A Prospective, Multicenter, Single-Arm Phase II Exploratory Study of Serplulimab Combined With Oncolytic Virus H101, Short-Course Radiotherapy, and XELOX Chemotherapy as Total Neoadjuvant Treatment for Locally Advanced (cT1-3N0M0) Rectal Cancer

Inclusion Criteria:

\- Participants must meet all of the following criteria:

1. Age 18 to 80 years, male or female.
2. Histologically confirmed low rectal adenocarcinoma, with tumor located ≤5 cm from the anal verge.
3. Clinical stage T1-3N0M0 according to AJCC staging criteria.
4. Mismatch repair-proficient (pMMR) or microsatellite stable (MSS) tumor confirmed by immunohistochemistry or genetic testing.
5. No prior anti-tumor treatment.
6. Eastern Cooperative Oncology Group (ECOG) performance status of 0-1, with an expected survival of at least 3 months.
7. Adequate organ function, including:

   * Absolute neutrophil count ≥1.5 × 10\^9/L
   * Platelet count ≥100 × 10\^9/L
   * Hemoglobin ≥9 g/dL
   * Serum albumin ≥3 g/dL
   * Total bilirubin ≤1.5 × upper limit of normal (ULN)
   * ALT and AST ≤2 × ULN
   * Serum creatinine ≤1.5 × ULN or creatinine clearance ≥60 mL/min
   * INR or PT ≤1.5 × ULN, and aPTT ≤1.5 × ULN
8. Thyroid function within normal range or adequately controlled.
9. Negative pregnancy test for women of childbearing potential.
10. Willingness to use effective contraception during the study and for 12 months after treatment.
11. Ability to understand and willingness to sign a written informed consent form.
12. Willingness and ability to comply with study procedures and follow-up.

Exclusion Criteria:

* Participants will be excluded if they meet any of the following criteria:

  1. Histology other than rectal adenocarcinoma (e.g., gastrointestinal stromal tumor, lymphoma).
  2. Prior pelvic radiotherapy.
  3. Prior treatment with PD-1, PD-L1, or CTLA-4 inhibitors.
  4. Known allergy to study drugs or their components.
  5. History of other malignancies, except adequately treated non-melanoma skin cancer, carcinoma in situ of the cervix, or papillary thyroid carcinoma.
  6. Active autoimmune disease or history of autoimmune disease requiring systemic treatment.
  7. Immunodeficiency, including HIV infection, or history of organ transplantation.
  8. History of interstitial lung disease or non-infectious pneumonitis.
  9. Active tuberculosis infection or history of untreated tuberculosis.
  10. Active hepatitis B or hepatitis C infection.
  11. Severe cardiovascular, pulmonary, or renal disease.
  12. Uncontrolled hypertension despite medication.
  13. History of substance abuse (alcohol or drugs).
  14. Active uncontrolled infection.
  15. Use of systemic immunosuppressive therapy.
  16. Pregnant or breastfeeding women.
  17. Any condition that, in the investigator's opinion, may interfere with study participation or safety.