Radiobiology is a branch of science which combines the basic principles of physics and biology and is concerned with the action of ionizing radiation on biological tissues and living organisms. The study of basic radiobiological mechanisms deals with biological effects produced by energy absorption in small volumes corresponding to single cells or parts of cells. All living entities are made up of protoplasm, which consists if inorganic and organic compounds dissolved or suspended in water. The smallest unit of protoplasm capable of independent existence is the cell, the basic microscopic unit of all living organisms. A group of cells that together perform one or more functions is referred to as tissue. A group of tissues that together perform one or more functions is called an organ. A group of organs that perform one or more functions is an organ system or an organism. Cells contain: • Inorganic compounds (water and minerals). • Organic compounds (proteins, carbohydrates, nucleic acids, lipids). The two main constituents of a cell are the cytoplasm and the nucleus: • Cytoplasm supports all metabolic functions within a cell. • Nucleus contains the genetic information (DNA). Human cells are either somatic cells or germ cells. The germ cells are either a sperm or an egg, all other human cells are called the somatic cells. Cells propagate through division: • Division of somatic cells is called mitosis and results in two genetically identical daughter cells. • Division of germ cells is called meiosis and involves two fissions of the nucleus giving rise to four sex cells, each possessing half the number of chromosomes of the original germ cell. When a somatic cell divides, two cells are produced, each carrying a chromosome complement identical to that of the original cell. New cells themselves may undergo further division, and the process continues producing a large number of progeny. Radiation is classified into two main categories: • Non-ionizing radiation (cannot ionize matter) • Ionizing radiation (can ionize matter) Ionizing radiation contains two major categories • Directly ionizing radiation (charged particles) electrons, protons, alpha particles, heavy ions • Indirectly ionizing radiation (neutral particles) photons (x rays, gIn radiobiology and radiation protection the linear energy transfer (LET) is used for defining the quality of an ionizing radiation beam. In contrast to the stopping power, which focuses attention on the energy loss by a charged particle moving through a medium, the LET focuses attention on the linear rate of energy absorption by the absorbing medium as the charged particle traverses the medium. The ICRU defines the LET as follows: “LET of charged particles in a medium is the quotient , where dE is the average energy locally imparted to the medium by a charged particle of specified energy in traversing a distance .”