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2.: Effects of ionizing radiation on biological tissues, especially the ocular structures

The primary processes in the interaction of ionizing radiation with matter are electronic excitation and ionization. Absorption of a photon can result in a direct ionization of a target molecule, or an indirect effect may occur accomplished through formation of free radicals. In biological tissues water is the molecule in highest concentration and therefore the ionization or radiolysis of water is the most likely, and most important, first effect of any radiation treatment. The ionization of water results in formation of different kinds of free radicals, which can readily attack the biological target molecules. Free electrons also react with oxygen by which more toxic radicals, as well as hydrogen peroxide, are formed. The effect of ionizing radiation is a random one. It depends on atomic number and density of the irradiated tissue. The molecular structure and the electric conductivity of the matter, so important for the effect of longer wavelengths (UV, visible light, IR and microwaves), has little influence on the absorption of x and gamma rays, charged particles and neutrons (Rink 1980). The molecular excitation and ionization may take different forms depending upon the photon energy. With low energy X-rays only a coherent scattering of the photons occurs. With higher energies, the photon energy is transferred to an electron by photoelectric absorption. The ejected electron, usually from an inner shell of the atom, is called a photoelectron. With still higher energies, the Compton effect predominates, Here, only part of the energy of the incident photon i transferred to the electron (recoil or Compton electron), and the photon, now with lower energy, continues in a different direction (is scattered). With energies higher than 1.02 MeV, the production of an electron and a positron becomes possible (pair production). 1.02 MeV is the energy equivalent of the rest masses of the electron and the positron. Thus, photoelectric absorption, the Compton effect and pair production are the three main processes whereby Xand gamma radiation transmit energy to the tissue and initiate chemical changes. Only with energies of 10 MeV or more can interaction with the atomic nucleus become possible. For clinical purposes, Xor gamma radiation may be regarded as a method of introducing fast electrons into a biological tissue. Electrons may also be introduced directly from an external source e.g. a linear accelerator or a P-emitting atom. The charged heavy particles (protons, deuterons, alpha-particles) in general behave similarly to photons and lose most of their energy by causing electronic excitation and ionization. The transfer of the energy as a function of the depth in the tissue may vary, allowing for different clinical applicabilities. The uncharged neutrons do not react with orbital electrons, and therefore cannot directly cause ionization. They interact with the atomic nucleus, preferably one of low atomic weight. Neutrons, therefore, have a much greater biological effect compared to Xor gamma rays, since biological tissues are mainly composed of water.