PET, or Positron Emission Tomography, is a scan that constructs images of organ tissues in the body.PET takes advantage of the metabolic differences between cancerous and normal tissue to show tumors from a metabolic perspective. Small amounts of radioactive substances are injected into the body and these substances can be absorbed by the organs and tissues with the highest energy metabolism in the body. Cancerous tissues are metabolically active and thus can absorb higher amounts of radioactive substances. The scanner detects the radioactive elements and thus creates an image of the body. Clinically, the test is fused with the traditional CT examination, i.e. PET-CT, to show tumors from a metabolic perspective as well as to accurately localize cancerous tissues. It is superior for early detection of malignant tumors. So, under what circumstances is PET-CT examination necessary? Broadly speaking, there are the following cases: 1. It can be used for early tumor diagnosis and benign-malignant differentiation: the important characteristic of tumor tissue is rapid growth and high metabolism, which shows high uptake in PETCT, and if there is no metabolic increase performance, it suggests that benign lesions are more likely. Especially, other detection means can be considered when the nature of tumor is difficult to determine. 2.It helps to clarify the staging: PET can perform whole body tomography imaging at one time, which is not possible by 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 to the staging of tumor. 3.Early identification of tumor recurrence and re-staging of 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-staging 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.Search for primary tumor lesion: Through rapid whole-body PET/CT scan, the primary lesion is searched for unexplained metastatic tumors. However, PET-CT examination is not a panacea. Certain inflammatory and traumatic changes can also have abnormal uptake, thus causing false positives; meanwhile, certain tumors with inactive metabolism can also show low uptake, thus causing false negatives. Overall, such false negatives and false positives account for about 5-8%. In addition, PET-CT examination is an expensive test and is generally not recommended for patients who can be diagnosed clearly by CT and bronchoscopy.