Analysis of treatment outcomes according to the cycles of adjuvant chemotherapy in gastric cancer: a retrospective nationwide cohort study

BACKGROUND: One-year S-1 or six-month capecitabine/oxaliplatin (CAPOX) has been the standard adjuvant chemotherapy for gastric cancer (GC). We investigated outcomes according to the cycles of adjuvant chemotherapy, using data from the Korean Health Insurance and Assessment Service. METHODS: A total of 20,552 patients, including 13,614 patients who received S-1 and 6,938 patients who received CAPOX extracted from 558,442 patients were retrospectively analyzed. The five-year overall survival rate was evaluated according to the duration of adjuvant chemotherapy. RESULTS: The five-year overall survival rate gradually increased according to the increase in adjuvant chemotherapy cycles in both the S-1 (≤ 5 cycles: 48.4%, hazard ratio [HR] 4.06, 95% confidence interval [CI] 3.74–4.40, P < 0.0001; 5 < cycles ≤ 6: 55.4%, HR 3.08, 95% CI 2.65–3.57, P < 0.0001; 6 < cycles ≤ 7: 64.1%, HR 2.11, 95% CI 1.84–2.41, P < 0.0001; 7 < cycles < 8: 71.1%, HR 1.60, 95% CI 1.39–1.84, P < 0.0001; ≥ 8 cycles: 77.9%) and the CAPOX groups (≤ 4 cycles: 43.5%, HR 3.20, 95% CI 2.84–3.61, P < 0.0001; 5 cycles: 45.3%, HR 2.63, 95% CI 2.11–3.27, P < 0.0001; 6 cycles: 47.1%, HR 2.09, 95% CI 1.76–2.49, P < 0.0001; 7 cycles: 55.3%, HR 1.63, 95% CI 1.35–1.96, P < 0.0001; ≥ 8 cycles: 67.2%). CONCLUSIONS: Reducing the treatment cycles of adjuvant chemotherapy in GC with S-1 or CAPOX showed inferior survival outcomes. Completing the standard duration of adjuvant chemotherapy with S-1 or CAPOX would be strongly recommended.