Background: Curative gastrectomy followed by adjuvant S-1 chemotherapy for 12 months is one of the standard treatments for patients with pathological stage (p-stage) II or III gastric cancer. Although some patients have difficulty maintaining compliance with adjuvant S-1, the risk factors for poor compliance are unknown. Methods: We retrospectively analyzed the data of patients at 21 institutions who underwent curative gastrectomy followed by adjuvant S-1 for p-stage II or III gastric cancer. Patients who had a recurrence within 12 months after surgery were excluded from the analysis. Associations between clinicopathological factors and both 12-month compliance and the cumulative continuation rate of S-1 were analyzed. Results: Of 359 patients, 252 (70.2%) continued adjuvant S-1 until 12 months after surgery. Older age (>65 years) and postoperative infectious complications (Clavien–Dindo grade III or higher) were significantly correlated with low compliance with S-1 for 12 months (p = 0.008 and p = 0.042). These two factors also showed significant associations with low cumulative continuation rate (log-rank p < 0.001 and p = 0.018). Continuation rates at 12 months after surgery in patients aged ≤60 years, 61–65, 66–70, 71–75, and 76–80 years were 81.5, 75.9, 65.4, 58.7, and 62.9%, respectively. Type of gastrectomy or body weight loss at 1 month after surgery did not affect either 12-month compliance or the cumulative continuation rate of S-1. Conclusions: Older age, especially over 65 years, and postoperative infectious complications were independent risk factors for poor compliance with adjuvant S-1 chemotherapy for gastric cancer.
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