Brachytherapy, i.e., internal irradiation, is now mostly performed using the post-loading technique: the radiation source is inserted directly into the tumor (e.g., skin cancer, tongue cancer, etc.) or placed into the organ cavity (e.g., esophagus, trachea, rectum, vagina, etc.) through a source applicator – the former is called inter-tissue irradiation and the latter is intracavitary irradiation. Distant treatment, or external irradiation, is performed at a distance from the patient and focuses on a pre-determined area. The difference between internal and external irradiation: the intensity of the radiation source for internal irradiation is small, so the treatment distance is short and most of the energy is absorbed; most of the radiation energy for external irradiation is screened and only a small portion of the energy reaches the tissue. External irradiation of radiation passes through the skin and normal tissues to reach the tumor. According to the tolerated dose and sensitivity of different tumors, different types of radiation with different energy are selected; at the same time, multi-wild irradiation techniques are used to protect the normal tissues around the tumor.