Three-dimensional conformal radiation therapy is a kind of high-precision radiation therapy. It uses CT images to reconstruct the tumor structure in three dimensions, and by setting a series of different irradiation fields in different directions and using conformal lead blocking that is consistent with the shape of the lesion, the distribution shape of the high-dose area is consistent with the shape of the target area in three dimensions (anterior-posterior, left-right, and superior-inferior directions), while making the exposure of the normal tissues around the lesion reduced. The ideal state of tumor radiotherapy is to irradiate only the tumor without irradiating the normal tissues around the tumor. With the development of computer technology and tumor imaging technology, virtual 3D reconstruction and display technology on tumor and its surrounding normal tissues and structures have been generated. In conventional radiation therapy, the radiation treatment we do cannot be validated effectively and we do not know whether the dose distribution in the target area is achieving the expected effect. In the 3D planning system, we can calculate the real situation of the dose distribution on the virtual image based on the patient entity, evaluate the irradiation effect in time and optimize it. This improves the accuracy of the radiotherapy planning process, maximizes the irradiation of the tumor, and best protects the normal tissue around the tumor. Three-dimensional conformal radiotherapy is currently the mainstream technology in radiation therapy and is applicable to most tumors, especially in brain tumors, head and neck tumors (including laryngeal cancer, maxillary sinus cancer, oral cancer, etc.), lung cancer, mediastinal tumors, liver tumors, prostate cancer, etc.