Radiation proctitis is a common complication after radiation therapy for pelvic tumors. The investigators found that live bifidobacterium and lactobacillus tablets combined with compound glutamine enteric-coated capsules can significantly relieve the symptoms of radiation proctitis through preliminary clinical practice, but the mechanism is unknown. Through a prospective randomized controlled study, this study intends to investigate the incidence of grade 2 or higher acute radiation proctitis in patients of locally advanced rectal cancer after radiotherapy with the combined therapy. And through various scales, next-generation sequencing methods and other methods to evaluate the clinical symptoms, colonoscopy, imaging, and changes in the species and abundance of intestinal flora before and after treatment. To further explore the related pathways and mechanisms affecting radiation proctitis.

Inclusion Criteria:

• locally advanced rectal adenocarcinoma diagnosed by histopathology; clinical stage T3/4 or N+(AJCC,8th), no metastasis; and planned to receive preoperative neoadjuvant concurrent chemoradiotherapy.
• without other malignant tumors;
• Those without serious heart, liver, kidney and other diseases;
• ECOG score: 0-1.

Exclusion Criteria:

• Complicated with ulcerative colitis or Crohn's disease;
• Complicated with severe organ insufficiency, severe diabetes, and heart disease;
• Pregnant and lactating women;
• Abnormal bowel movements with systemic and metabolic diseases;
• The patient has a history of using systemic antibiotics and intestinal active bacteria in the past 1 month.