ABSTRACT
Ionizing radiation refers to photon energy that can knock out a negatively charged electron from the orbit of an atom leaving it positively charged and, thus, creating an ion pair after every such interaction. The process of creating an ion pair is referred to as ionization. Ionizing radiation has been divided into two categories, low linear energy transfer (LET) radiation and high LET radiation. The low LET radiation includes x-rays, gamma-rays, and beta-rays that cause cellular damage primarily (about two-thirds of damage) by generating free radicals during radiation exposure. The high LET radiation includes proton radiation, neutron radiation, alpha-particle radiation, and other heavy particle radiation that cause initial cellular damage primarily by ionization, while free radicals play a minor role. High LET radiation is generally 5-20 times more effective than the low LET radiation in causing damage, depending upon the type of radiation and criteria of radiation injury. The current unit used for low LET radiation dose is Gy (named after a famous radiobiologist, Dr. Gray), whereas the unit used for radiation protection recommendation is Sv (named after a famous health physicist, Dr. Sievert). The unit Sv accommodates any difference in relative biological efciency (RBE) between low and high LET radiation. RBE is a ratio of a dose to produce an effect by low LET radiation and a dose to produce the same effect by high LET radiation. The above basic radiobiology concepts have been described in detail in several books including the one referred to here.1
