Characteristics And Prognostic Value of Lymph Node Metastasis Along Recurrent Laryngeal Nerves in Thoracic Esophageal Squamous Cell Carcinoma


 Background: Though the value of lymph node (LN) dissection along bilateral recurrent laryngeal nerve (RLN) has been debated and emphasized in recent years in thoracic esophageal squamous cell carcinoma (ESCC). However, the characteristics of nodal metastasis along RLN chain has not been clarified. This study aimed to investigate the characteristics of nodal metastasis along recurrent laryngeal nerves and the influence of these metastasis on the prognosis of thoracic ESCC.Patients and Methods: 339 eligible patients with thoracic ESCC who underwent esophagectomy with a three-field(3-FL) or two-field(2-FL) lymph node dissection from March 2015 to December 2018 were included in this study, consisting of 282 males and 57 females with a mean age of 60.6 years (range,40-80 years). The association of LN metastasis near RLN with clinicopathologic factors and its influence on survival were analyzed. Results: Among the 339 patients, 96 (28.3%) had LN metastasis along bilateral recurrent laryngeal nerves, 76 (22.4%) with positive LNs along right RLN and 47 (13.9%) along the left RLN. There was a significant difference in the metastasis rate between the LNs along right RLN and along the left RLN (P=0.004). The LN metastasis rate along RLN was significantly correlated with primary tumor locations (upper vs middle vs lower: 35.1% vs 30.9% vs 15.6%; P=0.015), tumor invasion depth (T3/T4 vs T1 vs T2: 36.2% vs 15.8% vs 26.2%, P=0.001 ) and degree of differentiation (well vs moderately vs poorly: 9.3% vs 29.3% vs 33.9%; P=0.009), subcarinal and left tracheobronchial lymph node metastasis (positive vs negative:58.1% vs 25.3%, P＜0.001), abdominal LN metastasis (positive vs negative:41.2% vs 24.0%, P=0.003 ), but was not significantly correlated with age, gender and tumor length. The median follow-up time for this study was 34 months. The cumulative 1-, 2- and 3-year overall survival rates were 95.7%, 86.6% and 82.2% in RLN-LN(-) group versus 81.5%, 67.4% and 53.7% in the RLN-LN(+) group, with a significant difference between two group (HR=2.975,95% CI:1.918-4.614, P＜0.01). Conclusions: The lymph node metastasis along RLNs was significantly correlated with primary tumor locations, tumor invasion depth, tumor differentiation, metastasis in the LNs of other stations, and indicate poor prognosis in ESCC.


Background
Lymph node (LN) metastasis has been reported as a signi cant prognostic factor in the patients with thoracic esophageal squamous cell carcinoma (ESCC) [1]. Cancer cells of the thoracic ESCC could spread widely to the cervical and abdominal lymph nodes besides mediastinal lymph nodes through rich longitudinal lymphoid vessels in the submucosa of the esophagus, and this usually leads to poor prognosis [2]. LNs near recurrent laryngeal nerve were reported as the most common sites of nodal metastasis of ESCC in the chest [3], and metastasis to these nodes usually indicated poor prognosis.
Historically, lymph node dissection (LND) along the recurrent laryngeal nerve chain has not been emphasized in China during the past decades because most patients with ESCC underwent esophagectomies through left thoracotomy instead of right thoracic approach. This leads to frequent recurrence in the cervicothoracic area, which was just the recurrences from the LN metastasis along RLN chain [4]. In recent years, as the video-assisted thoracic surgery (VATS) getting its popularity across China, LND along RLNs has been emphasized gradually and performed in the patients with thoracic ESCC as a standardized procedure in most hospitals [5]. However, the advantages and disadvantages of LND along RLN chain have not been fully clari ed either [6]. Therefore, in order to clarify these factors, we retrospectively analyzed the association of LN metastasis along RLNs with clinicopathologic factors, and its in uence on the overall survival in the thoracic ESCC patients who underwent 2-eld or 3-eld lymph node dissection via right thoracic approach in our center during the past years.

Methods
Patient enrollment:
High resolution and enhanced chest/abdominal CT scans, bone scanning, brain MRI/CT, neck ultrasonography; FOE and EUS were performed preoperatively in all patients in order to exclude distant metastasis and make a precise clinical TNM staging. All specimens were examined pathologically at the Department of Pathology in our institution. The seventh edition UICC esophageal cancer staging criteria was used for pathological TNM classi cation. The ethic approval was waived by Ethic Committee of National Cancer Center of China after submitting the study protocol and discussion.

Principle Surgical Procedure
All 339 patients underwent McKeown or Ivor-Lewis esophagectomy via either open procedures through conventional three incisions (thoracotomy + midline laparotomy + left neck incision) or minimally invasive procedures (thoracoscopy/laparoscopy + neck incision). Lymph nodes in the chest including periesophageal lymph nodes, bilateral recurrent laryngeal nerve lymph nodes, subcarinal lymph nodes, and left tracheobronchial lymph nodes were all dissected. The abdominal lymph nodes consisting of paracardial, lesser curvature, and left gastric artery, splenic artery, common hepatic artery were all dissected. Next, the stomach conduit was made and pulled to the neck or the apex of the right thorax through esophageal bed, a handsewn or stapled gastro-esophageal anastomosis was carried out.

Statistical analysis
All statistical analysis was performed using IBM SPSS Statistics ver. 22.0(SPSS Inc, Chicago, IL, USA). Chi-square test or Fisher's exact test was used to compare categorical data. Student's t test was used for continuous data. OS was created using the Kaplan-Meier method and compared between two groups using log-rank test. Univariate Cox regression analysis was used to estimate the hazard ratios of OS.
Two-sided P values less than 0.05 were considered statistically signi cant.

Results
The clinical and pathologic characteristics of the study population are summarized in Table 1 (Fig. 4). Besides, cumulative OS in patients without any LNs metastasis was signi cantly higher than that in patients with only RLN LN metastasis (HR 2.841,95%CI 1.33-6.069, P=0.007) (Fig. 5). Compared to patients without any LNs metastasis and patients with only RLN LNs, OS of patients with RLN LNs and other LNs metastasis was signi cantly poorer (Fig. 6)

Discussion
The esophageal submucosa is rich in vertical and horizontal lymphatic vessels, which communicates with each other in the esophagus and in the mediastinal lymph network. Once tumor invades the submucosal layer, the tumor cells can spread along the longitudinal lymphatic vessels in the esophageal submucosa to the remote LNs, which result in distant or skip metastasis [5]. LN metastasis in the upper mediastinum and cervical region is the main cause of local recurrence of esophageal cancer in the thoracic esophageal cancer [7]. LN metastasis along bilateral RLN was reported to account for a major  -9]. In our study, the LN metastasis rate along bilateral RLNs was 28.3%, which was much higher than that in the other mediastinal lymph node stations. Our result is consistent with what has been reported in the literatures [10][11][12][13]. Furthermore, it was 22.4% for the nodes along the right RLN and 13.9% along the left RLN, with a signi cant difference between the right and left RLN (P=0.004). This suggests esophageal cancer has a predilection of metastasis to the nodes along bilateral RLNs, while the LNs along right RLN are the most common metastasis station. Therefore, LNs dissection along bilateral RLN is a necessary procedure in the patients with thoracic ESCC, which not only increase accuracy of tumor staging, but also improve R0 resection.
In this study, it was found that LN metastasis rate along bilateral RLNs was signi cantly correlated with primary tumor sites, tumor differentiation and depth of tumor invasion in the patients with thoracic ESCC, this is consistent with the results reported in the literatures [9][10][11]. Upper third thoracic esophageal tumors, T3/T4 tumors and poorly differentiated tumors have much higher risk of LN metastasis along RLNs, and deserve more extensive LN dissection. It was reported that lower third thoracic ESCC mainly metastasize downward to the LNs around paracardia and left gastric artery. However, our study showed that the lower third thoracic ESCC also had a high LN metastasis rate along RLNs. This implies that complete LN dissection near bilateral RLNs should also be emphasized as a necessary procedure in those patients. Our study also found that the LN metastasis rate along RLNs was signi cantly higher in the patients with the positive LNs of other stations such as subcarinal, left tracheobronchial and upper abdominal stations than those with negative LNs in the other stations. This suggest that more advanced stage tumors have much higher risk of LNs metastasis along RLNs.
It has been reported that LN metastasis was one of the most important factors affecting the prognosis of patients with esophageal cancer. Not only the number of metastatic LNs, but also the site of LN metastasis, particularly the RLN-LNs, was an important prognostic indicator [13]. Our study demonstrated that the OS and DFS of the patients with LN metastasis along RLNs were signi cantly poorer than that of those without and those with LN metastasis in other stations. Therefore, the LN metastasis along RLNs usually indicated an advanced stage. Another interesting nding was that the prognosis of the patients with single RLN nodal metastasis was much better than the patients with ≥2 nodal metastasis. All the above ndings suggested that LN metastasis along RLNs was an important prognostic factor for the patients with thoracic ESCC, and neoadjuvant therapy may be necessary for those patients with preoperative suspicious nodal metastasis along RLNs.
Besides, RLNP occurred in 38 patients (11.2%). The incidence of RLNP was related to the degree of lymph node dissection (3FL 24.0% versus 2FL 7.6%, P<0.001), and was not related to operation method, tumor T stage, and degree of tumor differentiation (P>0.05).
The limitation of this study is that this was a retrospective study, and all included patients were from a single center and operated by a group of surgeons with difference in surgical skills and experience, which may produce bias in the results. And this study lacks data on postoperative treatment of the included patients and the impact of postoperative treatment on survival was missing. Besides, factors cause the higher metastasis rate of the right RLN chain lymph node were also not explained in our study.
In conclusion, for the patients with thoracic ESCC, RLN-LNs are the most frequent metastatic sites; the metastasis in the RLN-LNs signi cantly correlated with tumor invasion depth, tumor locations and differentiation, and usually indicate an advanced stage and poor prognosis. Complete LN dissection of bilateral RLNs is strongly recommend for the patients with thoracic ESCC.
Declarations following the Declaration of Helsinki (as revised in 2013). All patients provided written informed consent before enrollment.

Consent for publication:
Yes Availability of data and material: The datasets used and analysed during the current study are available from the corresponding author on reasonable request.

Funding:
None