_______________________________ DCA05DR Dental Radiation Health, Safety, and Protection, 3rd Edition
Short-term , early , or acute effects of radiation occur within minutes, days, or weeks following the latent period. Typically, short-term effects are the result of high doses of radiation deliv- ered to the entire body, as when a nuclear power plant melts down or a nuclear weapon explodes. In these rare instances of high-level whole-body exposure, a particular sequence of biologic responses known as acute radiation syndrome occurs. This syndrome involves a subset of three syndromes: hemato- logical, gastrointestinal, and central nervous system death. A person with acute radiation syndrome will typically proceed through three stages (prodromal, latent, manifest illness) before reaching the final stage, which is recovery or death. During the prodromal period, the clinical symptoms may include nausea, vomiting, diarrhea, and leukopenia. After the initial period of radiation sickness, a latent period occurs in which there is absence of illness and seeming recovery. The latent period is followed by manifest illness, a dose-related period during which the three syndromes occur. It is important to emphasize that short-term or acute effects are not associated with diagnostic imaging, including dental radiographic exposures. Radiation effects that occur months, years, or decades after the exposure are referred to as long-term , late , or chronic effects. Chronic effects are the result of small amounts of radiation absorbed repeatedly and cumulatively over a long period. Receipt of these repeated low levels of radiation may result in the development of cancer or genetic abnormalities much later in life or the presence of birth defects in the affected person’s offspring. RADIATION MEASUREMENTS Two systems are used to quantify radiation: the Système Inter- national d’Unités , referred to as SI or international units, and traditional or standard units. International units are based on a modernized version of the metric system and are the preferred method for expressing radiation measurement. The term dosimetry refers to the scientific determination of the amount or dose of ionizing radiation. In measuring radiation, the terminology varies depending on whether the radiation is being measured at the source, the radiation is absorbed by a patient, or the risk of detrimental health effects is being estimated. The common units used to measure radiation include exposure, absorbed dose, equivalent dose, effective dose, and radioactivity. EXPOSURE Exposure refers to the measurement of radiation quantity or the ability of x-rays to ionize air. In terms of dental radiography, exposure represents the amount of radiation emitted from the tubehead that reaches the patient. The measurement is taken at the surface of the skin before the radiation has penetrated the tissues. The SI unit of measurement for exposure is coulombs per kilogram (C/kg). The traditional unit of exposure is the roentgen , named after the scientist who discovered x-rays. The roentgen measures the amount of energy that reaches the skin
surface, rather than the amount of radiation that is absorbed by the patient. ABSORBED DOSE The absorbed dose measures the radiation energy absorbed per unit mass of matter, regardless of the type of ionizing radiation and the type of matter. This is the actual amount of radiation absorbed by the tissues when a person is exposed to radiation. The biologic effects of radiation are most commonly related to the absorbed dose. The SI unit used to measure the absorbed dose is the gray (Gy). The traditional unit is the radiation absorbed dose (rad). One Gy equals 100 rads. EQUIVALENT DOSE The equivalent dose compares the biologic effects of different types of ionizing radiation on living tissues and organs. Some types of ionizing radiation produce more biologic damage than others. The degree of damage varies according to the linear energy transfer rate (LET) for each type of radiation. As the LET increases, the frequency of ionization and the ability to produce biologic damage also increase. This effect is quanti- fied by a factor referred to as the relative biologic effectiveness or RBE (Tarrence, 2020). The RBE is the weighting factor used to reflect the differences in biologic damage as a consequence of the variation in the LET. For example, the RBE (or weighting factor) for fast neutrons (produced by nuclear weapons and some nuclear reactors) is 10, while the RBE (or weighting factor) for x-rays is 1. This means that fast neutrons produce 10 times more biologic damage than x-rays. The purpose of the equivalent dose is to compare the biologic effects of these differing types of radiation and to bring all forms of radiation to the same dose level (Jones & Hacking, 2020). The equiva- lent dose (H T ) is the product of absorbed dose averaged over a tissue or organ (D T ) and the weighting factor (W R ) for the type and energy of the radiation: H T = W R × D T (Jones & Hacking, 2020). The SI unit for the equivalent dose is the sievert (Sv). The traditional unit is the roentgen equivalent man (rem). For the purposes of conversion, 1 Sv equals 100 rem. To summarize using the previous example, the equivalent dose would imply that 1 mSv of fast neutron radiation would produce the same amount of biologic damage as 1 mSv of x-radiation despite their difference in LET. EFFECTIVE DOSE The effective dose takes into account the type of radiation and the radiosensitivity of the tissue being irradiated. The effec- tive dose is used to estimate radiation risk or to evaluate the biologic consequences of radiation in humans. The effective dose was developed for use in radiation protection, but it also facilitates comparison of the risk of radiation exposure from one area of the body to another (Martin et al., 2020). The effective dose (E) is the sum of the products of the equivalent dose to each tissue or organ (H T ) and the tissue-weighting fac- tor (W T ), a numerical value given to different tissues such as the brain, gonads, bone marrow, thyroid gland, and skin: E = Σ W T × H T . The tissue-weighting factors are defined by the International Commission on Radiological Protection (ICRP).
89
EliteLearning.com/Dental
Powered by FlippingBook