The dangers of radiotherapy for rectal cancer are mainly radiocutaneous dermatitis, bone marrow suppression, abnormal urination and defecation and other side effects. 1. Radioactive dermatitis: it can lead to redness, itching, pigmentation and roughness of skin at the radiation site. It may even cause skin folds to molt, blisters appear, and then break out and erupt. 2. Bone marrow suppression: it can lead to blood abnormalities, such as decrease in white blood cells, red blood cells and platelets, and then secondary infection, generalized weakness, bleeding and other symptoms. 3. Abnormal urination and defecation: patients may have urinary abnormalities such as frequent urination, urinary urgency, increased nocturia; as well as symptoms of urgency and heaviness, increased frequency of bowel movements, blood in stools and so on. Patients do not have to worry too much, after the end of radiation therapy, the symptoms will generally improve gradually, if more serious patients need to consult a doctor in time, timely treatment.