Radiotherapy for rectal cancer is generally carried out under two conditions, as follows: 1. Pre-operative radiotherapy: when the tumor is large, pre-operative radiotherapy can achieve the purpose of reducing the lesion, and because the tumor is large, surgical resection is not easy to operate, and the resection area becomes smaller after radiotherapy, thus reducing the damage to the patient’s intestinal function; 2. Late radiotherapy: patients with late rectal cancer will have abdominal pain, more than blood in stool and other symptoms, and radiotherapy can be used to achieve the purpose of pain and hemostasis, thus relieving the patient’s physical discomfort.