The benefits of 18F-fluorodeoxyglucose (18FDG) positron emission tomography (PET) cancer screening are expected to include a large population of examinees and are intended for a healthy group. Therefore, we attempted to determine the benefit/risk ratio, estimated risk of radiation exposure, and benefit of cancer detection. We used software that embodied the method of the International Commission on Radiological Protection (ICRP) to calculate the average duration of life of radiation exposure. We calculated the lifesaving person years of benefit to be obtained by 18FDG PET cancer screening detection. We also calculated the benefit/risk ratio using life-shortening and lifesaving person years. According to age, the benefit/risk ratio was more than 1 at 35–39 years old for males and 30–34 years old for females. 18FDG PET cancer screening also is effective for examinees older than this. A risk-benefit analysis of 18FDG-PET/computed tomography (CT) cancer screening will be necessary in the future.