Previously, preliminary results, from a subgroup analysis of STARS-RC03 (NCT04906044) conducted by our research team, showed that the 6-cycles consolidation chemotherapy combining with anti-PD-1 therapy had a better tumor regression advantage with a restricted safety profile contrasted with 3-cycle counterparts. Herein, we designed this study to further evaluate the short-term efficacy (such as pCR rate, R0 resection rate, etc.) and long-term survival (including DFS, OS, etc.) of 6-cycles consolidation therapy.

The combination of total neoadjuvant treatment (TNT) and immunotherapy has shown a significant improvement in the pCR rate compared to the standard of care (SOC) or TNT alone for pMMR LARC. On this basis, we believe that this treatment mode will offers the opportunity of organ preservation for subperitoneal "Bad" or "Advanced" patients with LARC, who are initially assessed as unresectable or difficult to obtain R0 resection. Previously, preliminary results, from a subgroup analysis of STARS-RC03 (NCT04906044) conducted by our research team, showed that the 6-cycles consolidation chemotherapy combining with anti-PD-1 therapy had a better tumor regression advantage with a restricted safety profile contrasted with 3-cycle counterparts. Herein, we designed this study to further evaluate the short-term efficacy (such as pCR rate, R0 resection rate, etc.) and long-term survival (including DFS, OS, etc.) of 6-cycles consolidation therapy.

Inclusion Criteria:

1. The patients and their families are able to understand and are willing to participate in this clinical study, and sign an informed consent form.
2. Age: 18~75 years old, no gender limit;
3. Pathologically diagnosed rectal adenocarcinoma: differentiated into Grade 1-3, that is, high, medium, and poorly differentiated tubular adenocarcinoma; classified as pMMR/MSS.
4. The initial TNM risk category (from Rectal cancer: ESMO Clinical Practice Guidelines, 2017 edition) is as follows: 1) "Bad": cT3c/d or very low localisation levators threatened, MRF clear; cT3c/d mid-rectum, cN1-N2 (extranodal), EMVI+, limited cT4aN0; 2) "Advanced": cT3 with any MRF involved, any cT4a/b, lateral node+.
5. The lower edge of the tumor is located below the peritoneal reflex;
6. No distant transfer;
7. ECOG PS score 0-1 within 7 days before the first medication;
8. Hepatitis B Surface Antigen (HBsAg) (-) and Hepatitis B Core Antibody (HBcAb) (-). If HBsAg (+) or HBcAb (+), hepatitis B virus deoxyribonucleic acid (HBV-DNA) must be less than 1000 copies/mL or 200 IU/mL before entering the group.
9. HCV antibody (-)
10. The main organ function is normal.
11. No history of pelvic radiotherapy;
12. No history of rectal cancer surgery or chemotherapy;
13. Not accompanied by systemic infections requiring antibiotic treatment;
14. Heart, lung, liver, and kidney functions can tolerate surgery;
15. Others, based on the results of previous medical history, vital signs, physical examination or laboratory examination, the research doctor judges that you are suitable for participating in this clinical study.

Exclusion Criteria:

1. Recurrent rectal cancer;
2. Patients who are planning to undergo or have previously received organ or bone marrow transplantation;
3. Myocardial infarction or poorly controlled arrhythmia (including QTc interval ≥ 450 ms for males and ≥ 470 ms for females) occurred within 6 months before the first medication (QTc interval is calculated by Fridericia formula);
4. Existence of NYHA standard grade III to IV cardiac insufficiency or color Doppler ultrasound examination: LVEF (left ventricular ejection fraction) <50%;
5. Human immunodeficiency virus (HIV) infection;
6. Suffer from active tuberculosis;
7. Past and present patients with interstitial pneumonia, pneumoconiosis, radiation pneumonia, drug-related pneumonia, severely impaired lung function, etc., which may interfere with the detection and treatment of suspected drug-related lung toxicity;
8. Patients with active or suspicious autoimmune disease, or with a history of that;
9. Received treatment with live vaccines within 28 days before the first administration; except for inactivated viral vaccines for seasonal influenza;
10. Have received other antibody/drug treatments against immune checkpoints in the past, such as PD-1, PD-L1, CTLA4, etc.;
11. Known to have a history of severe allergies to any monoclonal antibody or research drug excipients;
12. In the past 5 years, patients have suffered from malignant tumors whose survival rate is significantly lower than the historical data of our rectal cancer survival rate (properly treated basal cell carcinoma, skin squamous cell carcinoma, small kidney cancer, breast cancer, and papillary thyroid carcinoma are not included here. range);
13. The patient has had arterial embolism diseases in the past 6 months, such as angina pectoris, MI, TIA, CVA, etc.;
14. Have received other types of anti-tumor or experimental treatments;
15. The patient is a female during pregnancy or lactation;
16. The patient has other diseases or abnormal mental states, which may affect the patient's participation in this study;
17. There are patients who may increase the risk of participating in research and research medication, or other severe, acute and chronic diseases, who are not suitable for clinical research based on the judgment of the investigator.