Difference Between Signet Ring Cell Gastric Cancers and Non-Signet Ring Cell Gastric Cancers: A Systematic Review and Meta-Analysis

Background: There is controversy about the characteristics and prognostic implications of signet ring cell gastric cancers and non-signet ring cell gastric cancers. Objective: This study aims to evaluate clinicopathological characteristics and prognoses of signet ring cell carcinoma (SRCC) and non-signet ring cell carcinoma (NSRCC) of stomach. Methods: Studies compared between SRCC and NSRCC of the stomach after gastrectomy and published before September 1st, 2020, in the PubMed, Cochrane, and Embase databases, were identified systematically. Results: A total of 2,865 studies were screened, and 36 studies were included, with 19,174 patients in the SRCC group and 55,942 patients in the NSRCC group. SRCC patients were younger in age (P < 0.001), less likely to be male patients (P < 0.001), more afflicted with upper third lesions (P < 0.001), and presenting with more Borrmann type IV tumors (P = 0.005) than NSRCC patients. Lymph nodes metastasis was similar between SRCC and NSRCC patients with advanced tumor stage (OR: 0.86, 95% CI: 0.671.10, P = 0.23), but lower in the SRCC than NSRCC patients with early tumor stage (OR: 0.73; 95% CI: 0.560.98, P = 0.02). SRCC patients had comparable survival outcomes with NSRCC patients for early gastric cancers (HR: 1.05, 95% CI: 0.651.68, P < 0.001) but had significantly poor prognosis for patients with advanced tumor stage (HR: 1.50, 95% CI: 1.281.76, P < 0.001). Conclusions: Signet ring cell carcinomas of the stomach are an increasingly common histopathological subtype of gastric cancers. These kinds of patients tend to be younger in age and more often female. Although, signet ring cell gastric cancer is a negative prognostic factor for patients with advanced stage. The difference is that for early stage of signet ring cell gastric cancers, it has low lymph nodes metastasis rate and comparable prognosis with non-signet ring cell cancers.


INTRODUCTION
As one of the most common malignancies of the world, gastric cancer has a higher incidence in East Asian countries (1)(2)(3)(4). The signet ring cell carcinoma (SRCC) of the stomach is one of histological subtypes of gastric adenocarcinomas. According to the World Health Organization (WHO) histological classification, the SRCC is an adenocarcinoma in which more than 50% of the tumor cells are scattered malignant cells containing intracytoplasmic mucin (5,6). Besides, in the other histological classification of gastric cancers, SRCC is also can be classified as "diffused type" by Lauren classification and "undifferentiated type" by Japanese Gastric Cancer Classification (7,8). Some studies reported that the SRCC of the stomach has unique and distinct clinicopathological characteristics with other types of carcinomas of the stomach (9)(10)(11). Some scholars have stated that the SRCC patients are further younger and include more female patients, while easily have lymph nodes metastasis and distal metastasis than non-signet ring cell carcinoma (NSRCC) patients (11)(12)(13). Besides, the prognostic implication of SRCC is still with controversies. Some studies reported that SRCC has better survival outcomes than NSRCC patients (14)(15)(16). Also, some studies have presented that the survival outcomes of SRCC were similar and even poorer than NSRCC patients (17)(18)(19). With respect to these controversies, some scholars attribute the differences to the different components of the tumor stage between SRCC and NSRCC patients (20).
In view of the foregoing, we performed this study aiming to systematically ascertain and comprehensively clarify the characteristics of signet ring cell gastric cancers. The primary outcomes of this study were the survival outcomes of SRCC patients. Other clinical characteristics, such as age, sex, and tumor stage, were also analyzed.

METHODS AND MATERIALS Search Strategy and Study selection
A comprehensive literature search was performed in the Web of Knowledge, PubMed/Medline, Cochrane Collaborative Central Register of Controlled Trials, and Embase databases on September 1st, 2020, using the terms "gastric cancer, " "gastric carcinoma, " "gastric neoplasm, " "signet ring cell, " and restricted to title, abstract, and keywords. Previously published meta-analysis and systematic reviews were searched as well. Relevant articles were manually checked from the reference lists of the retrieved articles. Titles, abstracts, and subsequently full-text articles were screened by two authors (C Zhang and R Liu) based on the inclusion and exclusion criteria of this study.

Inclusion and Exclusion Criteria
The present study included those studies comparing SRCC with NSRCC (either well-controlled, moderated, and or/poorly differentiated cancers) on at least one outcome of interest.

Data Extraction
The data were independently extracted by two authors (C Zhang and R Liu) from the studies included. For each study, we recorded the name of first author, year of publication, country, study

Quality Assessment
The quality of studies included was independently assessed by two authors (C Zhang and R Liu), according to the Newcastle-Ottawa Scale (NOS) (21). If there existed disagreement on the assessment, the consensus was reached by a discussion with supervisors (WH Zhang and JK Hu). All of those studies included were ranked with a maximum of 9 points, studies with a Newcastle-Ottawa Scale score lower than 6 were considered as a moderate or low-quality study.

Statistical Analysis
The meta-analysis was performed according to the Cochrane guidelines (22). Category data were analyzed using the Mantel-Haenszel method. Continuous data were presented as the mean ± standard deviation (SD) and analyzed by the inverse variance method. For those studies which only reported median values and ranges for continuous variables, the means and standard deviations were converted according to the method reported by Hozo et al. (23). The odds ratio (OR), mean difference (MD), and hazard ratio (HR) were used to evaluated dichotomous data, continuous data, and survival outcomes, respectively. All of the OR, HR, and MD were reported with 95% confidence intervals (CIs). Egger's regression and the funnel plot were used to test the publication bias. Heterogeneity was assessed using by the I 2 statistic. When I 2 < 30%, it was considered to be low heterogeneity; 30 and <50% were considered to be moderate heterogeneity, and ≥50% was considered to be considerable heterogeneity. In the case of considerable heterogeneity, the random-effects model was used. For data with low or moderate heterogeneity, the fixed-effects model was used. Subgroup analyses based on different tumor stages were performed to identify potential differences between SRCC and NSRCC patients. The source of heterogeneity was explored with the meta-regression analysis. Possible parameters (publication year, sample size, study region, and tumor stage) were tested to explore potential origin of heterogeneity. All of the statistical analysis was performed by the "metafor" and "meta" packages of R software, version 3.2.4 (R Foundation for Statistical Computing, Vienna, Austria) and Review Manager software, version 5.3 (Cochrane, London, UK). A P-value < 0.05 was considered statistically significant in the present study.

Publication Bias
Meta-regression was performed to illuminate the origin of heterogeneity. We examined the year of publication, sample size, region of study, and tumor stage in a meta-regression model. The resulting analyses indicated that publication year (P = 0.039) and stage of the tumor (P = 0.002) were significant sources of heterogeneity for overall survival outcomes ( Table 4).
The publication bias is evaluated by Funnel plots and Egger's test. The result found there was no publication bias for the early gastric cancer subgroup (P = 0.667) or the advanced gastric cancer subgroup (P = 0.629) for overall survival outcomes. The funnel plot and results of Egger's test of the early gastric cancer and advanced gastric cancer subgroup are presented in Figures 5A,B and  Figures 6A,B.

DISCUSSION
Gastric cancer is one of the most common malignant diseases of the digestive system all over the world, and East Asian countries, such as Japan, Korea, and China have the highest incidence (1)(2)(3)(4). Although the incidence of gastric cancer is declining, an increasing trend of signet ring cells in gastric cancer was obvious in recent decades (49). According to the previous report, signet ring cell carcinoma of the stomach has significantly different clinicopathological characteristics from other histological subtypes of gastric cancer (11)(12)(13). However, the prognostic meaning of SRCC is still controversial; for example, SRCC is a poor prognostic risk factor for overall survival outcomes (9)(10)(11). In the results of the present study, SRCC patients tended to be younger, more proportionally female, and more afflicted with middle and lower third tumors than NSRCC. As for survival outcomes, there were entirely different long-term survival outcomes of different tumor stages of SRCC when compared with NSRCC.
It is an acknowledgment that the SRCC patients are lower in age than NSRCC patients, and only a few studies reported that the mean age was similar between SRCC and NSRCC patients (19,30,40). In the pooled analysis, age was significantly lower for SRCC patients than NSRCC patients. Besides, we also found earlystage cancer patients have greater age variance than advanced stage patients between the SRCC and NSRCC groups. Younger cohorts, tend to have a greater proportion of female patients, which is another clinicopathological characteristic of SRCC patients. However, the essential reason for a high proportion of female patients is unclear. Some studies have concluded that this phenomenon is due to the sex hormones of SRCC patients (50,51).
We conducted an analysis of lymph node metastasis of SRCC and NSRCC patients. The results were that there was no significant difference for advanced gastric cancer patients between SRCC and NSRCC patients (OR: 0.86, 95% CI: 0.67-1.10, P = 0.23). However, for early gastric cancer patients, the results showed that SRCC patients had significantly lower incidence of lymph node metastasis than NSRCC patients. These results are consistent with results of the previous clinical study, in which Korean scholars deemed that the lymph node metastasis risk is low when the SRCC tumor was confined in the mucosa layer, but the risk of lymph node metastasis increases significantly, once the tumor penetrates the submucosa layer to the deep layers (52,53).
The dispute about survival outcomes of SRCC patients is a major controversy when compared with NSRCC patients. During recent decades, scholars generally consider that SRCC patients have poorer survival outcomes than NSRCC patients, due to poor tumor behavior. However, the studies published in recent years have reported that the survival outcomes of SRCC patients should be evaluated and adjusted by tumor stage (20). For early gastric cancer, majority studies reported that SRCC was a good prognostic factor (15,24,43,45). Besides, some also reported that the survival outcomes were comparable between SRCC and NSRCC patients (13,17). In the pooled analysis of our study, SRCC patients have similar survival outcomes to those of NSRCC patients in both early gastric cancer patients and stage-I patients and with low heterogeneity. At least we can show that, for early gastric cancers, the long-term prognosis of SRCC patients is not worse than that of NSRCC patients. It needs to be mentioned that the present study only included patients who underwent surgical treatment. Those SRCC and NSRCC patients who had endoscopic mucosa resection or endoscopic submucosa dissection are not included in this study.
For advanced gastric cancer patients, the prognostic meaning of signet ring cancer cell content is controversial. The general consensus is that the SRCC patients had poorer survival outcomes than the NSRCC patients (19). But does the evidence support this consensus? Some scholars claimed that SRCC patients had similar survival outcomes as NSRCC patients, and the survival evaluation between SRCC and NSRCC patients should adjust the differentiated degree and tumor stage (11,45). A Korean study found that SRCC and NSRCC patients had similar survival outcomes after adjusting for the tumor stage by propensity score matching (20). In the pooled survival outcomes of advanced tumor stage patients, we found SRCC patients had significantly poorer survival outcomes than NSRCC patients (HR 1.27, 95% 1.04-1.55). However, according to the TNM staging system of gastric cancer, advanced gastric cancers included tumors with T2-4, N-/+, Mx stages. Therefore, we performed a survival analysis according to the TNM stage, and we found that SRCC patients had similar survival outcomes in stage I and stage IV patients, and poorer survival outcomes in stage II and stage III SRCC patients with the comparison with NSRCC patients. Therefore, the prognosis of stage I and stage IV SRCC patients can be considered almost equal to that of NSRCC patients; but for the locally advanced stage (stages II-III) patients, the prognosis of SRCC patients is significantly poorer than for NSRCC patients.
Most of the studies included were retrospective studies. The quality of different retrospective studies varies, which is inevitable. Because of this, we use the NOS scoring system to evaluate the quality of each study included. Among the 36 retrospective studies included, two had a NOS score of 5. We eliminated these two studies with relatively poor quality and conducted a subgroup analysis. In the end, we found that the results were not statistically different from those before the elimination. Through careful statistical analysis, 36 studies were finally included.
There is no consistent evidence about the appropriate chemotherapy treatment strategies for signet ring cell gastric carcinoma to improve prognosis. In previous studies, signet ring cell gastric carcinoma of the stomach was generally considered to be insensitive to chemotherapy, but there was no definite clinical evidence to support it. The comparison of chemosensitivity between signet ring cell gastric carcinoma and non-signet ring cell gastric carcinoma is still limited. Our previous study found that not all signet ring cell gastric cancers were insensitive to chemotherapy, and its chemosensitivity was related to the CLDN18-ARHGAP26/6 fusion gene (54). Li explored the survival of stage II-III primary signet ring cell gastric carcinoma by adjuvant chemoradiotherapy (55). In this study, SRCC patients with stage II-III experienced improved overall survival after receiving adjuvant chemoradiotherapy, which provides several treatment implications. Therefore, more clinical trials will be needed to verify the conclusion.
However, there were several limitations in the present metaanalysis. First, all studies included are associated with long time spans and different versions of tumor stage classification. The stage migration and corresponding outcomes bias were exactly included among these studies and may result in the high heterogeneity in the pooled analysis. Second, the studies included were from different countries, the different treatment strategies from eastern and western countries were bias factors. Besides, different stage compositions between eastern and western countries also have an influence on the survival analysis. Third, all of the studies included are retrospective studies. The natural limitation and quality of the retrospective studies were another factor resulting in bias. Fourth, there is no indication of radical surgery for stage IV gastric cancer. And the reason for surgery is mostly because of complications caused by tumors such as bleeding and obstruction, rather than the tumor itself. So fewer patients with stage IV gastric cancer were included.
The heterogeneity test has been completed, and its purpose is to minimize the impact that heterogeneity may have on the quality of research and results.

CONCLUSIONS
Signet ring cell carcinoma of the stomach is one of the specific histological types of gastric carcinomas. The signet ring cell gastric cancer is predominantly found among younger people and females than non-signet ring cell gastric cancer. The prognostic features of signet ring cell carcinoma are significantly correlated with tumor stage. For gastric cancer patients with T1 stage or TNM stage-I, the prognosis of SRCC patients is comparable to that of NSRCC patients. For patients with T2-T4 stages and TNM stages II-III, the prognosis of SRCC patients is significantly worse than for NSRCC patients.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

AUTHOR CONTRIBUTIONS
W-HZ and J-KH designed the study. CZ, RL, W-HZ, X-ZC, KL, KY, X-LC, L-YZ, Z-XC, Z-GZ, and J-KH collected information and analyzed and interpreted the data. W-HZ and J-KH supervised this study. All authors contributed to the article and approved the submitted version.