A Single-center, Phase II Study on Efficacy & Safety of SCRT+CAPOX+Serplulimab+Bevacizumab for MSS Rectal Cancer

Inclusion Criteria: * Patients who have a desire to preserve the anus and are willing to receive the entire course of neoadjuvant therapy. * Aged between 18 and 75 years, with no gender restrictions. * Diagnosed via pelvic MRI and rectoscopy with a tumor located ≤10 cm from the anal verge, with a clinical stage of cT3-4N0/+M0, and lymph nodes confined within the mesorectum. * Histologically diagnosed with rectal adenocarcinoma; genetic testing indicates MSS or MSI-L, or immunohistochemistry of tumor biopsy indicates pMMR (positive for MSH1, MSH2, MSH6, and PMS2 proteins). * ECOG performance status score of 0-1. ⑥ No prior antitumor therapy, immunotherapy, anti-angiogenic therapy, or pelvic radiotherapy before inclusion. ⑦ Laboratory tests must meet the following criteria: i. White blood cell count ≥3.5×10⁹/L, absolute neutrophil count ≥1.8×10⁹/L, platelet count ≥100×10⁹/L, and hemoglobin ≥100 g/L; ii. INR ≤1.5, and APTT ≤1.5 times the upper limit of normal, or partial thromboplastin time (PTT) ≤1.5 times the upper limit of normal; iii. Total bilirubin ≤1.25 times the upper limit of normal; ALT and AST ≤3 times the upper limit of normal; serum albumin ≥28 g/L; iv. 24-hour creatinine clearance rate ≥50 mL/min or serum creatinine ≤1.5 times the upper limit of normal. ⑧ Willing to participate in this study voluntarily, sign the informed consent form, and comply with the scheduled outpatient visits and related procedural requirements to complete follow-up. Exclusion Criteria: * In addition to a confirmed diagnosis of rectal cancer, there is a current or past history of active malignant tumors. * Patients with metastases to other sites indicated by preoperative staging. * Patients with suspicious positive lymph nodes in non-draining areas of the rectum, such as the internal and external iliac lymph nodes, as assessed by preoperative MRI or CT. * Patients requiring emergency surgery due to complications such as intestinal obstruction, intestinal perforation, or intestinal hemorrhage. * Patients with severe comorbid conditions and an estimated life expectancy of ≤5 years. * Patients with known allergies to any components in the study. ⑦ Patients who have received immunosuppressive or systemic corticosteroid therapy for immunosuppressive purposes within 30 days prior to the initiation of study treatment. ⑧ Patients who have received any other investigational drug treatment (including immunotherapy) or participated in another interventional clinical trial within 30 days prior to screening. ⑨ Patients with other factors that may affect the study results or lead to premature termination of the study, such as alcoholism, drug abuse, other severe diseases requiring combined treatment (including psychiatric disorders), and severe laboratory abnormalities. ⑩ Patients with congenital or acquired immunodeficiency (such as HIV infection). ⑪ Vulnerable populations, including individuals with psychiatric disorders, cognitive impairments, critically ill patients, minors, pregnant women, illiterate individuals, etc. ⑫ Other circumstances where, in the investigator's judgment, the patient is deemed unsuitable to participate in the clinical trial.