_______________________________ DCA05DR Dental Radiation Health, Safety, and Protection, 3rd Edition
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 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. 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 (Hoehn et al., 2024). 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 (Hoehn et al., 2024). Among the primary macromolecules, DNA is the most criti- cal 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 40% of radiation-induced biologic damage results from the direct effect of radiation (Ibanez et al., 2024). 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 mol- ecules. 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 with- out producing any change. However, free radicals are highly reactive and seek to combine with molecules. The hydroxyl (OH•) free radical can combine with the hydrogen (H•) free radical to form hydrogen peroxide (H 2 O 2 ), which is toxic to the cell and is capable of producing widespread cellular alterations (Ibanez et al., 2024). 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).
Self-Assessment Question
1. Free radicals produced as an indirect effect of radiation: A) Cannot migrate through the tissue once they are formed. B) Are highly reactive. C) Can only produce injury at the site of the initial radia- tion exposure. D) Do not combine with other molecules. 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. Deter- ministic 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 maxil- lofacial diagnostic imaging. Stochastic effects involve biologic responses to radiation in which the probability or incidence of such effects occurring increases as the radiation dose increases. Stochastic effects result from damage to DNA. These effects are associated with repeated low-dose radiation exposures received over a long period and are not dependent on a threshold dose. The primary stochastic effects are cancers or genetic (heritable) changes. With regard to stochastic effects, no radiation dose is considered safe. The idea that the probability of experienc- ing effects of radiation goes up as the dosage goes up over time—called the linear nonthreshold ( LNT , or linear no-threshold ) hypothesis—has been widely accepted for establishing policy in radiation safety and protection with the sum of several small radiation exposures causing the same damage as one large exposure (Laurier et al., 2023). Stochastic effects are therefore more important than deterministic effects when considering risk in maxillofacial diagnostic imaging. OTHER FACTORS THAT INFLUENCE THE EFFECTS OF RADIATION Several other factors influence the effects of radiation on the human body. The type of tissue exposed, the sensitivity of the tissue to radiation, and the area of exposure are all important considerations.
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