Radiation therapy can kill cancer cells. The killing effect of radiation on cancer cells is divided into direct killing effect and indirect killing effect. The direct killing effect is mainly through X-rays acting on DNA, the genetic material of tumor cells, causing them to be damaged, necrotic and lose their ability to proliferate. The indirect effect is through the production of hydrogen peroxide, a free radical harmful to cells, which causes the cells to die from poisoning. According to the World Health Organization, 18% of tumors can be cured by radiotherapy, such as early stage nasopharyngeal, laryngeal, esophageal and basal cell carcinomas, which have a chance to be cured in early stage. With the continuous updating of radiotherapy equipment, the accuracy of radiotherapy is also improving, which can effectively control the growth of tumors. The dose of radiation given to tumor cells is large enough to achieve the purpose of killing tumor cells, considering the tolerated dose of surrounding normal tissues, the dose should not be too high. Clinically, the best effect is to achieve the dose of controlling tumor cells within the tolerated range of normal tissues.