PET-CT is more than 90% used for tumor disease examination: Firstly, if from the perspective of tumor screening, patients have a family history of tumor or high risk factors for cancer, such as long-term smoking, more than 20 cigarettes per day, or exposure to some carcinogenic substances, they are patients with high risk of tumor, and are suitable for PET-CT for tumor screening. Because PET-CT can detect tumor at an early stage after injecting posionuclide imaging agent. Usually, other imaging examinations may detect tumor only when it grows to a certain degree and is larger, but PET-CT can detect it at an early stage through metabolic imaging, which refers to this stage of tumor screening; secondly, if the patient has already found lung nodule or lung occupancy through other methods, such as lung, but is not sure whether it is PET-CT is also needed for the differential diagnosis of benign and malignant; thirdly, if malignant tumor has been identified, treatment is needed, currently surgery is a better way to treat malignant tumor, but not all patients can undergo surgery, so staging is needed before surgery, PET-CT is an internationally recognized better staging means, which can find the extent of tumor involvement in the whole body. For example, a nodule in the lung, though small, may not be detected by chest CT only, but PET-CT can detect small lymph nodes and bone metastases far from the chest, which are not suitable for surgery; fourthly, the scar after surgery and the recurrence of tumor after surgery cannot be distinguished by conventional CT, whose performance is all isodensity shadow, but PET-CT can distinguish them by tumor PET-CT can identify whether it is recurrence or scar tissue by the degree of malignant metabolism of the tumor; fifthly, it can detect metastasis and recurrence at an early stage after surgery, and it is difficult to detect small metastatic lesions by conventional means, including the judgment of its prognosis, so PET-CT is very useful in the whole process from diagnosis to treatment of tumor.