Disitamab Vedotin Combined With SOX Regimen Versus SOX Alone as Adjuvant Therapy for HER2-Moderate/High Expressing Stage Ⅲ Gastric Cancer: A Prospective, Multicenter, Randomized, Phase Ⅱ Clinical Trial

Inclusion Criteria: * Aged between 18 and 75 years. * Have undergone radical resection with D2 or wider lymph node dissection and achieved R0 resection status. * No prior systemic anti-tumor treatment (i.e., neoadjuvant therapy) before surgery. * Histopathologically confirmed gastric adenocarcinoma. * Pathological stage III gastric and gastroesophageal junction adenocarcinoma patients (according to the 8th edition of the American Joint Committee on Cancer \[AJCC\] staging system). * HER2 moderate-to-high expression (IHC 3+ or 2+). * Eastern Cooperative Oncology Group Performance Status (ECOG PS) of 0 or 1. * Expected survival time ≥ 3 months. * Able to understand the study protocol, voluntarily participate in the study, and provide written informed consent; * Good compliance and able to cooperate with the treatment regimen specified in this study; * Possesses complete imaging and pathological clinical data; * Adequate organ and bone-marrow function. Exclusion Criteria: * Primary stage III gastric or gastroesophageal-junction adenocarcinoma not histologically or cytologically confirmed; * Unable to receive disitamab vedotin or SOX chemotherapy; * Unable to comply with the required follow-up schedule; * Unable to accept the treatment regimen specified in this protocol; * Unable or unwilling to undergo mandated response assessments (e.g., CT imaging); * Active autoimmune disease; * History of psychoactive-substance abuse that cannot be discontinued, or any severe/uncontrolled psychiatric disorder, or any severe/uncontrolled systemic disease; * Any concomitant condition that, in the investigator's opinion, poses significant risk to the subject or could compromise study completion; * Other malignancies within 5 years before screening, except adequately treated cancers considered cured (e.g., thyroid cancer, cervical carcinoma in situ, basal/squamous-cell skin cancer, or ductal carcinoma in situ of the breast treated with curative surgery); * Lactating women; * Prior neoadjuvant therapy or intra-operative intraperitoneal chemotherapy.