PET-CT examination is needed for the following four types of patients: 1. Patients whose existing information is difficult to clearly diagnose malignant tumor and to identify benign and malignant tumor: the important feature of tumor tissue is rapid growth and high metabolism, which shows high uptake in PETCT. Especially, other detection means can be considered when the nature of tumor is difficult to determine. 2. Patients with staging difficulties: PET can perform whole body tomographic imaging at one time, which is not possible with other imaging devices. In addition to detecting the primary lesion, it can also detect whether there is metastasis in other parts of the body, which is very helpful for the staging of tumor. 3.Patients who are difficult to judge whether they have recurrence or not: PET-CT can identify tumor recurrence at an early stage and re-stage tumor: PET can make early diagnosis of tumor residue or recurrence after treatment and differentiate it from fibrosis and necrosis after treatment, as well as re-stage according to the distribution of lesions after treatment, which is difficult to do by structural information-based imaging means such as CT and MRI. 4.Difficulty in diagnosing the primary foci of malignant tumors: The rapid whole-body PET/CT scan is used to find the primary foci for unexplained metastatic tumors with semi-quantitative effect.