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20 Radiation for Radionuclide Users D. Biological Effects of Ionizing Radiation Cellular Effects The energy deposited by ionizing radiation as it interacts with matter may result in the breaking of chemical bonds. If the irradiated matter is living tissue, such chemical changes may result in altered structure or function of constituent cells. Mechanisms of Radiation Damage Less than 20% of the energy deposited in cells by ionizing radiation is absorbed directly by macromolecules. This is due to the fact that such molecules make up a relatively small proportion of the cell’s mass. More than 80% of the energy deposited in the cell is absorbed by water molecules which make up the majority of the cell’s mass. This results in the formation of highly reactive free radicals: 2H 2 0 radiation 2H •+2H0 • → H 2 0 2 + H ↑ These radicals and their products (e.g., hydrogen peroxide) may initiate numerous chemical reactions which result in damage to macromolecules and a corresponding alteration of structure or function. Damage produced within the cell by the radiation induced formation of free radicals is described as being by indirect action of radiation. Changes in Structure and Function As a result of the chemical changes in the cell caused by the direct or indirect action of ionizing radiation, large biological molecules may undergo a variety of structural changes which lead to altered function. Some of the more common effects which have been observed are inhibition of cell division, denaturation of proteins and inactivation of enzymes, alteration of membrane permeability, and chromosome aberrations. Radiosensitivity The cell nucleus is the major site of radiation damage leading to cell death. This is due to the importance of the DNA within the nucleus in controlling all cellular function. Damage to the DNA molecule may prevent it from providing the proper template for the production of additional DNA or RNA. This hypothesis is supported by research which has shown that cells are most sensitive to radiation damage during reproductive phases (i.e., during DNA replication). In general, it has been found that cell radiosensitivity is directly proportional to rate of cell division and inversely proportional to