Prediction and applications of lymph nodes posterior to the right recurrent laryngeal nerve metastasis in patients with papillary thyroid carcinoma

Purpose Residues from the LN-prRLN are one of the main reasons for persistence or recurrence in patients with papillary thyroid carcinoma (PTC). However, the methods for preoperative assessment of LN-prRLN status, as well as the correlation with other clinicopathological features remains unclear. Methods Clinicopathological feature of 493 patients with PTC were retrospectively reviewed. Univariate and multivariate analyses were performed to identify the independent risk factors of LN-prRLN metastasis. The implications of positive LN-prRLN on right lateral lymph node and contralateral central lymph node metastasis were also investigated. metastasis; cN0, lymph cN1, clinical lymph node positive; ETE, extrathyroidal extension; LN-arRLN, lymph node anterior to the right recurrent laryngeal nerve; LN-prRLN, lymph nodes posterior to the right recurrent laryngeal nerve; Right-CLNM, right central lymph node metastasis; Right-LLNM, right lateral lymph node metastasis; SD, standard deviation. with positive LN-prRLN accompanied by right-LLNM, which was 14.35 times higher than in negative LN-prRLN patients. In addition, approximately 95.0% PTC patients with positive LN-prRLN accompanied by con-CLNM, which was 21.13 times higher than in negative LN-prRLN patients. Our study rstly found that LN-prRLN metastasis has a considerable relationship with right-LLNM and con-CLNM, which means LN-prRLN metastasis can indicate that the higher risk of right-LLNM and con-CLNM.


Introduction
Thyroid carcinoma (TC) is one of the most rapidly increasing malignancies during the past 40 years worldwide, and the estimated incidence rate increased from 4.56 to 14.42 per 100,000 person-years between 1974-1977 and 2010-2013 [1,2]. Papillary thyroid carcinoma (PTC) is the most common pathological type of TC, which accounts for approximately 80%-90% [3]. Thyroidectomy with lymph nodes dissection (LND) is the main treatment of choice for PTC, but the correct management of nodal excision remains controversial [4]. Although PTC shows a favorable prognosis, and with an average 10-years overall survival (OS) rate of around 90%, however, central lymph node metastasis (CLNM) has been proved to exist in 21-82% of PTC patients during the rst treatment [5][6][7]. In addition, residual positive lymph nodes by negative lymph node dissection will increase the risk of recurrence and distant metastasis, which greatly affects the quality of life after surgery and increases the cost of retreatment again [8,9]. Thus, complete lymph node dissection (LND) during the initial operation will help to improve the prognosis of patients with PTC.
Based on the physiological characteristics of speci c anatomy of neck, central compartment is different between the left and right. According to American Thyroid Association (ATA), lymph nodes in central compartment consist of Delphian, pretracheal and paratrachea-esophageal lymph nodes [10]. Since the right recurrent laryngeal nerve is located in lymph nodes and brofatty tissues, the lymph nodes in the right central compartment are divided into two parts and labeled separately based on American Head and Neck Society (AHNS), including lymph nodes posterior to the right recurrent laryngeal nerve (LN-prRLN) and lymph nodes anterior to the right recurrent laryngeal nerve (LN-arRLN) [11], which is different from the left central compartment. Nowadays, it is controversial whether to dissect the LN-prRLN, as relatively lower prevalence of LN-prRLN metastasis and higher morbidities such as recurrent laryngeal nerve injury has been observed during the operation [12][13][14][15]. Accurate preoperative and intraoperative evaluation of LN-prRLN metastasis will de nitely help in determining the extent of CLND, thereby reducing the risk of residual positive lymph nodes and surgery-related complications. However, the ability to evaluate the status of LN-prRLN is limited.
Few studies have found some relationship between the clinicopathological characteristics and status of LN-prRLN, but the application was limited due to insu cient study individuals and shortage of genetic background [16]. Our previous study has found BRAF V600E mutation was the independent predictors of LNM in PTC [17]. Given these data, in this study 493 PTC patients were divided into LN-prRLN metastasis positive and LN-prRLN metastasis negative groups according to the status of LN-prRLN from postoperative histopathologic records, and identify the risk factors associated with LN-prRLN metastasis in PTC by investigating preoperative clinicopathological and BRAF V600E status retrospectively. In addition to this, our study also evaluated the association between LN-prRLN and right lateral lymph node metastasis (right-LLNM) and contralateral central lateral lymph node metastasis (con-CLNM), proposed to guide surgical decision making.

Materials And Methods
Patients and study design This study was approved by the Ethics Review Committee of Xiangya Hospital, Central South University and all patients gave written informed consent for research prior to surgery. We enrolled the patients who underwent thyroid surgery between January 2015 and June 2021 at the department of general surgery, Xiangya hospital, central south university. The inclusion criteria were: pathologically proven PTC located in the right lobe, patients underwent thyroidectomy and lymph node dissection (LND). Patients will be excluded if they with any of the following conditions: mixed thyroid carcinomas, history of thyroid surgery, distant metastasis, unknown LN-prRLN status, and incomplete clinical information. Finally, totally 493 patients with PTC were enrolled in this study.

Surgical strategy
Total thyroidectomy (TT) was performed for bilateral PTC or unilateral PTC patients with tumor size > 1 cm, extrathyroidal extension (ETE), or regional suspicious LN metastasis detected during preoperative or intraoperative examination (cN1), according to the ATA management guidelines [10]. We also performed thyroidectomy for papillary thyroid microcarcinoma (PTMC) patients with unfavorable features (age < 45 years, multifocality, ETE and tumor size > 0.5 cm) for lymph nodes metastasis based on the patient's wishes [18,19]. We routinely perform at least ipsilateral CLND for unilateral PTC patients. For bilateral PTC patients, cN1 patients, and locally advanced tumor (T3/T4), bilateral CLND will be performed.
Therapeutic lateral lymph nodes dissection (LLND) was done only for patients with clinically suspicious lateral LN metastasis which was con rmed by ne needle aspiration biopsy and/or US, CT before surgery [10]. All surgical procedures performed by one surgical team.
The neck central compartment was bounded superiorly by the hyoid bone, inferiorly by the suprasternal notch, laterally by the carotid artery, medially by midline of trachea, and dorsally by the prevertebral fascia. In the right central compartment, the paratrachea-esophageal lymph nodes posterior to the right recurrent laryngeal nerve were marked as paraesophageal lymph nodes also known as LN-prRLN, and the paratrachea-esophageal lymph nodes anterior to the right recurrent laryngeal nerve were marked as paratracheal lymph nodes. After surgery completed, surgeons subdivided the right central lymph node into Delphian, pretracheal, paratracheal, and LN-prRLN groups depending on the anatomical location.

Clinicopathological characteristics evaluation
The clinicopathological characteristics include age at diagnosis, gender, BRAF V600E mutation, hashimoto's thyroiditis (HT), tumor size and location in the right lobe, capsule invasion and ETE of the primary lesion in the right lobe, and LN metastasis status. Multifocality is de ned as two or more lesions in the right lobe regardless of bilateral tumors. Tumor size was calculated as the largest tumor diameter in the right lobe. Capsular invasion and ETE was evaluated by intraoperative gross and pathological ndings. Other clinical features include preoperative sonographic characteristics of the primary lesion in the right lobe such as echogenicity, composition, shape, margin, microcalci cations and oval.

Statistical analysis
Statistical analyses were performed by SPSS 26.0 (IBM SPSS, Inc. Chicago, IL, USA). Continuous variables were presented as mean ± standard deviation (SD) while categorical variables were presented as the number of cases, percentages (%). The continuous variables used Student's t-test to compare differences whereas the categorical variables used chi-square test Fisher's exact test to compare differences. Univariate and multivariate analysis were used to determine the independent predictors of LN-prRLN metastasis. Subsequently, the prediction model was developed based on the independent predictors. The area under the receiver operating characteristic curve (AUC) was used to assess the discrimination ability between the present model in our study with other models. p-value < 0.05 was considered as statistically signi cant.
It is worth noting that both tumor size and the number of positive LN-arRLN were positively correlated with the risk of positive LN-prRLN in patients with PTC (R 2 = 0.9652, p < 0.001; R 2 = 0.9109, p < 0.001, respectively) ( Fig. 1a and 1b). Based on the receiver operating characteristic curve (ROC) analysis, we found that the number of positive LN-arRLN and tumor size were the well predictors for positive LN-prRLN ( Fig. 1c). In our results, the area under the receiver operating characteristic curve (AUC) of number of positive LN-arRLN in positive LN-prRLN group was 0.853 (the optimal cutoff value = 1, 95% CI: 0.818-0.887), indicating that the accuracy of the test was good, which means the number of positive LN-arRLN ≥ 1 was a well predictor of positive LN-prRLN (Fig. 1c). The optimal cutoff value of tumor size in positive LN-prRLN group was de ned as 10.05 mm (AUC = 0.779, 95% CI: 0.635-0.824) and means PTC patients with tumor size > 10 mm was at high risk for positive LN-prRLN (Fig. 1c).
To As several predictive models for positive LN-prRLN were reported in previous studies, we summarized the characteristics of these different models in Supplemental Table 1. In order to verify the superiority of the present model, and to compare the predictive ability between different models, the concordance index (Cindex) was assessed by ROC curve. Our results showed that the AUC of our present model was the largest (AUC = 0.915), which means the present model was more appropriate in predicting positive LN-prRLN. In addition, net reclassi cation improvement index (NRI) was performed to quantify improvements in predictive performance of present model relative to other models. We found that adopt the present model resulted in signi cant improvement in reclassi cation for predict positive LN-prRLN (NRI > 0, all) (Fig. 2).
Other parameters related to the diagnostic value were summarized in Supplemental Table 1.

Discussion
Generally, central LNs are the rst and the most commonly involved regions, and the relationship between insu cient CLND after initial surgery and the recurrence in PTC patients has been well proved [9,20].
Since LN-prRLN metastasis is not common, the decision regarding the appropriate range of CLND should take into consideration both for the reduction of local recurrence and the incidence of postoperative complications. Besides, reoperation may increase patient's postoperative complications [21]. However, the predictors for LN-prRLN metastasis remains unclear. Consequently, it is imperative to identify patients with high risks of positive LN-prRLN for surgeons to choose the most appropriate operative approach. In the present study, we are the rst to evaluate risk factors of LN-prRLN metastasis in PTC based on clinicopathological characteristics and genetic background, and identi ed four independent risk factors signi cantly associated with LN-prRLN metastasis which are male sex, ETE, tumor size > 10 mm, the number of LN-arRLN metastasis and present right-LLNM.
In our study, the incidence of LN-prRLN metastasis was 32.0%, which was slightly higher than previous studies reporting LN-prRLN metastasis rates of 11-26.7% [13,14], because of we recruited some PTC patients with clinical lymph nodes positive. Moreover, we observed the risk of LN-prRLN metastasis in male was 1.978 times higher compared to female, which also matches with previous studies [13]. In addition, LN-prRLN metastasis accruing to 56.4% patients with CLNM and 71.5% patients with LLNM, which was close to the previously reported results [22,23]. It is worth noting that 94.9% (150/158) of patients with LN-prRLN metastasis simultaneous with pathologically con rmed LN-arRLN metastasis.
Thus, complete CLND including LN-prRLN was necessary for PTC patients when any evidence of LN-arRLN metastasis existed.
Ultrasonography (US) and computed tomography (CT) are the primary examination for lymphadenopathy. Zou M et al. found that US-detected lateral compartment lymph node metastasis (LLNM) was the independent risk factors of LN-prRLN metastasis [23]. However, owing to its anatomically deep position, emerging studies indicated that both the sensitivity of ultrasonography and computed tomography are not su cient enough to directly detect LNM in the central compartment [24], especially for LN-prRLN metastasis. Other sonographic characteristics like microcalci cations were reported as signi cantly related to positive LN-prRLN [23]. However, in our results, all sonographic characteristics including solid composition, hypoechogenicity, irregular shape, poorly marginal, oval, microcalci cations were de ned as not an independent risk factor for positive LN-prRLN.
Tumor size played an important role in LN-prRLN metastasis and was indicated as an independent risk factor of LN-prRLN metastasis in almost all reported models [12,22,23,[25][26][27]. In our study, we found that tumor size is positively correlated with LN-prRLN metastasis, and tumor size > 10mm was an independent predictor for LN-prRLN metastasis. Consistent with our research result, the relationship also has been well proved by Wang 27]. However, in our results, capsular invasion, multifocality and bilateral tumor were de ned as not associate with positive LN-prRLN. Consistent with previous reports, we found that ETE was an independent risk factor for LN-prRLN metastasis. Furthermore, we summarized all reported models for predicting LN-prRLN metastasis. Surprisingly, in the seven predictive models that have been reported, there is no common risk factor. In order to further testify its superiority, we compared the predictive performance of the present model with all reported models based on the AUC (C-index) and net reclassi cation improvement index (NRI). Our results showed that the AUC of the present models was the largest (AUC = 0.915) and the NRI shows signi cant improvement in reclassi cation relative to others (NRI > 0, all), which means the present model demonstrates the excellent performance and can be effectively used to discriminate the LN-prRLN metastasis.
Similarly, we con rmed that the positive LN-prPLN shows a signi cant correlation with right-LLNM and con-CLNM, relative to right-CLNM and positive LN-arPLN. Consistent with our results, a number of recent studies have reported that the ipsilateral CLNM was an independent predictor for ipsilateral LLNM and con-CLNM in PTC patients [29][30][31]. However, there is no study which stated the association among LN-prRLN metastasis with right-LLNM and con-CLNM. In this study, the prevalence of right-LLNM was 144 (29.2%), that is consistent with results from other studies [32]. Out of the 79 patients underwent bilateral CLND, 40 (50.6%) have LNM in the contralateral central compartment. This comparatively higher incidence of con-CLNM in our study could be explained by the following reason, most of bilateral CLND was performed for these advanced PTC patients (T3/T4) and clinically involved lateral neck lymph nodes (cN1b). In our study, we observed that approximately 78.5% PTC patients with positive LN-prRLN accompanied by right-LLNM, which was 14.35 times higher than in negative LN-prRLN patients. In addition, approximately 95.0% PTC patients with positive LN-prRLN accompanied by con-CLNM, which was 21.13 times higher than in negative LN-prRLN patients. Our study rstly found that LN-prRLN metastasis has a considerable relationship with right-LLNM and con-CLNM, which means LN-prRLN metastasis can indicate that the higher risk of right-LLNM and con-CLNM.
There are some limitations to this study, including the inherent design aws due to its nonrandomized retrospective study. Besides, although we determine the risk factors for LN-prRLN metastasis in our study, we still cannot be sure whether or not routine LN-prRLN dissection can improve the prognosis of patients with PTC, further studies with longer follow-up periods from multicenter is needed.