• • Voluntarily signs a written informed consent form.

‣ Aged between 18 and 75 years at the time of enrollment.

⁃ Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.

⁃ Expected survival of more than 2 years.

⁃ Histologically confirmed rectal adenocarcinoma.

⁃ Tumor biopsy indicates proficient mismatch repair (pMMR), defined by positive immunohistochemical staining for MSH1, MSH2, MSH6, and PMS2, or molecular testing confirms microsatellite stability (MSS).

⁃ Clinical stage T3-4N0M0 or TanyN+M0 based on the 8th edition of the AJCC TNM classification, as evaluated by high-resolution MRI ± endoscopic ultrasound/transrectal ultrasonography, with the tumor located in the mid-to-lower rectum below the peritoneal reflection.

⁃ Prior to enrollment, a qualified surgical attending physician must assess the patient's medical history and confirm eligibility for curative R0 resection.

⁃ No prior systemic or local anti-tumor treatment for rectal cancer, including radiotherapy, chemotherapy, immunotherapy, biologics, or small-molecule targeted therapy.

⁃ Agrees to provide tumor tissue and peripheral blood samples during screening and throughout the study for research purposes.

⁃ Adequate organ function, defined as follows:

⁃ Hematologic (without use of blood components or growth factors within 7 days prior to treatment initiation):

⁃ Absolute neutrophil count (ANC) ≥ 1.5 × 10⁹/L

⁃ Platelet count ≥ 100 × 10⁹/L

⁃ Hemoglobin ≥ 90 g/L

⁃ Renal:

⁃ Calculated creatinine clearance (CrCl) ≥ 50 mL/min using the Cockcroft-Gault formula:

∙ CrCl (mL/min) = \[(140 - age) × weight (kg) × 0.85 (if female)\] / (72 × serum creatinine \[mg/dL\])

• Urine protein \< 2+ on dipstick or \< 1.0 g per 24-hour collection

• Hepatic:

• Total bilirubin ≤ 1.5 × ULN

• AST and ALT ≤ 2.5 × ULN

• Serum albumin ≥ 28 g/L

• Coagulation:

• INR and APTT ≤ 1.5 × ULN

• Cardiac:

• Left ventricular ejection fraction (LVEF) ≥ 50%

• Women of childbearing potential must have a negative urine or serum pregnancy test within 3 days prior to initiating study treatment. If the urine test is inconclusive, a serum test must confirm the negative result. Women of childbearing potential who are sexually active with non-sterilized male partners must agree to use highly effective contraception from screening through 120 days after the last dose of study drug. The need for continued contraception beyond this period should be discussed with the investigator.

• Women of childbearing potential are defined as those who are not surgically sterile (i.e., bilateral tubal ligation, bilateral oophorectomy, or hysterectomy) and have not undergone menopause (defined as ≥12 months of amenorrhea without alternative medical cause, with FSH levels in the postmenopausal range).

• Highly effective contraception methods are those with \<1% failure rate per year when used consistently and correctly (e.g., hormonal contraceptives). In addition to barrier methods, hormonal contraception is required. Periodic abstinence and the calendar method are not considered acceptable.

• Willing and able to comply with scheduled visits, treatment plans, laboratory tests, and other study requirements.