thyroid nodules that had benign FNA results but were pathologically proven to be malignant [24]. In our research, suspicious US features, including hypoechogenicity, taller-than-wider, irregular boundary and microcalcification, were predictors of malignancy in cytology benign nodules and the combined of these features had better performance (the AUC was 0.892).

The combination of FNA and BRAF V600E were well known to effectively improve the diagnostic accuracy, especially for predicting the results of atypia or follicular lesion of undetermined significance (AUS/FLUS) cytology samples and had a high specificity [25]. The BRAF V600E detection was proposed to refine the risk of malignancy, and surgical management would change based on the presence of positive results [26]. In a meta-analysis, the findings suggested that better risk stratification and treatment options were possible for MPTC with BRAF V600E mutations than those without BRAF V600E mutations [27]. For thyroid nodules found to be benign with FNA cytology, it was unpractical and did not meeting the ATA guideline to do BRAF V600E test. In our study, twenty of 27 thyroid nodules with benign FNA and malignant results pathologically confirmed had BRAF V600E mutations, yielding the good performances with AUC of 0.848 (95% CI: 0.733, 0.962). However, the application of the BRAF V600E detection is expensive and not widely available in clinical and BRAFV600E mutations can only be found in 50–85% PTC. Our study retrospectively analyzed not only the molecular marker (BRAFV600E), but also the CUS and SWE features to predict malignancy in benign cytology nodules.

Elastography is another important noninvasive method for preoperative diagnosis of thyroid nodules. Many subsequent prospective and retrospective studies had interpreted the diagnostic efficacy of elastic assessments for benign and malignant thyroid nodules with both qualitative and quantitative modes and two-dimensional and three-dimensional techniques [28]. Elastography of ARFI also showed the special diagnostic superiority in thyroid nodules less than 10 mm [29, 30]. At present, there is no final conclusion on which elastic imaging technology is more standard. The most commonly used elastography technique is SWE in clinical. The advantage is that it can represent both quantitative and qualitative indicators at the same time, which were displayed in numerical values and color images respectively [31]. It was reported that benign nodules showed lower values than carcinomas when SWE were performed, which offered new possibilities for the differentiation malignancy when combined CUS, Color Doppler and contrast enhanced ultrasound (CEUS) [32]. To date, few research had assessed the diagnostic value of SWE in identifying thyroid nodules with benign cytology. Based on our study, we concluded that SWE max≥32.9 KPa and SWE mean≥17.1 KPa were predictors for malignancy in thyroid nodules with benign cytology. It was consistent with previous researches that that SWE max was an important predictor of benign and malignant nodules [33].

The part of the reason for the FNA misdiagnosis might be that several cells of the thyroid nodule cytology sample were unable to reflect the morphology of the nodule tissue and that of the adjacent to the surrounding tissues. In our study, 77% (22/27) malignant nodules had a maximum diameter less than 10 mm and 25.9% (7/27) less than 5 mm. The size of nodules may also be one of the factors that account for the accuracy of FNA, which was considered an FNA omission due to the small diameter of the cancer [34, 35].Although the guidelines recommend that nodules less than 5 mm were not suggested for FNA, 7 of 8 nodules less than 5 mm were malignant proved pathologically and 1 was benign in our study. Two suspicious CUS features at least were associated with this nodule and both were accompanied with BRAF V600E mutations.

Our research analyzed the characteristics of FNA benign nodules with positive BRAF V600E results or not respectively. For the malignant nodules with FNA benign results and BRAF V600E mutation, 90% nodules had 2 or more suspicious CUS features and as similar as the conclusion that was reported previously [36]. For malignant nodules pathologically proved without mutational FNA benign results, the conclusion was consistent with the mutational nodule.

The forty-nine thyroid nodules with benign FNA results were surgical performed for reasons as followed: Twenty-one thyroid nodules were operated because of positive BRAF V600E results and 6 nodules with diameters more than 20 mm because of the sense of oppression, and 22 nodules because of higher TI-RADS classification and the operations needs of patients. To avoid non-diagnostic or atypical outcomes of FNA, additional BRAF V600E tests were performed after FNA with the consent of patients.

There were some limitations in our study. Firstly, although the reliability of benign and malignant results were guaranteed by the inclusion of surgical patients merely, but the loss of patients who were followed up after FNA could also cause bias in the results. Secondly, Kim et al and DiLorenzo et al had reported false-positive cases of BRAF V600E and the cause of the result was high sensitivity of BRAF V600E detection. Similarly, in our study, one nodule with Bethesda II classification and preoperative BRAF V600E mutation, but benign result was obtained from histopathology. The other reason was that the BRAF V600E was mutated but the cellular tissue morphology had not changed significantly and cannot be diagnosed malignancy by histology [37 –39]. However, there is no conclusive evidence that it was false negative of FNA and histopathology or false positive of BRAF V600E. Additionally, mutational panels have been expanded but molecular marker of BRAF V600E was used in this study. Thirdly, this is a retrospective study and 49 cases were included in the study merely. Additionally, only the type of PTC was included in the malignant cases. we need more cases and prospective studies to verify our results in future studies.

5 Conclusions

Hypoechogenicity, taller-than-wider, irregular boundary, microcalcification, SWE max≥32.9 KPa, SWE mean≥17.1 KPa and BRAF V600E mutation may be predictors for malignancy in thyroid nodules with benign FNA results, which may help to make better preoperative decision. Further clinical decisions should be considered for nodules with 2 or more suspicious CUS features and SWE parameters greater than cut-off values whether BRAF V600E is mutational or not. The results of this study were helpful to improve the current FNA strategy of thyroid nodules.

Conflict of interest

These authors do not have any conflicts of interest.

Footnotes

Acknowledgments

This research was financially supported by Science and Technology Commission of Shanghai Municipality (Grant No.20ZR1443400) and National Natural Science Foundation of China (Grant No. 81772849, No. 82171942 and No. 81801702).

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