Maintenance Chemotherapy With S-1 vs. Observation After Adjuvant Therapy for Resected Pancreatic Cancer With High Risk of Recurrence/Metastasis

The goal of this real-world study is to learn if maintenance chemotherapy with Tegafur, Gimeracil, and Oteracil Potassium (S-1) can improve disease-free survival (DFS) compared to follow-up observation in patients with resected pancreatic cancer at high risk of recurrence or metastasis after adjuvant therapy. The main questions it aims to answer are: * Does maintenance therapy with S-1 improve disease-free survival (DFS) compared to follow-up observation after standard treatment for resected high-risk pancreatic cancer? * Does S-1 maintenance therapy improve overall survival (OS), distant disease-free survival (DDFS), and local recurrence-free survival (LRFS) compared to observation? * What are the safety and tolerability profiles of S-1 maintenance therapy compared to observation? Researchers will compare two groups: the S-1 maintenance therapy group and the observation-only group, to see if S-1 improves survival outcomes and safety. Participants will: * Receive maintenance chemotherapy with S-1 based on body surface area dosing or be assigned to the observation group without drug intervention. * Undergo imaging evaluations every 12 weeks to monitor for disease recurrence or metastasis. * Report side effects and any adverse events during the study.