After the diagnosis of tumor, it needs to be treated actively, and only active treatment can ensure the patients’ life span can be prolonged. However, there is a phenomenon in real life: even though the treatment is going well and the effect after treatment is good, and the tumor has no sign of recurrence, patients and family members are still looking for various methods to “eliminate the root of the tumor”, is this right? The first thing is to treat tumor effectively, but to give unscientific treatment without considering the actual situation is over-treatment. In many cases, the tumor recurred and metastasized, or the patient’s body became weaker and weaker, which eventually led to death. Because many measures to kill tumor are also harmful to the healthy body, therefore, when choosing treatment strategies, it is important to weigh the pros and cons so that the tumor and the human body can reach a dynamic balance. Therefore, in order to have a thorough understanding of the tumor lesion and determine whether the next step of treatment is needed, a very accurate and comprehensive examination is also needed, and this is where PET-CT can play a big role. PET-CT examination combines the advantages of PET and CT technology, which can scan the whole body and give a detailed description of the location, shape and malignancy of the tumor lesions. After treatment, PET-CT examination can also be used to predict the prognosis and determine the effect of treatment, and can also indicate whether further treatment is needed, playing a role in guarding against over-treatment. Therefore, PET-CT examination can guide the direction of tumor treatment, ensure the effectiveness of treatment, help to develop “appropriate” treatment plan, avoid the harm caused by over-treatment, and thus prolong the survival of patients.