Rectal cancer is mainly operated, but radiotherapy plays a great role in reducing recurrence and preserving function. Because: 1. 50-75% of patients who failed surgery for rectal cancer had local recurrence. Among patients who died of recurrence and metastasis within 5 years after surgery, about 49% died of local recurrence, so improving the local control rate is the key to prolong survival time; 2. 80% of rectal cancer occurs below the peritoneal fold, and local recurrence after surgery is mainly in the small pelvis and perineal area, where the scope of surgical removal is limited. 3. Pre-operative radiotherapy can shrink the tumor and make use of surgical removal, and at the same time, it can make the original patients who cannot retain the anus change to retain the anus after radiotherapy and improve the quality of life. Pre-operative radiotherapy for rectal cancer includes: 1.Pre-operative radiotherapy: to shrink the tumor and improve the resection rate; to reduce post-operative recurrence, to increase the 5-year survival rate by 10-15%, and to improve the anal preservation rate. 2.Intraoperative radiotherapy: for the masses that cannot be resected or the “high-risk area” with clear tumor residue, the small intestine and ureter should be pushed open and the tumor should be exposed to give about 15Gy at one time to improve the local control rate. 3.Postoperative radiotherapy: for the patients with lymph node metastasis, cancer infiltrated outside the intestinal wall and cannot be removed cleanly, the recurrence rate of these patients after surgery is very high. The postoperative radiotherapy can reduce the local recurrence rate. 4.Palliative radiotherapy: It is used to relieve pain, stop bleeding, shrink tumor and reduce symptoms.