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

Inclusion Criteria: 1. Histologically confirmed pancreatic cancer (originating from the pancreatic ductal epithelium); 2. Meets one of the following conditions: Pathologically confirmed as moderately differentiated, moderately to poorly differentiated, or poorly differentiated; or pathologic staging as: T3N0M0, T1-3N1-2M0, T4N0-2M0; or one or more surgical margins are R1 resected (R0 resection if no tumor cells are found more than 1mm from the margin, otherwise R1 resection); or involvement of the portal vein and/or superior mesenteric vein resection; or pre-adjuvant chemotherapy CA19-9 \> 90 U/mL; or pre-adjuvant chemotherapy ctDNA testing positive; 3. Completed standard treatment: Received radical resection, postoperative adjuvant therapy (including adjuvant chemotherapy based on gemcitabine or fluorouracil), and radiotherapy (if applicable); 4. Eligible for oral medication; 5. Age ≥18 and ≤75, male or female; 6. ECOG performance status: 0 to 2; 7. Normal function of major organs as per the following criteria within 14 days prior to starting treatment:① Neutrophil count ≥ 1.5×10\^9/L;② Platelet count ≥ 75×10\^9/L;③ Hemoglobin ≥ 9.0 g/dL;④ AST ≤ 2.5×UNL (upper normal limit) (if liver metastasis present, AST ≤ 5×UNL);⑤ ALT ≤ 2.5×UNL (if liver metastasis present, ALT ≤ 5×UNL);⑥ Total bilirubin ≤ 1.5×UNL;⑦ Creatinine clearance (calculated using the Cockcroft-Gault formula) \> 60 mL/min or serum creatinine ≤ 1.5×UNL; 8. Women of childbearing potential must have used reliable contraception within 7 days prior to enrollment and have a negative pregnancy test, and must be willing to use appropriate contraception during the study and for 6 months after the last dose of the investigational drug. For men, they must be surgically sterile or agree to use appropriate contraception during the study and for 3 months after treatment; 9. Expected survival time ≥6 months; 10. Voluntary participation in the study, signed informed consent, and demonstrated good compliance and cooperation during follow-up. Exclusion Criteria: 1. Pancreatic cancer originating from non-pancreatic ductal epithelium, including pancreatic neuroendocrine tumors, pancreatic acinar cell carcinoma, pancreatoblastoma, and solid-pseudopapillary tumor; 2. Incomplete macroscopic resection (R2 resection); 3. Presence of distant metastasis (including malignant ascites and pleural effusion, peritoneal metastasis) or locally recurrent pancreatic cancer; 4. CA19-9 \> 180 U/mL within 21 days before enrollment; 5. Severe liver dysfunction (AST/ALT \> 3.5 times the upper limit of normal, alkaline phosphatase \> 6 times the upper limit of normal), with liver drainage; 6. Known peripheral neuropathy (CTCAE ≥ Grade 2); 7. Participation in another clinical trial of cytotoxic drugs, targeted therapies, immunotherapies, etc., within the past 4 weeks, or received systemic chemotherapy, radiotherapy, or biological therapy within the past 4 weeks; 8. Concurrent or metachronous cancers with disease-free survival ≥ 5 years (excluding pancreatic cancer), except for cancers that have been cured or can be potentially cured with local excision (e.g., esophageal cancer, gastric cancer, colorectal cancer, cervical cancer, non-melanoma skin cancer, bladder cancer); 9. Factors that significantly affect oral drug absorption, such as difficulty swallowing, chronic diarrhea, or gastrointestinal obstruction; uncontrolled Crohn's disease or ulcerative colitis; 10. Clinically symptomatic serous effusions (including pleural effusion, ascites, pericardial effusion) requiring symptomatic treatment; 11. Pregnant or breastfeeding women; patients of childbearing potential unwilling or unable to take effective contraceptive measures; 12. Known allergy to the investigational drug, the class of the investigational drug, or its components; 13. Need for systemic corticosteroid treatment (except for local steroid pre-treatment); 14. History of interstitial lung disease (including interstitial pneumonia, pulmonary fibrosis, etc.) or CT findings of interstitial lung disease; 15. Active local or systemic infection requiring treatment; 16. Heart failure NYHA classification ≥ II or severe heart disease; 17. Known HIV infection or history of acquired immunodeficiency syndrome (AIDS) or active hepatitis B or C; 18. Toxicity not recovered (CTCAE \> Grade 1) or previous anticancer surgery not fully recovered; 19. Patients deemed unsuitable for this study by the investigator.