Occult Nodal Metastasis Dened by PET-CT Identies a Unique Clinical Subtype of Lung Cancer: A Retrospective Multicenter Study

Purpose To investigate the surgical prognosis and ecacy of adjuvant therapy in non-small cell lung cancer (NSCLC) with occult lymph node metastasis (ONM) dened by positron emission tomography-computed tomography (PET-CT). Methods A total of 3537 NSCLC patients receiving surgical resection were included in this study. The prognosis between patients with ONM and evident nodal metastasis, ONM patients with and without adjuvant therapy were compared, respectively. Results


Introduction
Lymph node staging is a critical determinant for the therapeutic strategy in patients with non-small cell lung cancer (NSCLC). The occurrence of lymph node metastasis generally heralds a more guarded prognosis [1] and therefore calls for a more aggressive treatment [2]. For NSCLC with lymph node involvement, surgery alone cannot provide adequate oncological e cacy, adjuvant therapy have been proved to confer additional survival bene ts [3][4][5][6][7][8].
In the clinical practice of lymph node staging, there is a highly specialized population of occult lymph node metastasis (ONM), in whom lymph nodal metastasis is ignored by preoperative staging modalities but unexpectedly recognized during surgery. ONM presents speci c clinicopathologic characteristics and may represent a distinct invasive extent from clinically evident nodal metastasis [9,10], which implies the prognostic and therapeutic uniqueness of ONM.
However, the prognosis and treatment strategy of ONM have not been clari ed. For one thing, the controversy continues on the oncological results of ONM. Previous publications tended to favor ONM, demonstrating that ONM yielded better prognosis than clinically evident nodal metastasis [11,12].
Conversely, there were also several studies drawing a negative conclusion, revealing ONM was not a signi cant prognostic factor in NSCLC with lymph node involvement [13,14]. And the existing evidences only limited in the N2 subgroup, the signi cance of ONM in N1 population remains ambiguous. For another, the therapeutic strategy of ONM has not been fully investigated yet. The most common option for ONM is probably to proceed with surgery and administer adjuvant therapy. However, the bene ts of adjuvant therapy remain an issue of contention [12][13][14][15].
In addition, ONM in prior studies were mainly de ned according to the computed tomography modality.
Positron emission tomography-computed tomography (PET-CT), which simultaneously provides the functional and anatomical information of tumors, has emerged as a more effective staging modality [16].
In the era of PET-CT, whether ONM de ned by PET-CT represents a unique clinical subtype of NSCLC requires further investigation.
In such instances, this study aims to reveal the heterogeneity of prognosis between ONM and clinically evident nodal metastasis and tentatively explore the e cacy of adjuvant therapy in NSCLC with ONM. Patients were excluded when meeting the following criteria: history of malignancy, sublobar resection, non-R0 resection, insu cient lymphadenectomy (numbers of resected lymph nodes < 6 or resected mediastinal stations < 3), carcinoma in situ or minimally invasive carcinoma, pathological N3 involvement, distant metastasis, pathological lymph node metastasis but without preoperative Positron emission tomography-computed tomography (PET-CT) and conduction of neoadjuvant therapy. Finally, a total of 3537 NSCLC patients were included (Figure 1).

Preoperative evaluation and surgical treatment
Routine evaluation before surgery included chest X-ray and computed tomography (CT) scan, abdominal ultrasound, pulmonary function test, exible bronchoscopy, assessment of cardiac function. Magnetic resonance imaging (MRI) of cerebrum and bone scintigraphy and were applied to rule out the distant metastasis. Considering PET-CT was not a routine administration in our institutions, we excluded patients with pathological lymph node metastasis but without preoperative PET-CT to con rm the reliability of the clinical staging in lymph node involved patients. Lymph nodes with short-axis diameter > 1cm on CT or maximum standardized uptake value ≥ 2.5 on PET were de ned as suspected lymph node metastasis [10]. The tumor stages were re-assessed according to eighth edition of the TNM staging system [1].

Adjuvant therapy and follow-up
Adjuvant chemotherapy was conducted for stage IB diseases with high-risk factors and stage IIA-IIIB tumors after surgery. Platinum-based doublet regimens were given for 4-6 cycles (3 weeks per cycle) after surgery. Adjuvant radiotherapy was administrated for stage III-N2 diseases. A radiotherapy dose of 50-60 Gy in 1.8-2.0 Gy per fraction was delivered for 5-6 weeks.
Follow-up was conducted at 3, 6, 12 months within the rst postoperative year and then at one-year interval. Chest CT scan and abdominal ultrasound were routinely implemented. MRI scan for cerebrum and bone were adopted to excluded the distant metastasis. The PET-CT scan or/and endobronchial ultrasound-guided transbronchial needle aspiration (EBUS-TBNA) were recommended when recurrence was suspected. Survival data were acquired from the outpatient visit and telephone follow-up. The overall survival (OS) was estimated as the duration since the day of surgery until the day of death or last followup visit. Recurrence-free survival (RFS) was de ned as the time elapsed between the date of surgery and the date of progress or death or last follow-up visit. All patients completed follow-up survey up to September 2021.

Statistical analysis
Categorical data were presented as frequency (percentage) and compared by Pearson χ2 test or Fisher exact test. Normally distributed continuous parameters were exhibited as mean ± standard deviation and analyzed using Student t-test, and continuous variables in skewed distribution were described as median (interquartile range [IQR]) and assessed by Mann-Whitney U test. The Kaplan-Meier method and Log-rank test were used to estimate the survival outcomes. Cox proportional hazards regression model was performed to identify the risk factors for the prognosis. All statistical analyses were conducted via SPSS 23.0 (IBM Corporation, Armonk, NY).

Adjuvant therapy bene ts of ONM
In the ONM population (n=352), adjuvant therapy was administrated to 292 (83%) patients, of them, 264 (75%) cases received adjuvant chemotherapy and 89 (25.3%) cases underwent adjuvant radiotherapy. The baseline between patients with and without adjuvant therapy did not differ signi cantly ( Table 4). As illustrated in Figure 3 & Table 5, patients receiving adjuvant therapy were associated with improved prognosis comparing to those without adjuvant therapy (5-

Discussion
ONM is a highly specialized group of lung cancer, in whom nodal metastasis evaded from the preoperative monitoring and only discovered during surgery. The current study raises several questions: should ONM de ned by PET-CT be considered as a different category from evident nodal metastasis? If such is the case, how is the prognosis of ONM population? Could the adjuvant therapy confer additional bene ts for ONM?
It is presumed that compared to evident nodal metastasis, ONM may be associated with lower extent of nodal involvement and tumor metastasis burden. In early studies, favorable prognostic impacts have been initially observed in ONM based on chest x-ray and bronchoscopy [11]. Thereafter, Andre et al. [12] claimed that occult N2 disease de ned by CT and mediastinoscopy yielded better OS than evident N2 disease (evident N2 disease versus occult N2 disease: HR=1.8, p<0.001). In the era of modern staging modality, a recent study [13] revealed that N2 metastasis negative on PET-CT achieved excellent surgical prognosis with 5-year OS of 48%, despite better than N2 disease positive on PET-CT, the results did not differ signi cantly (p=0.457). The current study included larger sample size of occult N2 disease and concurrently investigated the prognosis of occult N1 metastasis, revealing that ONM was associated with signi cantly better prognosis than evident lymph node metastasis whether for patients with N1 ( Lymph node staging determined the optimal therapeutic strategy of NSCLC [2]. The occurrence of lymph node metastasis would signi cantly reduce the oncological e cacy of surgery alone, postoperative adjuvant therapy was proved to confer additional survival bene ts [4,5]. However, controversy continues on the role of postoperative adjuvant therapy in the ONM. In the study of Andre et al. [12] and Kim et al. [13], adjuvant therapy was not a signi cant prognostic predictor for the survival outcomes of occult N2 disease. In contrast, Kim et al. [15] analyzed 115 pathological N2 disease negative on PET-CT, concluding that patients receiving adjuvant therapy were associated with better 5-year OS rate, but the difference did not reach statistical signi cance. Our results revealed that adjuvant chemotherapy did provide signi cant survival improvements whether for the occult N1 or N2 population, we speculated that it was attributable to the large sample size of our study, which enhanced the statistical e cacy of the results.
There are several limitations of our study. Firstly, due to the retrospective nature, this study suffered from its inherent selection bias. Another limitation was represented by the heterogeneity in therapeutic regimens. The current study included the fact that not all patients received adjuvant therapy, and different drugs protocols and irradiation courses were administrated. Thirdly, in handing the lymph nodes in the N1 region, station 12-14 nodes were not routinely dissected, which potentially resulted in the underestimation of N1 metastasis. In such instances, the generalization and robustness of our conclusion await validated by a randomized controlled trial.
In conclusion, our study demonstrated that ONM de ned by PET-CT was a distinct group of NSCLC, which correlated to signi cantly better prognosis than clinically evident nodal metastasis. In treatment, adjuvant therapy could signi cantly improve the prognosis of ONM patients.     Overall survival (A) and recurrence-free survival (B) curves of N classi cations for the entire cohort HR, hazard ratio; OS, overall survival; RFS, recurrence-free survival