Postoperative recurrence is a major cause of death in patients with gastric cancer. However, no established prognostic factors, other than TNM, have been identified. Therefore, it is important to identify prognostic factors for gastric cancer recurrence. While designing the study, several pathological factors were considered important, but only the SIRI was significantly associated with gastric cancer recurrence.
The relationship between inflammation and cancer has been extensively studied over the past 20 years. Immune cells have high plasticity and may show antitumor activity or, conversely, may have pro-tumorigenic activity[29–32]. In particular, systemic inflammation caused by innate/adaptive immunity is complex and highly variable, causing controversy. However, several epidemiological studies have shown that chronic inflammation causes tumor development. One prospective study showed that the cancer incidence rate in patients with elevated circulating inflammatory markers (e.g., C-reactive protein) during routine checkup was more than twice that of the control group[33]. According to a 2018 survey, 42% of adult cancers in North America are caused by correctable risk factors, all of which cause local or systemic inflammation[34]. Large-scale clinical studies have also provided evidence that non-specific inflammation inhibition using non-steroidal anti-inflammatory drugs reduces the incidence and mortality of various cancers[35, 36].
Unlike acute inflammation, which promotes antitumor immunity, chronic inflammation is non-resolving[37]. Both acute inflammation and its resolution are complex programmed processes, which if not controlled, can lead to a chronic inflammatory state and immunosuppressive microenvironment through the recruitment of immunosuppressive cells and cytokines[38, 39]. As the relationship between inflammation and cancer has been revealed, interest in the tumor microenvironment (TME), which comprises tumor cells and surrounding non-malignant cells, has increased. Much research is being conducted on the signaling pathways of immune cells and mediators, such as cytokines and chemokines, which make up the TME, and recent developments in cancer treatments have also focused on related areas[40, 41]. The first attempt to classify cancer type in patients with colorectal cancer was based on the immune base rather than the TNM stage. Depending on the degree of cytotoxic T-cell infiltration around the tumor cells, it is classified as a hot, altered, or cold tumor. Cancers with high T-cell infiltration have an antitumor effect and a good prognosis[42, 43]. Moreover, a higher expression of Programmed Cell Death Protein1/Programmed Cell Death Ligand1 (PD-1/PD-L1) in tumor-associated immune cells showed a greater antitumor effect[44]. Accordingly, checkpoint inhibitors have been developed for various cancer types, and the PD-1 inhibitor nivolumab is also used clinically as a treatment for gastric cancer[45].
Analyzing the local immunity/inflammation of the TME requires tissue and pathological analyses, which are time-consuming and expensive. Moreover, in many cases, a partial biopsy is performed, which does not analyze the entire tumor tissue, and in such cases, analysis errors may occur. A clear relationship between systemic and local inflammation has not yet been revealed; however, the immune system of the human body may be related to both local and systemic inflammation. Therefore, studies on systemic inflammatory markers are ongoing. Many studies have reported on various types of systemic inflammatory markers such as neutrophil-lymphocyte, platelet-lymphocyte, and lymphocyte-monocyte ratios and systemic immune-inflammation index in several cancer types[46–50]. Among them, the SIRI is calculated from the number of neutrophils, monocytes, and lymphocytes in peripheral blood and is known to be associated with poor prognosis in various carcinoma types[51]. A high SIRI indicates an increase in blood neutrophil and monocyte levels and a decrease in lymphocyte levels, which is related to tumor progression, infiltration, and metastasis[52–54]. The SIRI includes neutrophils and monocytes, which are among the most studied innate immune cells related to cancer progression. Moreover, the SIRI was selected because it comprises three inflammatory markers, including lymphocytes, and is expected to complement the interactions between markers, including innate, and adaptive immunity.
This study had several limitations. First, we attempted to develop a recurrence prediction scoring system using prognostic factors for gastric cancer recurrence. However, we were unable to develop this model because fewer factors showed significant results than expected. To construct this model, further research should be conducted with a larger sample size to identify more factors that increase the risk of gastric cancer recurrence. Second, systemic inflammation might have been affected by various external factors such as infection and inflammatory diseases. To minimize the possible impact, the SIRI was obtained from the last blood test performed before gastric cancer resection surgery, and this was combined with other test results, including chest radiography, urinalysis, and vital signs to determine test results, such as infection and fever. Finally, most studies related to systematic inflammatory markers and cancer prognosis are limited to China; therefore, references related to various races are lacking. Moreover, the cut-off values were set differently for each study. Therefore, additional studies including more diverse races and larger numbers of patients are necessary.
Through this study, we confirmed that the SIRI could be independently used as a risk factor for gastric cancer recurrence, regardless of TNM stage. The fact that significant results were obtained after excluding TNM stage, the strongest risk predictor, provides grounds for recognizing the SIRI as an independent factor. Additionally, a proportional relationship between the SIRI and timing of gastric cancer recurrence was confirmed. To the best of our knowledge, this finding is a novel contribution that has not been reported in previous studies. Both the recurrence of gastric cancer and the period of disease-free survival greatly affect a patient's quality of life; thus, measuring the SIRI as a predictive factor can have clinical significance. The SIRI is simple and cost-effective to measure, meaning that it can be easily integrated into routine postoperative monitoring protocols for patients with stage II/III gastric cancer to provide early identification of individuals at higher risk of recurrence. Furthermore, the SIRI can be a tool to predict the prognosis of patients with gastric cancer undergoing adjuvant anticancer therapy after surgery. Such approaches would allow for personalized follow-up schedules and timely intervention strategies, potentially improving patient outcomes. Finally, the SIRI could be used to stratify patients in clinical trials, ensuring that those at a higher risk of recurrence receive more intensive monitoring and therapeutic approaches.