Adaptive Boost Radiotherapy to Primary Lesions and Positive Nodes in the Neoadjuvant Treatment of Locally Advanced Rectal Cancer

Inclusion Criteria: * Histopathologically confirmed rectal adenocarcinoma. * Tumor located ≤10cm from the anal verge. * Age ≥18 years. * Eastern Cooperative Oncology Group Performance Status (ECOG PS) 0-1. * Primary treatment-naive tumor confirmed by endorectal ultrasound (ERUS) or - * Magnetic resonance imaging (MRI) as cT3-4/N+ according to the 8th edition of AJCC staging. * Ability to provide tissue and blood samples for translational research. * Anticipated survival of ≥6 months. * Normal major organ function (within 14 days prior to enrollment) and suitability for receiving chemoradiotherapy. Exclusion Criteria: * History of prior chemotherapy, radiotherapy, or surgical treatment for rectal cancer, including transanal tumor resection. * Locally recurrent rectal cancer. * History of familial adenomatous polyposis. * Active Crohn's disease or ulcerative colitis. * Allergy or hypersensitivity history to 5-fluorouracil (fluorouracil) and/or oxaliplatin. * History of difficulty or inability to take or absorb oral medications. * Diagnosis of malignancy other than rectal cancer within the past 5 years (excluding completely cured basal cell carcinoma, squamous cell carcinoma of the skin, and/or in situ carcinoma treated with radical resection). * Confirmed distant metastasis, i.e., cM1, through imaging or biopsy. * History of pelvic radiotherapy. * Pregnant or lactating women. * Presence of any severe or uncontrollable systemic illness.