Low-Dose Radiotherapy to Sensitize Pucotenlimab Plus CAPEOX for pMMR Locally Advanced Rectal Cancer

Inclusion Criteria * Written informed consent provided prior to any study-specific procedures. * Age 18 to 75 years at the time of enrollment. * Histologically confirmed rectal adenocarcinoma. * Tumor located within 10 cm from the anal verge, as assessed by endoscopy or imaging. * Locally advanced disease, defined as clinical stage T2N+ or T3-T4a (any N) based on pelvic magnetic resonance imaging (MRI). * Proficient mismatch repair (pMMR) or microsatellite-stable (MSS) tumor status confirmed by immunohistochemistry or molecular testing. * No evidence of distant metastasis on preoperative imaging, including chest, abdominal, and pelvic computed tomography (CT). * Eastern Cooperative Oncology Group (ECOG) performance status 0-1. * Adequate organ function as defined by: * Absolute neutrophil count ≥1.5 × 10⁹/L * Platelet count ≥100 × 10⁹/L * Hemoglobin ≥90 g/L * Total bilirubin ≤1.5 × upper limit of normal (ULN) * Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤2.5 × ULN * Creatinine clearance ≥50 mL/min * Thyroid-stimulating hormone (TSH) within normal limits * Women of childbearing potential must have a negative pregnancy test and agree to use effective contraception during the study and for a protocol-defined period after the last dose. * Men with partners of childbearing potential must agree to use effective contraception during the study and for a protocol-defined period after the last dose. Exclusion Criteria * Clinical T4b disease, defined as tumor invasion into adjacent organs or structures on baseline imaging. * Evidence of distant metastasis outside the pelvis. * Prior pelvic or abdominal radiotherapy. * Prior treatment with immune checkpoint inhibitors or other systemic anticancer therapy for rectal cancer. * Active or history of autoimmune disease requiring systemic treatment, except for conditions considered low risk for recurrence (e.g., vitiligo, resolved childhood asthma). * Ongoing use of systemic immunosuppressive therapy, including corticosteroids equivalent to \>10 mg/day of prednisone, within 2 weeks prior to enrollment. * Known human immunodeficiency virus (HIV) infection. * Active hepatitis B virus infection with positive hepatitis B surface antigen and high viral load, or hepatitis C virus infection requiring treatment. * Uncontrolled active infection or other serious medical condition that, in the investigator's judgment, would compromise patient safety or study compliance. * History of another malignancy within 5 years, except for adequately treated basal cell carcinoma of the skin, cervical carcinoma in situ, or other malignancies with negligible risk of recurrence. * Known hypersensitivity or allergy to pucotenlimab, oxaliplatin, capecitabine, or any of their excipients. * Pregnant or breastfeeding women. * Any condition that, in the investigator's opinion, makes the participant unsuitable for study participation.