As an important pathological feature, NI is frequently found in several cancers, ranging from 6.8–75.6% in oncologic patients. The incidence of NI is approximately 20% in colorectal cancer(Peng, Sheng, Huang, Venook, Xu, Guan and Cai, 2011) and is reported much higher in biliary tract carcinoma (85–88%)(Mavros, Economopoulos, Alexiou and Pawlik, 2014; Nagakawa, Mori, Nakano, Kadoya, Kobayashi, Akiyama, Kayahara, Ohta, Ueno, Higashino and et al., 1993) and pancreatic cancer (50–80%)(Demir, Ceyhan, Liebl, D'Haese, Maak and Friess, 2010; Lenz, Karasek, Jarkovsky, Muckova, Dite, Kala, Veselska and Hermanova, 2011). As for gastric cancer, the median rate of NI positivity was 40.9% (6.8–75.6%) according to a meta-analysis from 24 studies containing 30,590 GC patients who underwent curative gastrectomy(Deng, You, Gao, Yu, Zhao, Zheng, Fang, Xu and Teng, 2014). In our study, NI was detected as positive in 270 of the 592 patients, which accounts for 45.61% in GC.
NI was reported to be associated with aggressive behavior, tumor recurrence, neoplastic pain, and poor survival in multiple tumors. In the study, we found NI positivity was associated with tumor size, tumor stage, lymph node metastasis, and blood vessel invasion, which were important features of advanced gastric cancer. Furthermore, we also detected the elevated incidences of NI in the poor-signet differentiated and diffused type GC which had more aggressive behavior.
The incidence of NI positivity in patients at pathological T1 and T2 stage is 12.03%, while increases to 68.66% with T3 and T4 lesions. Notably, 6/148 (4.05%) of the patients with T1 lesions presented with NI. T1 stage in GC can be further divided into T1a (mucosal GC) and T1b (submucosal GC). We found that all of these 6 patients were in T1b stage and no NI was detected in T1a stage patients. This phenomenon might be related to the different nerve distribution in gastric layer and need further investigation. More importantly, GC with T1 lesion is usually treated by Endoscopic mucosal resection (EMR) and Endoscopic submucosal dissection (ESD). Our data that identified the existence of NI in GC with T1 lesion indicated a potential non-curative risk of EMR or ESD, as NI represents a crucial route for local spread and potential recurrence.
Previous studies had demonstrated the features in epidemiological, clinical, and molecular aspects between Lauren diffuse histotype and intestinal histotype. In the study, elevated incidences of NI were detected in the diffused type gastric cancer patients, which is usually presented in younger patients with a lower male-female ratio.
Nagakawa et al. reported that the high ratio of NI in and biliary tract and pancreatic cancers might be associated with the high autonomic innervation in these organs(Nagakawa, Mori, Nakano, Kadoya, Kobayashi, Akiyama, Kayahara, Ohta, Ueno, Higashino and et al., 1993), which might also explain the high NI incidence in GC. Interestingly, we also found that NI positivity correlated with the tumor site of GC. The NI incidence of GC located in upper 1/3 (proximal cancer) was higher than the other parts, which was similar to the previous reports(De Franco, Marrelli, Voglino, Vindigni, Ferrara, Di Mare, Iudici, Marini and Roviello, 2018). This is possibly caused by the higher nerve density around the cardia, which support the notion of the correlation between organ innervation an NI. However, the underlying mechanism needs to be investigated in further studies.
Based on previous studies and our data, NI was highly related to the lymph node metastasis. NI positivity was detected only in 24.00% of patients without lymph node metastasis vs 61.40% of lymph node metastasis positive cases. Nevertheless, the perineural space is more likely to be considered as an independent route for tumor spread as it differs from lymphatic vessels in anatomy and ultrastructure(Takahashi, Ishikura, Motohara, Okushiba, Dohke and Katoh, 1997). Therefore, these 24.00% patients with N0 stage and positive NI further indicated NI as an independent pathological feature rather than the subsequent consequence of lymph node metastasis.
NI has been identified as a critical prognostic factor in several cancer including pancreatic cancer(Crippa, Pergolini, Javed, Honselmann, Weiss, Di Salvo, Burkhart, Zamboni, Belfiori, Ferrone, Rubini, Yu, Gasparini, Qadan, He, Lillemoe, Castillo, Wolfgang and Falconi, 2020), colorectal cancer(Al-Sukhni, Attwood, Gabriel, LeVea, Kanehira and Nurkin, 2017; Alotaibi, Lee, Kim, Lim, Yu, Kim, Kim and Kim, 2017) and cholangiocarcinoma(Zhang, Zhou, Hu, Wang, Wang and Huang, 2020). Apart from these cancers, NI is also related to the poor prognosis and a high risk of recurrence in GC. In this study, by univariate survival analysis, we found that the 5-year overall survival in NI negative patients was 80.00%, while it reduced to 56.5% in NI positive cases. Therefore, considering the significant prognostic value of NI in GC, increasing researchers have recommend to incorporate NI status into GC TNM staging system for amelioration patients’ stratification(Aurello, Berardi, Tierno, Rampioni Vinciguerra, Socciarelli, Laracca, Giulitti, Pilozzi and Ramacciato, 2017).
However, it remains a debate whether NI could serve as an independent factor for GC prognosis. Tanaka et al. reported that NI indicated a poor prognosis in gastric cancer patients(Tanaka, Yoshikawa, Okuno, Koh, Watatani, Matsumura and Yasutomi, 1997). Tianhang et al. and Bilici et al. found that NI was an independent prognostic factor for overall survival in patients with gastric cancer who underwent gastrectomy(Bilici, Seker, Ustaalioglu, Kefeli, Yildirim, Yavuzer, Aydin, Salepci, Oncel and Gumus, 2010; Tianhang, Guoen, Jianwei and Liye, 2008). Franco et al. detected that NI did not result as an independent prognostic factor in gastric cancer, but NI emerged as an independent prognostic factor in the subgroup of patients with intestinal histotype(De Franco, Marrelli, Voglino, Vindigni, Ferrara, Di Mare, Iudici, Marini and Roviello, 2018). Lee et al. showed that NI incidence was higher in the Signet Ring Cell Gastric Carcinoma (SRC) patients than in the non-SRC patients and SRC was an independent prognostic factor for the patients who were positive for NI(Lee, Son, Kim, Han and Hur, 2018).
In our study, at multivariate analysis, NI positivity was not an independent prognostic factor in total GC patients, which may limit its application. In spite of this, Lorenzo et al. demonstrated the independent prognostic value of NI in GC patients with intestinal type(De Franco, Marrelli, Voglino, Vindigni, Ferrara, Di Mare, Iudici, Marini and Roviello, 2018). Therefore, stratification analysis was performed according to different clinical-pathological variable. NI emerged as an independent prognostic factor in young (age < 60) and lymph node metastasis negative (pN0) patients. These results indicated that more aggressive treatment may be needed for these patients with NI. In addition, it was also reported that perineural invasion was independently associated with the early recurrence of GC patients after curative resection(Chen, Lin, Chen, Chen, Wang, Guo and Yu, 2020). However, whether neural invasion provides more information for the assistant of postoperative adjuvant therapy needs further research.
It is widely accepted that the combined application of clinical factors might provide more information than either factor alone. Such markers have been applied for the diagnostic and prognosis in GC patients, including neutrophil-to-lymphocyte ratio (NLR)(Fan, Wang, Zhang, Liu, Liu, Li, Ma, Li, Guan, Bai, Yang, Lou, Li, Wang and Li, 2021; Shimozaki, Nakayama, Takahari, Kamiimabeppu, Osumi, Wakatsuki, Ooki, Ogura, Shinozaki, Chin and Yamaguchi, 2021), platelet-to-lymphocyte ratio (PLR)(Zhang, Li, Zhang, Li, Zhang, Tang, Ge, Li, Xu, Guo and Shi, 2021), and C-reactive protein/albumin ratio (CAR)(Namikawa, Shimizu, Yokota, Tanioka, Munekage, Uemura, Maeda, Kitagawa, Kobayashi and Hanazaki, 2022). Moreover, it was reported that the concomitant existence of NI and lymphovascular invasion might serve as an independent prognostic factor for DFS and OS in GC patients. Therefore, we performed multivariate analysis with Cox regression according to the combination of NI status with other clinical-pathological variables. The results showed that the concomitant existence of tumor size ≥ 3cm with NI, TNM stage III with NI, and diffuse Lauren classification with NI independently predicted prognosis in GC patients. These combined markers could be used to predict the prognosis of GC patients who underwent curative gastrectomy more accurately and needs the further validation.
This study has some limitations. First, although NI has a significant prognostic value in GC, this feature can only be evaluated postoperatively. Therefore, this study was a retrospective study conducted in a single center which weakened the reliability of the data because of the nature of its collection. Second, the sample size was not large enough to distinguish the differences between different groups in subgroup analysis. Third, this study only included GC patients who underwent curative gastrectomy and did not involve IV stage GC, limiting our knowledge of the full spectrum of GC.
In summary, the retrospective analysis of a large series of GC patients who underwent curative gastrectomy confirmed the strong association between NI status and tumor size, tumor site, depth of invasion (T grade), lymph node metastasis, TNM stage, tumor differentiation, Lauren classification, and blood vessel invasion. Survival analysis revealed that NI predicted poor survival in GC patients, while it was not an independent prognostic factor according to Cox regression. Stratification analysis identified NI status as an independent prognostic factor in age < 60 subgroup and lymph node metastasis negative subgroup. Besides, the concomitant existence of tumor size ≥ 3cm with NI, TNM stage III with NI, and diffuse Lauren classification with NI independently predicted prognosis in GC patients. These new finding provided important information for detecting patients at high risk for poor prognosis after curative resection and for planning follow-up and treatment after surgery in GC patients.