As we all know, radiation is harmful to human body, therefore, every day we meet many patients and their families in outpatient clinics. When it comes to radiotherapy, many families are scared of it and worry that radiotherapy will have serious impact on patients’ quality of life, they will nervously consult what is radiation therapy, whether the side effects are particularly heavy, whether the efficacy is good, etc. As the name implies, radiation therapy is the use of radiation to treat tumors. Like surgery, radiation therapy is a local and regional treatment that uses the high energy of radiation to destroy DNA, the genetic material in the nucleus of tumor cells, and induce double stream breaks (DSB) in DNA, leading to the death of tumor cells. Radiation therapy is widely used in clinical practice. Since Roentgen discovered X-rays in 1895, radiation therapy has been used to treat tumors and has been one of the three major conventional treatments for tumors and is one of the important means of comprehensive tumor therapy. It has been reported in the literature that 60%-70% of tumor patients in the United States will receive radiation therapy during the course of their disease. According to the purpose of treatment, radiation therapy can be divided into radical radiotherapy, adjuvant radiotherapy and palliative radiotherapy. For many years, it has been the main radical treatment for tumors such as nasopharyngeal carcinoma, germ cell tumors, skin cancer, cervical cancer, esophageal cancer prostate cancer and inert lymphoma. Some tumors, such as bladder cancer, breast cancer, and laryngeal cancer, are more sensitive to radiation therapy and can be treated by giving local radiation therapy to avoid extensive surgery and preserve the patient’s vital functional organs. In recent years, a large number of studies at home and abroad have successively reported that early stage lung cancer patients who cannot tolerate surgery can achieve similar efficacy as surgical resection through stereotactic radiation therapy (SBRT). In addition, it is also an adjuvant for many tumors after surgery, such as head and neck tumors, breast cancer, lung cancer, esophageal cancer, rectal cancer, soft tissue sarcoma, etc. By combining postoperative adjuvant radiation therapy, local control of these tumors can be significantly improved and tumor recurrence can be reduced. Palliative care is also one of the important roles of radiation therapy. For example, a variety of tumors are associated with bone metastasis pain, causing patients to experience severe pain. Local radiation therapy is able to control 80% of bone metastatic pain and significantly improve the quality of life of patients. Patients with brain metastases have significantly reduced life treatment, and by giving patients radiation therapy to the brain, the symptoms of the vast majority of patients with brain metastases can be controlled. Radiation therapy is not tissue specific and thus can also damage normal tissues surrounding the tumor, producing radiation damage and leading to the side effects of treatment that patients and families fear. The toxicity of radiation therapy is mainly located within the range of irradiation, and damage outside the range of irradiation rarely occurs. The common early toxicity includes mucositis after irradiation of the pharynx and esophagus, nausea and vomiting after abdominal irradiation, dermatitis and rupture after skin irradiation, bone marrow suppression after bone irradiation, and radiation pneumonia, etc. Early toxicity can be classified as early toxicity or late toxicity. Early damage can improve and disappear in most patients after cessation of radiotherapy. Late toxicity mostly appears after 3-6 months of treatment and includes radiation pulmonary fibrosis, radiation brain necrosis, radiation renal consolidation and radiation osteonecrosis. Once late radiation toxicity appears, most of them are irreversible due to the lack of effective treatment, so some scholars point out that “prevention is the best treatment for late radiation injury”. Therefore, as many patients and family members worry, radiation therapy is a “double-edged sword”, which can indeed kill three thousand enemies and damage eight hundred. By applying advanced radiotherapy techniques and designing reasonable treatment plans, we can significantly reduce the toxicity of radiation therapy. For example, in the conventional treatment era, many patients with nasopharyngeal cancer experienced prolonged dry mouth at the end of radiotherapy, but after applying advanced intensity-modulated radiotherapy (IMRT) patients had more severe dry mouth for a short period of time after treatment, but the dry mouth was often significantly relieved six months after the end of treatment. The incidence of radiation lung injury was significantly reduced in lung cancer patients who received radiotherapy after evaluation using the 3D program. The incidence of treatment toxicity can be further reduced by developing an “individualized” treatment plan that takes into account the patient’s physical condition, tumor stage, and treatment history. In conclusion, as a traditional cytotoxic treatment, radiation therapy may lead to some treatment-related complications. By developing an “individualized” radiation treatment plan with the application of advanced radiation therapy techniques and adequate planning evaluation, the advantages can be used to avoid the disadvantages, and the efficacy can be maximized without increasing toxicity.