Metastatic Patterns of Mediastinal Lymph Nodes in Small-Size Non-small Cell Lung Cancer (T1b)

Background: Lymph node metastasis (LNM) status is critical to the treatment. Fewer studies has focused on LNM in patients with small-size non-small cell lung cancer (NSCLC). This study aims to investigate clinicopathological characteristics associated with skip N2 (SN2) and non-skip N2 (NSN2) metastasis, and their metastatic patterns in NSCLC with tumor size of 1–2 cm. Methods: We reviewed the records of NSCLC patients with tumor size of 1–2 cm who underwent lobectomy with systematic lymph node dissection (LND) between January 2013 and June 2019. Clinical, radiographical, and pathological characteristics were compared among N1, SN2, and NSN2 groups. Metastatic patterns of mediastinal lymph node were analyzed based on final pathology. Results: A total of 63 NSCLC patients with tumor size of 1–2 cm were staged as pN2, including 25 (39.7%) SN2 and 38 (60.3%) NSN2. The incidence rates of SN2 and NSN2 were 2.8% (25/884) and 4.3% (38/884), respectively. For all clinicopathological characteristics, no significant difference was observed among the groups of N1, SN2, and NSN2. For the tumor located in each lobe, specific nodal drainage stations were identified: 2R/4R for right upper lobe; 2R/4R and subcarinal node (#7) for right middle lobe and right lower lobe; 4L and subaortic node (#5) for left upper lobe; #7 for left lower lobe. However, there were still a few patients (10.9%, 5/46) had the involvement of lower zone for tumors of upper lobe and the involvement of upper zone for lower lobe. Conclusions: SN2 occurs frequently in patients with small-size NSCLC. Whether lobe-specific selective LND is suitable for all small-size patients deserves more studies to confirm. Surgeons should be more careful when performing selective LND for tumors located in the lower and upper lobes.


INTRODUCTION
An increasing incidence of small-size non-small cell lung cancer (NSCLC) has been identified in recent years (1). Lobectomy with systematic lymph node dissection remains the standard treatment for NSCLC ≤ 2 cm (2), but sublobar resection (wedge resection and segmentectomy) and non-surgical treatment have attracted growing attention. Although some studies reported no significant survival between standard lobectomy and sublobar resection (3,4), the incidence rate of occult lymph node metastasis (LNM) was high in patients with T1a-b NSCLC (5,6).
The new strategy of selective lymph node dissection (LND) has been considered for early-stage NSCLC. The American College of Surgery Oncology Group Z0030 study reported that no significant survival was observed between patients with early-stage NSCLC that received systematic LND and lymph node sampling. Furthermore, the sublevel dissection of lymph nodes could lead to the lower perioperative complications and morbidities in patients receiving selective LND, especially in elderly patients (7,8). However, little is known about the extent of selective LND, which has raised more interests for earlystage NSCLC (9,10). Therefore, it is important to elucidate the metastatic patterns of mediastinal lymph nodes in patients with NSCLC ≤ 2 cm. Previous studies reported the characteristics of mediastinal LNM in patients staged as pN2, but few of them focused on LNM in NSCLC ≤ 2 cm, and the metastatic patterns of NSCLC ≤ 2 cm remain unclear.

Study Population
This retrospective study reviewed 884 NSCLC patients with tumor size of 1-2 cm. TNM staging was based on the American Joint Committee on Cancer (AJCC) 8th edition TNM classification (27). All patients received lobectomy with systematic LND between January 2013 and June 2019 at Department of Thoracic Surgery, Peking Union Medical College Hospital (PUMCH). Pathological examination was according to the 2015 World Health Organization (WHO) classification (28). Patients were excluded for multiple lung nodules or receiving radiotherapy or neoadjuvant chemotherapy before surgery. This study has been approved by the Ethics Committee of PUMCH and informed written consents of all patients have been obtained.

Clinical, Radiographical, and Histological Characteristics
Medical records of clinical information were as follows: sex, age, smoking status, and serum tumor biomarker level (carcinoembryonic antigen, CEA). Radiographical characteristic including maximal tumor size, tumor imaging density, specific signs (spiculation, vessel convergence, lobulation, pleural indentation, and calcification). The maximal tumor size was measured at the lung window level. Based on imaging density, tumors were divided into 4 groups: pGGO, mixed GGO (mGGO; solid percentage < 50%), mGGO (solid percentage > 50%) and solid nodule. The solid percentage in mGGO was defined as the ratio of the maximal tumor diameter at the mediastinal window level to that at the lung window level. CT images were reviewed by one radiologist and two thoracic surgeons independently. Consensus reading was performed by them together when disagreement occurred. All patients underwent computed tomography (CT) or positron emission tomography/computed tomography (PET/CT) within 60 days before surgery. Patients who were highly suspected lung cancer nodule on CT were recommended to undergo PET/CT examination. None of patients underwent invasion examination such as endobronchial ultrasonography and mediastinoscopy before surgery.

Statistical Analysis
Statistical analysis was performed by using IBM SPSS 25.0 (SPSS Inc; Chicago, IL, USA). Two-category comparison was performed by Pearson's Chi square test or Fisher's exact test and quantitative data was compare using Mann-Whitney U-test. The non-parametric data was analyzed using Kruskal-Wallis test. Statistical significance was considered when p < 0.05.

DISCUSSION
Pathological N2 (pN2) stage is crucial to the management of patients with NSCLC, which was observed in ∼20-40% of all patients with NSCLC (17). In this retrospective study, 63 patients staged as N2 were identified, including 25 SN2 and 38 NSN2. Since the new surgical strategies of sublobar resection and selective lymph node dissection have been considered for earlystage NSCLC, it is crucial to reveal the metastatic patterns of mediastinal lymph nodes. Skip N2, one special sub-stage of N2 that was not furtherly subdivided in the 8th edition TNM classification, has attracted more attention in recent years because of its better survival than non-skip N2 (15,16,21,24,26). Many studies reported very different incidence rates of SN2 among N2 patients, ranging from 17 to 53% ( Table 1), but most of patients enrolled in their studies had a tumor size > 3 cm. To date, no previous study has focused on SN2 among patients with NSCLC ≤ 2 cm. In our study, 25 (2.8%, 25/884) patients were proved to be SN2, indicating a high incidence rate of SN2 (39.7%, 25/63) among pN2 patients with NSCLC ≤ 2 cm. Gorai et al. studied the clinicopathological features of SN2 among 422 patients with NSCLC ≤ 3 cm, in which 21 SN2 (40%) and 31 NSN2 (60%) patients were identified, but fewer patients had a tumor size ≤ 2 cm (23). By summarizing the studies about SN2 from 1994-2012, they found that the frequency of SN2 decreased over time and this might attribute to the technological advancement that increased the likelihood of diagnosing hilar lymph nodes. Several studies reported the higher incidence of SN2 was associated with larger size of tumors (19,20). Based on that, the incidence of SN2 should have decreased with the increasing early-detection rate of NSCLC. However, in our study, a high frequency of SN2 in patients with NSCLC ≤ 2 cm was obtained. Considering NSN2 patients' better survival than SN2 (15,16,21,24,26), it might be postulated that SN2 was an different stage from NSN2, and these patients staged as SN2 should be evaluated alone.
Previous studies have reported several factors for SN2 such as tumor size at the lung window and mediastinal window of CT, tumor location, pathological type, and pleural invasion (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26). However, in our study, we tried to enroll clinical, radiographical, and histological characteristics, but no significant difference was observed among these three groups for all variables (Table 3). However, in our previous study, many characteristics demonstrated significantly difference between node-positive and node-negative patients (30). Thus, this study indicated that patients with positive nodes, whether N1, SN2, or N2, shared certain consistency of manifested characteristics. Gorai et al. found that pleural invasion might be an important risk factor for N2 metastasis among cIA NSCLC (23). A hypothesis was put forward that tumors invaded toward the pleura and into the lymphatic ducts below the pleura, and metastasized to the mediastinal lymph nodes (23). Although our study did not obtain significance between mediastinal metastasis and pleural invasion because of the limited number of study population, patients with pleural invasion should be evaluated carefully, especially for some patients who have no positive lymph nodes of pathologic outcome. For them, N2 metastasis might occur.
Although the standard treatment for IA NSCLC remains lobectomy with SND (2), whether SND is necessary for smallsize NSCLC is not clear. With the deepening of research on mediastinal LNM, the metastatic pattern in NSCLC has been reported based on a predictive manner of lobe-specificity, so called "lobe-specific selective lymph node dissection, " which might be sufficient for small-size NSCLC (9,31,32). Compared to SND, SLND is more appropriate for elderly patients or those with poor pulmonary reserve. However, lobe-specific SLND has not been widely accepted, for which one important reason is that the metastatic pattern of small-size remains unclear. In our study, based on tumor-located lobes, the metastatic stations among SN2 and NSN2 patients were analyzed, respectively (Supplementary Figures 1, 2). It could be inferred that patients staged as SN2 and NSN2 had a similar metastatic patterns of mediastinal lymph nodes. Our study indicated that there were still some tumors in lower lobes that invaded into upper zone and some in upper lobe that invaded into lower zone by skip or non-skip metastasis. Thus, lobe-specific SLND might not be always adequate for lung cancer patients, even for small-size lung cancer. For the small-size tumors in RML, the resection of 2R/4R and #7 nodes might be reasonable. In the clinic, surgeons should be more careful when performing SLND. Whether lobe-specific selective LND is suitable for all small-size patients deserves more studies to confirm.
There were also some limitations in this study. First, the number of patients enrolled in this pilot study was limited. Future studies may consider a larger or multicenter study population. Second, this is a retrospective study and data bias could not be avoided. Prospective studies are needed for further identification of metastatic patterns of small-size NSCLC in the future. Third, there were very few patients with NSCLC < 1 cm who had positive lymph nodes, and they were excluded for avoiding bias. The following studies may focus on this population in terms of nodal metastasis.

CONCLUSIONS
In this study, 63 patients with NSCLC ≤ 2 cm were staged as pN2, including 25 SN2 (39.7%) and 38 NSN2 (60.3%). The incidence rates of SN2 and NSN2 were 2.8% (25/884) and 4.3% (38/884), respectively. The comparisons of clinicopathological characteristics among N1, SN2, and NSN2 groups indicated no significant difference. The analysis of the mediastinal LNM among pN2 patients revealed useful information about metastatic patterns of small-size NSCLC. Lobe-specific lymph node dissection may not be adequate for patients with a tumor in lower and upper lobes. Whether lobe-specific selective LND is suitable for all small-size patients deserves more studies to confirm. Surgeons should be more careful when performing selective LND for lung cancer patients, even for small-size lung cancer.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

ETHICS STATEMENT
The studies involving human participants were reviewed and approved by the Ethics Committee of Peking Union Medical College Hospital. The patients/participants provided their written informed consent to participate in this study.

ACKNOWLEDGMENTS
We would like to give our sincere thanks to Professor Hongsheng Liu, Yushang Cui, Zhijun Han, and Zhili Cao for their contributions to the clinical works.