MECHANISMS OF RADIATION INJURY
The human body at its most basic level is composed of atoms. Ionizing radiation can produce injury through interactions at the atomic level when x-ray photons remove or excite orbital electrons and deposit energy within the tissue. When an x-ray photon dislodges an orbital electron, the normal equilibrium, or ground state, of the atom is disrupted and ion pairs are formed. The ionized atom becomes positively charged ( +ion ) and the ejected electron carries a negative charge ( –ion ). To regain equilibrium, ions either recombine or combine with other atoms or molecules to return to neutral status. With excitation, a low-energy x-ray photon interacts with an outer-shell orbital electron. The energy of the photon is transferred to the electron, causing it to become excited but not ejected. When the x-ray photon ceases to exist, the excited electron releases a secondary photon of equivalent energy, and the atom returns to the ground state. The secondary photon is ejected and may produce ionization at another site. When an atom is ionized, its chemical binding properties are modified. If the ionized atom is part of a large molecule, ionization may result in breakage of the molecule or a change in location of the atom within the molecular structure (Tarrence, 2020). The altered molecule may not function normally, or it may stop functioning altogether, causing severe impairment or cell death. Radiation injury can be reversed by gaining free electrons at the atomic level or through enzymatic repair at the molecular level.
The body has a tremendous capacity to repair and regenerate affected cells and tissues to aid recovery. However, the effects of radiation are additive, and some damage remains unrepaired. The cumulative effect of radiation is residual, unrepaired injury that accrues with repeated radiation exposure. Figure 1 provides an overview of possible events following radiation exposure. Figure 1: Response of Human Tissue to Radiation Exposure
Note . Adapted from Bushong, S. C. (2013). Radiologic science for technologists: Physics, biology, and protection (10th ed.). St. Louis, MO: Mosby.
DIRECT AND INDIRECT EFFECTS OF RADIATION
Radiation injury occurs both directly and indirectly through the mechanisms of ionization and free radical production. The direct effect is the result of x-ray interactions that ionize biologic Direct effect of radiation The primary macromolecules in the human body are proteins, lipids, carbohydrates, and nucleic acids which contain a range from 100 to over 10,000 subunits (Marieb and Hoehn, 2018). The direct effect of radiation occurs when biologic macromolecules absorb the energy of x-ray photons and form free radicals, which are extremely unstable, highly reactive, and short-lived molecules responsible for aging, tissue damage, and possibly some diseases. Free radicals immediately seek stability through the processes of dissociation (breaking apart) or cross-linking (joining with another molecule or fusing with itself in the wrong place), which produce altered molecules in terms of both structure and function (Marieb and Hoehn, 2018). Indirect effect of radiation Water is a major component of the body and accounts for approximately 60% of a person’s body weight. The indirect effect of radiation occurs through the ionization of water molecules. Because such a large percentage of the body’s molecules are water molecules, irradiation of water is the primary pathway for human radiation injury. Following irradiation, a series of intermediary reactions takes place that results in the radiolysis of water. The radiolysis of water produces ion pairs and free radicals, predominantly hydrogen (H•) and hydroxyl (OH•) free radicals (Tarrence, 2020). Ion pairs may recombine without producing any change. However, free radicals are highly reactive
macromolecules, while the indirect effect is the result of the ionization of water. These interactions are random, and both direct and indirect interactions take place during x-radiation.
Among the primary macromolecules, DNA is the most critical and radiosensitive. DNA is considered the critical target molecule because it carries the genetic information that controls the development and function of every cell. When radiation directly ionizes DNA, a number of alterations can occur – including chromosomal aberrations, uncontrolled metabolic activity, breakage of DNA strands, disruption of molecular bonds, changes in base sequence, loss of a base, and cross- linking (Tarrence, 2020). Double-strand breakage is thought to be responsible for the most cell death, cancer induction, and genetic effects (Tarrence, 2020). Approximately one third of radiation-induced biologic damage results from the direct effect of radiation (White & Pharoah, 2014). and seek to combine with molecules. The hydroxyl (OH•) free radical can combine with the hydrogen (H•) free radical to form hydrogen peroxide (H2O2), which is toxic to the cell and is capable of producing widespread cellular alterations (Iannucci & Howerton, 2017). Free radicals and altered molecules can migrate freely throughout the tissues. Thus, the indirect effect of radiation can produce injury at sites far removed from the original exposure and causes the majority of radiation-induced biologic cellular and tissue damage while the indirect effect of radiation comprises the minority of radiation-induced tissue damage (Tarrence, 2020).
DETERMINISTIC AND STOCHASTIC EFFECTS OF RADIATION
The biologic effects of radiation are categorized as either deterministic or stochastic. The deterministic or nonstochastic effects of radiation are those in which the biologic response increases in severity as the radiation dose increases. Deterministic effects are the result of cell death or malfunction and are associated with high-dose radiation exposure and early biologic response (in which the effects of radiation appear soon
after exposure). A threshold dose, or a certain minimum level of radiation, must be reached before a biologic effect is observed. All individuals will exhibit an effect when the dose is above the threshold. Erythema and mucositis are examples of deterministic effects that may manifest after a threshold dose is reached (e.g., during a course of radiotherapy to the oral cavity). These effects are never realized in oral and maxillofacial diagnostic imaging.
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