Radiation dose is a quantity expressing the value per unit mass of the energy released by ionizing radiation particles. It is also used as an absorbed dose or a dose only. In the SI unit system, its unit is the gray (Gy). Gray refers to the energy transferred into the medium in joules per kilogram of mass. In the former CGS unit system, its unit is rad. However, the use of the rad unit is not common. Radiation dose is a macroscopic quantity and is used as a measure to predict subsequent physical changes or damage in the environment due to particle migration. If the physical environment is a biological material or if the tissues or organs of a living organism have been exposed to radiation, radiobiological effects should also be considered in estimating the damage.