In this study, primary gastric cancer patients were classified into the EP group, aged ≥ 80 years, and the YP group, aged < 80 years. The significantly higher proportion of women in the EP group was thought to be due to women’s longer average lifespan [13]. However, Arakawa et al. also stated that male gastric cancer patients aged 75 years or older have more upper gastric cancer, more frequent postoperative complications, and a worse prognosis than female patients [14]. Furthermore, the EP group was characterized by poor performance status, respiratory function, immune function, and nutritional condition. The investigation of operative factors identified a significantly higher rate of laparoscopic surgery in the YP group, but no such significant difference was evident in the EP group. This may have been both because the YP group included more Stage I cases, and because more patients in the EP group had preoperative comorbidities that made it desirable to reduce operating time. However, in the era when only open surgery was performed, shortening the operation time might have been an important factor in making the procedure less invasive, but this concept may no longer apply now that laparoscopy has become commonplace.

The present examination of the type of gastrectomy showed that there were fewer total gastrectomies in the EP group, and that, though there was no significant difference in the lymph node dissection level, fewer D2 lymph node dissections were performed. Kiuchi et al. reported that postoperative pneumonia was associated with poor long–term outcomes in patients with gastric cancer [15]. Suzuki et al. stated that overall survival was poor for patients aged ≥ 75 years with ASA-PS scores of 3 who underwent D2 lymph node dissection, and that postoperative pneumonia was implicated in their poor prognosis, with D2 lymph node dissection being a significant risk factor for postoperative pneumonia [16]. Kimura et al. also showed that D2 lymph node dissection was an independent risk factor for postoperative pneumonia in 75-year-old patients [17]. In the present study, the incidence of postoperative pneumonia was significantly higher in the EP group than in the YP group. Based on preoperative data, the incidence of pneumonia may have been higher because respiratory function in the EP group was already significantly lower preoperatively, and there was no difference in lymph node dissection level.

The tendency for PNI to be lower after PSM indicates that nutritional status in the EP group was poor, and this may have increased the incidence of anastomotic leakage. The present result that PNI was identified as one of the factors contributing to survival on multivariate analysis that included all factors also suggests that poor nutritional status can trigger these complications and affect survival rates. This seems to support the possibility that there is a relationship. Rosenberg et al. defined sarcopenia as age-related loss of muscle mass [18], but secondary sarcopenia may also occur as a result of causes such as reduced activity, disease, and malnutrition [19], and older gastric cancer patients may have both these forms of sarcopenia simultaneously. Fukuda et al. reported that preoperative sarcopenia was a risk factor for severe postoperative complications in gastric cancer patients aged ≥ 65 years [20], whereas Wang et al. stated that preoperative sarcopenia and diabetes mellitus are predictors of complications after gastric cancer surgery [21]. In the present study, the EP group also included a higher proportion of patients who had a high ASA-PS score. This means that, because the sarcopenia-like condition suffered by older gastric patients may cause postoperative complications, particularly anastomotic leakage, a range of perioperative nutritional therapies is being used [22, 23].

However, cancer stage also affects the prognosis of older patients, and though some studies recommend proactively conducting curative surgery [24, 25], others encourage the use of less invasive procedures that prioritize operative safety to prevent postoperative complications [8, 26]. Konishi et al. reported that DSS was significantly higher in older patients with cStage II gastric cancer who underwent curative gastrectomy with lymph node dissection [27]. Matsunaga et al. also stated that older Stage III gastric cancer patients (≥ 75 years old) had significantly poorer DSS than younger gastric cancer patients (≤ 74 years), and that this was due to the former’s poor nutritional status and immune function, as well as their lower rates of D2 lymph node dissection and adjuvant chemotherapy [7]. However, the present data did not show a difference in DSS between patients at different stages, suggesting that it may be better to minimize surgical invasion as much as possible. Arakawa et al. reported that reduction surgery without postoperative complications had a better prognosis than standard surgery with postoperative complications [14]. The JLSSG0901 study demonstrated that laparoscopic distal gastrectomy with D2 lymph node dissection is not inferior to open distal gastrectomy for locally advanced gastric cancer [28]. Laparoscopic surgery also reportedly reduces the incidence of a range of complications than open surgery for gastric cancer patients in poor general condition with ASA-PS ≥ 3 [29]. Tanaka et al. used PSM to show that laparoscopic surgery shortened the length of hospital stay and reduced postoperative complications in gastric cancer patients aged ≥ 80 years compared with open surgery [30]. In the present study, laparoscopic surgery also improved the survival rate in the YP group and tended to reduce postoperative complications. Although there was no improvement in the survival rate after PSM in the EP group, there was a significant decrease in postoperative complications.

However, the death rate was significantly higher in the EP group, and this was characterized by the occurrence of more deaths from other diseases. The significant difference in the survival rate seen in stage II patients in the EP group also disappeared after death from other diseases was excluded. Kakeji et al. not only showed that both 5-year overall survival (OS) and 5-year DSS were poor in patients aged ≥ 80 years, but they also showed that there was a large difference in both OS and DSS between patients aged ≥ 80 years and those aged < 80 years, and that death from other diseases had a major effect above the age of 80 years [31].

However, although underlying disease was investigated with the aim of identifying factors causing death from other diseases in older patients, numerous significant, serious, underlying diseases were present in those who died from other diseases, with cerebrovascular and cardiovascular diseases being particularly common. This tendency was more pronounced in the YP group. However, the final cause of death from other diseases did not necessarily correspond to these underlying diseases, and it tended to correspond to respiratory disease more frequently in the EP group. From this, it is thought that, in elderly patients, attention should be paid to respiratory complications associated with decreased respiratory function in the early postoperative period and also in the long term postoperatively. Kamiya et al. reported that death from other diseases is not related to preoperative complications, but rather to postoperative complications and open gastrectomy [32]. Laparoscopic surgery, which reduces postoperative complications, is likely to be beneficial in reducing the number of deaths from other diseases, which has decreased the survival rate of elderly patients, as shown in the results. Although the benefit of laparoscopy was not demonstrated in the EP group, it is thought that, by proactively introducing minimally invasive surgery in the future, the survival rate will improve, as in the YP group.

Despite the significantly higher number of patients at a high pStage in the EP group, fewer of these patients underwent postoperative adjuvant chemotherapy, which may have been because of their lower postoperative performance status due to age-related underlying conditions and reduced organ function [33]. The merits and disadvantages of adjuvant chemotherapy for older gastric cancer patients are the subject of debate. Wakahara et al. recommended active treatment, such as surgery and adjuvant chemotherapy, if possible, and reported improved survival in older adult patients with advanced gastric cancer who received adjuvant chemotherapy for > 3 months [34]. Meanwhile, Schendel et al. reported that surgery alone improved survival compared to conservative treatment in older adult patients who were ineligible to receive chemotherapy [35]. The present data also showed that, since only 20.0% of patients in the EP group underwent adjuvant chemotherapy, its efficacy was difficult to evaluate, but it may be better to consider the use of adjuvant chemotherapy in patients not at risk of death from other disease, that is, those with good ASA-PS and no pre-existing comorbidities such as cerebrovascular or cardiovascular diseases, rather than on the basis of age. There is little evidence for the use of adjuvant chemotherapy in older gastric cancer patients, but in the ACTS-GC clinical trial [36], which demonstrated the efficacy of S-1 adjuvant chemotherapy for Stage II or III gastric cancer, patients aged ≥ 80 years were excluded. Phase III clinical trials to confirm the value of modified S-1 adjuvant chemotherapy following gastrectomy for frail pStage II/III older gastric cancer patients (JCOG1507, BIRDIE) are currently underway [37], and their results are awaited.

This study had a number of inherent limitations. The first was its retrospective nature. Second, follow-up was insufficient for some patients. Third, it was conducted as a single-center study of a comparatively small number of patients. Further prospective studies involving more patients in more institutions are desirable in the future. In particular, for ASA-PS, which showed a tendency to be associated with mortality on multivariate analysis of all cases, prospective studies involving a larger number of elderly patients are needed.

In conclusion, older gastric cancer patients aged ≥ 80 years may have sarcopenia associated with poor nutrition and decreased immune function. This means every effort should be made to improve their preoperative nutritional status as much as possible to prevent anastomotic leakage, and since their respiratory function is also decreased, it is also necessary to attempt minimally invasive surgery with D1 + lymph node dissection so that surgery is neither excessive nor insufficient. Beyond these efforts, it is believed that it will be possible to prevent death from other diseases that reduce the survival rate of elderly gastric cancer patients.