PET-CT mainly examines whether there is systemic metastasis of malignant tumors or makes qualitative diagnosis for some lesions of unknown nature. At present, the diagnosis of malignant tumors mainly relies on tumor markers, imaging and, if necessary, puncture biopsy. PET-CT is a qualitative diagnosis of tumor by combining nuclear medicine and radiology and using isotope-labeled tracers with tumor-specific uptake characteristics. PET-CT is generally not used as a routine medical examination because of its high radioactivity, which may cause some harm to the body.