Therapeutic Efficacy of Dose-Reduced Adjuvant Chemotherapy with S-1 in Patients with Pancreatic Cancer: a Retrospective Study

S-1 adjuvant chemotherapy is the standard treatment in Asia for resectable pancreatic ductal adenocarcinoma. The relative dose intensity of adjuvant chemotherapy influences survival in pancreatic cancer but does not precisely reflect treatment schedule modifications. We investigated the effects of total dose intensity of S-1 adjuvant chemotherapy on the survival of patients with pancreatic cancer and the permissible dose reduction. Patients who underwent surgical resection during 2011–2019 for pancreatic cancer were selected. We determined the total dose intensity cut-off value that predicted tumor recurrence within 2 years postoperatively using receiver operating characteristic curves and compared the outcomes between the high and low total dose intensity groups. Patients with total dose intensity ≥ 62.5% (n = 53) showed significantly better overall survival than those with total dose intensity < 62.5% (n = 16) (median survival time: 53.3 vs. 20.2 months, P < 0.001). The median survival of patients without adjuvant chemotherapy (total dose intensity = 0, n = 28) was 24.8 months. Univariate analysis identified lymphatic involvement (P = 0.035), lymph node metastasis (P = 0.034), and total dose intensity (P < 0.001) as factors affecting survival. On multivariate analysis, total dose intensity (P < 0.001) was an independent predictor of worse survival. Maintaining a total dose intensity of at least 60% in S-1 adjuvant chemotherapy seems important to achieve a long postoperative survival in patients with pancreatic cancer.