This is an open-label, prospective, multicenter phase II clinical trial to evaluate modified short-course radiation (Radiation targeting the tumor bed without irradiating surrounding tumor-draining lymph nodes) combined with CAPOX and PD-1 Inhibitor (Tislelizumab) for patients with MSS middle and low rectal cancer. A total of 32 patients will be enrolled in this trial. The primary endpoint is the rate of pathological complete response (pCR). The organ preservation rate, tumor regression grade, long-term prognosis, and adverse effects will also be analyzed.

Inclusion Criteria:

1. Patients who have a strong willingness to preserve the anus and are willing to receive neoadjuvant therapy.
2. Male or Female aged 18-75.
3. Patients diagnosed with low rectal cancer within 10 cm from the lower edge of the tumor to the anal verge by pelvic MRI and anorectoscopy, the clinical stage is cT2N+M0/cT3-4aN0/+M0, the lymph nodes are limited to the mesorectum, the circumferential resection margin is negative.
4. Histologically confirmed rectal adenocarcinoma; Genetic testing suggests MSI-L or MSS, or tumor biopsy immunohistochemistry reveals pMMR, that is, MSH1, MSH2, MSH6, and PMS2 are all positive.
5. Eastern Cooperative Oncology Group (ECOG) 0-1.
6. No previous treatment(including anti-tumor therapy、immunotherapy or pelvic radiation).
7. Adequate hematologic, hepatic, renal, thyroid and cardiac function: white blood cells ≥3500/mm3, neutrophils ≥1800/mm3, platelets ≥100,000/mm3, hemoglobin ≥100 g/L; activated partial thromboplastin time, prothrombin time and international normalized ratio ≤1.5 × ULN; aspartate aminotransferase and alanine aminotransferase ≤3.0 × upper limit of normal (ULN), bilirubin ≤1.25 × ULN, serum albumin ≥28 g/L. creatinine clearance ≥50 mL/mi, creatinine ≤1.5 × ULN;
8. Informed consent form signed.

Exclusion Criteria:

1. Patients with a previous history of malignant tumors besides rectal cancer.
2. Patients with distant metastases before enrollment.
3. Patients with positive internal or external iliac lymph nodes are assessed by MRI or CT.
4. Patients with obstruction, perforation, or bleeding that require emergency surgery.
5. Patients with severe concomitant diseases and estimated survival time ≤ 5 years.
6. Allergic to any component of the therapy.
7. Patients with poorly differentiated adenocarcinoma, signet ring cell carcinoma, or mucinous adenocarcinoma.
8. Patients who received immunosuppressive or systemic hormone therapy for immunosuppressive purposes within 1 month prior to the initiation of therapy.
9. Patients who have received any other experimental drug (including immunotherapy) or participated in another interventional clinical trial within 30 days before screening.
10. Factors leading to study termination, such as alcoholism, drug abuse, other serious illnesses (including psychiatric disorders) requiring combination therapy, and patients with severe laboratory abnormalities.
11. Patients with congenital or acquired immune deficiency (such as HIV infection).
12. Vulnerable groups, including mentally ill, cognitively impaired, critically ill patients, minors, pregnant or lactating women, illiterate, etc.
13. Other conditions that investigators consider not suitable for this study.