The absence of malignant tumors in PET-CT examination may have some impact on the radiotherapy dose, and the specific situation is related to the degree of the patient’s disease. PET-CT examination is an advanced diagnostic imaging technology with high sensitivity, which can accurately determine the location of primary tumor, regional metastatic lymph nodes and distant metastatic foci as well as their specific conditions, and plays an important role in the formulation of tumor radiotherapy plan, the effect of radiotherapy and the assessment of prognosis. Before the formal radiotherapy, the doctor will usually use PET-CT examination to grade the infiltration range of the lesion and the extent of the tumor, so as to determine the dose of radiotherapy. If the PET-CT examination does not show any malignant tumor, then the dose of radiotherapy may be relatively reduced. However, for some early stage, the tumor size is still relatively small cancer is difficult to detect, the examination does not see malignant tumors does not mean that there is no cancer, so it does not just through the PET-CT examination to adjust the radiotherapy dose. Therefore, in order to better confirm the diagnosis of malignant tumor disease, then in addition to PET-CT examination, it is also necessary to combine with other examinations to make a comprehensive judgment, such as physical examination, MRI and pathology examination.