In this review, I would like to discuss the effects of ionizing radiation on human beings, and protection against it. X-rays or γ-rays are electromagnetic radiation with short wavelengths, and have sufficient energy to cause the ejection of one or more orbital electrons from an atom or molecule. This process is called ionization, and this kind of radiation is referred to as ionizing radiation. There are other high-energy particles that have an ionizing effect, and they are all regarded as ionizing radiation. Ionizing radiation can be artificially made, or originate from radioisotopes or space.
Ionizing radiation can cause cell death or genomic mutation, which is related to carcinogenesis. Only ionizing radiation over the threshold dose can cause symptoms related to cell death (deterministic effects), whereas we suppose that there is no threshold for carcinogenesis (stochastic effects) by radiation in terms of safety. A dose limitation has been established for occupational and public exposure, with the aim of preventing deterministic effects and reducing risks of stochastic effects to the extent reasonably achievable. Radiation therapy is one of the most useful treatment modalities for cancers. Currently, we can use many modern treatment modalities in radiation therapy in addition to standard external-beam therapy.