Adjuvant Chemotherapy Trial of S-1 for breast cancer with ER-positive and HER2-negative

INTERVENTION: Patients in this group receive standard postoperative endocrine therapy for 5 years. Patients in this group receive standard postoperative endocrine therapy for 5 years and oral S‐1 for 1 year. CONDITION: Breast cancer PRIMARY OUTCOME: Invasive Disease‐free survival SECONDARY OUTCOME: Overall survival; Distant Disease‐free survival; Disease‐free survival; Safety INCLUSION CRITERIA: 1) Women with a histopathological diagnosis of invasive breast cancer 2) Curative surgery was performed for an initial diagnosis of stage I to IIIA or stage IIIB breast cancer 3) ER‐positive breast cancer 4) HER2‐negative breast cancer 5) Moderate or high risk of recurrence 6) If preoperative chemotherapy, preoperative endocrine therapy, or postoperative chemotherapy was given as previous treatment, the regimen(s) was/were conducted in accordance with the protocol. 7) If postoperative endocrine therapy was being received, the regimen was conducted in accordance with the protocol. 8) If the conserved breast, thoracic wall, or regional lymph nodes underwent radiotherapy, at least 2 weeks have elapsed since the completion of treatment. 9) Within 1 year after surgery and within 6 months after the start of postoperative endocrine therapy 10) An age of 20 years to 75 years 11) A performance status (Eastern Oncology Cooperative Group) of 0