DAMAGE MECHANISMS

a. It is often difficult to envision how radiation, which cannot be seen, heard, smelled, or felt, can cause injury or death to a healthy man. The answer is not fully understood, but a partial insight into the damage mechanism is gained when we realize that man is composed of millions of microscopic cells and that radiation damage occurs on the cellular and subcellular level.

b. When radiation is absorbed by a living cell, the primary damage is caused by ionization and excitation of the atoms and molecules of that cell. These interactions occur with any type of matter, whether living or nonliving; but in a living cell, the ionized or excited atoms and molecules may be highly reactive chemically. Under these circumstances, secondary reactions will occur, resulting in changes in cellular structure, damage to essential constituents, and observable biological injury.

c. In addition to the direct action of radiation, molecules damaged by radiation can also produce cellular injury. In fact, the formation of free radicals from water is the primary means of cell injury by ionizing radiation. The most frequently formed water radical, the hydroxyl radical attacks neighboring molecules that are important for homeostasis. The resulting damage to the genetic material of the cell is considered to be the major cause of cell death.

d. It is the combination of these primary and secondary reactions that results in acute and chronic radiation injury. The interrelationship of causes and effects is extremely complex and is dependent not only on the energy of the radiation, but also on the total dose, dose rate, presence of oxygen, sex, nutritional status, and other physiological factors which affect the body.

e. These factors in combination lead to the observation that the cells of the body which seem most radiosensitive are those which reproduce most rapidly and are in a state of high-metabolic activity. Regions of the body such as blood-forming organs, gonads, and hair follicles show injury at much lower dosages than slow or nonreproducing tissues such as nerve tissue.