With the popularization of large medical equipment Positron Emission Tomography /Computed Tomography (PET/CT), more and more people are coming into contact with and accepting PET/CT tests and tumor screening. With the media’s publicity on nuclear accidents abroad and nuclear military knowledge, the public has received more radiation safety knowledge, and some people have started to realize the radiation problem of PET/CT examinations like other radiological examinations, and have questions about the radiation safety of PET/CT examinations, “Is PET/CT examination safe? How long is the interval between exams?” Questions such as “Is it safe to have PET/CT examinations?” become the primary consultation question for most people who are ready to undergo examinations, and even create fear. In fact, radiation caused by military-industrial events is fundamentally different from medical radiation because of the wide range of environmental radiation contamination, radiation exposure is systemic, continuous and relatively high dose, which may cause certain health hazards; whereas medical exposure is local, transient and low dose, which is safe, and medical examinations can help physicians solve clinical problems and facilitate the development of disease diagnosis and treatment strategies to benefit patients. As PET/CT equipment continues to be updated, the dose of nuclide used for PET is getting smaller and smaller, the scanning speed of CT is getting faster and faster, and the radiation dose control is getting better and better, there is also the necessity to re-understand the issue of radiation dose and safety of PET/CT examinations. The source of radiation dose of PET/CT comes from radionuclide and X-CT. Among them, most of the radioactive radiation used in PET comes from fluorine-18 (18F), which emits positrons and produces high-energy γ-rays, and has a certain radiation dose to human body, which is about 2.1×10-2mSv/MBq, and generally 222-370 MBq is used in one examination, i.e. However, with the modernization of the equipment, the dose of radionuclides used in the third generation of PET/CT products can be reduced to about 185 MBq, and the radiation dose to the patient is reduced to about 3.9 mSv. The radiopharmaceuticals used are analogues of physiological metabolites. The radionuclide used is an analog of a physiological metabolite, such as 18F-FDG, which is an analog of glucose and is non-sensitizing and does not pose a risk to humans. The radionuclide used is an ultra-short half-life isotope that decays rapidly and disappears completely from the body within a few hours. By drinking more water and accelerating the excretion of the drug, the total radiation dose received by the patient from the nuclide can also be reduced appropriately. The second part of the radiation dose from PET/CT examinations is mainly from CT scans, but the CT used for PET/CT is lower in dose than conventional CT. The average radiation dose from X-ray CT during oncology health screening at 43 institutions surveyed by the Japanese Medical Association was 10.1 ± 7.85 mSv. The radiation dose from older generation CT examinations in the past was much higher, generally above 18 mSv, while CT scans in PET/CT generally use lower current acquisition, and the CT radiation dose from whole-body scans is lower than that from conventional local CT scans. However, with the modernization of PET/CT, PET/CT is also equipped with more advanced CT, which shortens the scanning time and will further reduce the radiation dose. This makes the radiation dose of a whole body examination by PET/CT reduced from about 15mSv to about 7.5mSv (7.0 to 10.2). However, if high mA scan mode or enhanced scan mode is used, the CT dose can be as high as 14.1~18.6mSv, while the radiation dose of radiopharmaceuticals is relatively stable in a certain level range, so the increased dose in PET/CT scan mainly depends on the CT scan mode, and low dose (low mA) scan mode is generally recommended for CT scan in routine PET/CT imaging, and suspicious organs The CT scan in conventional PET/CT imaging is generally recommended to adopt a low dose (low milliamp) scan mode, and the suspected organ is locally scanned with a diagnostic dose to minimize the radiation dose to the recipient. The radiation of the PET/CT subjects to the surrounding population mainly comes from the radiopharmaceuticals. If 185MBq fluorine-18 labeled drug is used, after 2 hours of drug injection, most of the radionuclides in the body have decayed and excreted, and the radiation dose to the surrounding 1m population has dropped to less than 0.01mSv/h, which is also very safe, and the dose is even lower with time extension. In fact, radiation is everywhere and there are many radiation sources in nature, such as natural radiation including cosmic rays, rays from the earth itself, radionuclides in house decoration materials, decay products of radioactive radon in the air, and various naturally existing radionuclides contained in food and drinks. It is generally believed that Radiation doses below 50mSv are considered safe; over 100mSv may produce direct radiation damage (risk of radiation deterministic effects); up to 250mSv is a subclinical dose (asymptomatic excess radiation, which may cause a small amount of biological cell damage that can be repaired or compensated by human body without clinical symptoms); over 500mSv radiation exposure may If the radiation dose exceeds 500 mSv, it may cause radiation damage to 5% of the exposed persons, and if the radiation dose exceeds 1000 mSv, it may cause radiation damage to 25% of the exposed persons. The radiation dose of PET/CT is much lower than the above-mentioned safe dose, and it is a very safe medical examination. Although there is a small amount of radiation in PET/CT examinations, the risk of random effects of radiation is extremely low. In fact, other clinical medical examinations and treatments also have certain side effects, such as taking medicine, which can treat diseases but also has certain side effects, so it is necessary to weigh the pros and cons when using it; and surgery can effectively treat diseases but may also bring about certain structural or functional damage to tissues and organs. Any medical treatment should be aimed at patient benefit, i.e., to find the best node in cost-benefit. For malignant tumors, the earlier the detection and precise evaluation, the more beneficial to the treatment and recovery of tumors. PET/CT examination is useful in early diagnosis and differential diagnosis of tumors; staging and re-staging of tumors, degree of differentiation; identification of tumor recurrence; guidance of tumor treatment plan; evaluation of tumor treatment efficacy; assessment of tumor prognosis; tumor primary and metastatic foci in tumor high-risk individuals such as blood tumor markers are continuously increased and tumor families. The benefits of PET/CT examination for patients with indications are significantly greater than the very low probability of radiation risk, and even gives the patient a chance to save his life. The decision of whether to repeat the examination for patients with suspicious lesions or patients with changing conditions can be based on the condition and treatment decision needs. In conclusion, PET/CT examinations need to be used for the right indications and to be justified and optimized to help physicians solve clinical problems and to benefit the patient. The new generation of PET/CT has improved the sensitivity of PET, reduced the use of radiopharmaceuticals, and adopted the intelligent low-dose scanning technology, which makes PET/CT lower and safer in radiation dose.