Radiation Biology

Mechanisms of Injury

In diagnostic radiography, not all x-rays pass through the patient and reach the dental x-ray film; some are absorbed by the patient’s tissues. Absorption, as defined in Chapter 2, refers to the total transfer of energy from the x-ray photon to patient tissues. What happens when x-ray energy is absorbed by patient tissues? Chemical changes occur that result in biologic damage. Two specific mechanisms of radiation injury are possible: (1) ionization and (2) free radical formation.

Theories of Radiation Injury

Damage to living tissues caused by exposure to ionizing radiation may result from a direct hit and absorption of an x-ray photon within a cell or from the absorption of an x-ray photon by the water within a cell accompanied by free radical formation. Two theories are used to describe how radiation damages biologic tissues: (1) the direct theory and (2) the indirect theory.

Dose–Response Curve

If all ionizing radiations are harmful and produce biologic damage, what level of exposure is considered acceptable? To establish acceptable levels of radiation exposure, it is useful to plot the dose administered and the damage produced. With radiation exposure, a dose–response curve can be used to correlate the “response,” or damage, of tissues with the “dose,” or amount, of radiation received.

When dose and damage are plotted on a graph, a linear, nonthreshold relationship is seen. A linear relationship indicates that the response of the tissues is directly proportional to the dose. A nonthreshold relationship indicates that a threshold dose level for damage does not exist. A nonthreshold dose–response curve suggests that no matter how small the amount of radiation received, some biologic damage does occur (Figure 4-4). Consequently, there is no safe amount of radiation exposure. In dental radiography, as mentioned earlier, although the doses received by patients are low, damage does occur.

Most of the information used to produce dose–response curves for radiation exposure comes from studying the effects of large doses of radiation on populations, for example, atomic bomb survivors. In the low-dose range, however, minimal information has been documented; instead, the curve has been extrapolated from animal and cellular experiments.

Stochastic and Nonstochastic Radiation Effects

Biologic effects from radiation can be classified as stochastic or nonstochastic. Stochastic effects occur as a direct function of dose. The probability of occurrence increases with increasing absorbed dose; however, the severity of effects does not depend on the magnitude of the absorbed dose. As in the case of nonthreshold radiation effects, stochastic effects do not have a dose threshold. Examples of stochastic effects include cancer (i.e., tumor) induction and genetic mutations.

Nonstochastic effects (deterministic effects) are somatic effects that have a threshold and that increase in severity with increasing absorbed dose. Examples of nonstochastic effects include erythema, loss of hair, cataract formation, and decreased fertility. Compared with stochastic effects, nonstochastic effects require larger radiation doses to cause serious impairment of health.

Sequence of Radiation Injury

Chemical reactions (e.g., ionization, free radical formation) that follow the absorption of radiation occur rapidly at the molecular level. However, varying amounts of time are required for these changes to alter cells and cellular functions. As a result, the observable effects of radiation are not visible immediately after exposure. Instead, following exposure, a latent period occurs. A latent period can be defined as the time that elapses between exposure to ionizing radiation and the appearance of observable clinical signs. The latent period may be short or long, depending on the total dose of radiation received and the amount of time, or rate, it took to receive the dose. The more radiation received and the faster the dose rate, the shorter is the latent period.

After the latent period, a period of injury occurs. A variety of cellular injuries may result, including cell death, changes in cell function, breaking or clumping of chromosomes, formation of giant cells, cessation of mitotic activity, and abnormal mitotic activity.

The last event in the sequence of radiation injury is the recovery period. Not all cellular radiation injuries are permanent. With each radiation exposure, cellular damage is followed by repair. Depending on a number of factors, cells can repair the damage caused by radiation. Most of the damage caused by low-level radiation is repaired within the cells of the body.

The effects of radiation exposure are additive, and unrepaired damage accumulates in the tissues. The cumulative effects of repeated radiation exposure can lead to health problems (e.g., cancer, cataract formation, birth defects). Table 4-1 lists disorders that may result from the cumulative effects of repeated radiation exposure on tissues and organs.

Determining Factors for Radiation Injury

In addition to understanding the mechanisms, theories, and sequence of radiation injury, it is important to recognize the factors that influence radiation injury. The factors used to determine the degree of radiation injury include the following:

Short-Term and Long-Term Effects

Radiation effects can be classified as either short-term or long-term effects. Following the latent period, effects that are seen within minutes, days, or weeks are termed short-term effects. Short-term effects are associated with large amounts of radiation absorbed in a short time (e.g., exposure to a nuclear accident or the atomic bomb). Acute radiation syndrome (ARS) is a short-term effect and includes nausea, vomiting, diarrhea, hair loss, and hemorrhage. Short-term effects are not applicable to dentistry.

Effects that appear after years, decades, or generations are termed long-term effects. Long-term effects are associated with small amounts of radiation absorbed repeatedly over a long period. Repeated low levels of radiation exposure are linked to the induction of cancer, birth abnormalities, and genetic defects.

Somatic and Genetic Effects

All the cells in the body can be classified as either somatic or genetic. Somatic cells are all the cells in the body except the reproductive cells. The reproductive cells (e.g., ova, sperm) are termed genetic cells. Depending on the type of cell injured by radiation, the biologic effects of radiation can be classified as somatic or genetic.

Somatic effects are seen in the person who has been irradiated. Radiation injuries that produce changes in somatic cells produce poor health in the irradiated individual. Major somatic effects of radiation exposure include the induction of cancer, leukemia, and cataracts. These changes, however, are not transmitted to future generations (Figure 4-5).

Genetic effects are not seen in the irradiated person but are passed on to future generations. Radiation injuries that produce changes in genetic cells do not affect the health of the exposed individual. Instead, the radiation-induced mutations affect the health of the offspring (see Figure 4-5). Genetic damage cannot be repaired.

Radiation Effects on Cells

The cell, or basic structural unit of all living organisms, is composed of a central nucleus and surrounding cytoplasm. Ionizing radiation may affect the nucleus, the cytoplasm, or the entire cell. The cell nucleus is more sensitive to radiation than is the cytoplasm. Damage to the nucleus affects the chromosomes containing DNA and results in disruption of cell division, which, in turn, may lead to disruption of cell function or cell death.

Not all cells respond to radiation in the same manner. A cell that is sensitive to radiation is termed radiosensitive; one that is resistant is termed radioresistant. The response of a cell to radiation exposure is determined by the following:

Cells that are radiosensitive include blood cells, immature reproductive cells, and young bone cells. The cell that is most sensitive to radiation is the small lymphocyte. Radioresistant cells include cells of bone, muscle, and nerve (Table 4-2).

Radiation Effects on Tissues and Organs

Cells are organized into the larger functioning units of tissues and organs. As with cells, tissues and organs vary in their sensitivity to radiation. Radiosensitive organs are composed of radiosensitive cells and include the lymphoid tissues, bone marrow, testes, and intestines. Examples of radioresistant tissues include the salivary glands, kidney, and liver.

In dentistry, some tissues and organs are designated as “critical” because they are exposed to more radiation than are others during radiographic procedures. A critical organ is an organ that, if damaged, diminishes the quality of a person’s life. Critical organs exposed during dental radiographic procedures in the head and neck region include the following:

Units of Measurement

Radiation can be measured in the same manner as other physical concepts such as time, distance, and weight. Just as the unit of measurement for time is minutes, for distance miles or kilometers, and for weight pounds or kilograms, the International Commission on Radiation Units and Measurement (ICRU) has established special units for the measurement of radiation. Such units are used to define three quantities of radiation: (1) exposure, (2) dose, and (3) dose equivalent.

The dental radiographer must know radiation measurements to discuss exposure and dose concepts with the dental patient.

At present, two systems are used to define radiation measurements: (1) The older system is referred to as the traditional system, or standard system; and (2) the newer system is the metric equivalent known as the SI system, or Système International de’Unités (International System of Units).

The traditional units of radiation measurement include the following:

The SI units of radiation measurement include the following:

This text uses both the traditional and SI units of measurement; the dental radiographer should be familiar with both systems and know how to convert measurements from one system to the other (Table 4-3). In addition, the dental radiographer must be familiar with a number of physics terms used in the definitions of both traditional and SI units of radiation measurement (Table 4-4).

Exposure Measurement

The term exposure refers to the measurement of ionization in air produced by x-rays. The traditional unit of exposure for x-rays is the roentgen (R). The roentgen is a way of measuring radiation exposure by determining the amount of ionization that occurs in air. A definition follows:

In measuring the roentgen, a known volume of air is irradiated. The interaction of x-ray photons with air molecules results in ionization, or the formation of ions. The ions (electrical charges) that are produced are collected and measured. One roentgen is equal to the amount of radiation that produces approximately two billion, or 2.08 × 10⁹, ion pairs in one cubic centimeter (cc) of air.

The roentgen has limitations as a unit of measure. It measures the amount of energy that reaches the surface of an organism but does not describe the amount of radiation absorbed. The roentgen is essentially limited to measurements in air. By definition, it is only used for x-rays and gamma rays and does not include other types of radiation.

No SI unit for exposure that is equivalent to the roentgen exists. Instead, exposure is simply stated in coulombs per kilograms (C/kg). The coulomb (C) is a unit of electrical charge. The unit C/kg measures the number of electrical charges, or the number of ion pairs, in 1 kg of air. The conversions for roentgen and coulombs per kilogram can be expressed as follows:

Dose Measurement

Dose can be defined as the amount of energy absorbed by a tissue. The radiation absorbed dose, or rad, is the traditional unit of dose. Unlike the roentgen, the rad is not restricted to air and can be applied to all forms of radiation. A definition follows:

Using SI units, 1 rad is equivalent to 0.01 joule per kilogram (0.01 J/kg). The SI unit equivalent to the rad is the gray (Gy), or 1 J/kg. The conversions for rad and Gy can be expressed as follows:

Dose Equivalent Measurement

Different types of radiation have different effects on tissues. The dose equivalent measurement is used to compare the biologic effects of different types of radiation. The traditional unit of the dose equivalent is the roentgen equivalent (in) man, or rem. A definition follows:

To place the exposure effects of different types of radiation on a common scale, a quality factor (QF), or dimensionless multiplier, is used. Each type of radiation has a specific QF based on different types of radiation producing different types of biologic damage. For example, the QF for x-rays is equal to 1.

The SI unit equivalent of the rem is the sievert (Sv). Conversions for the rem and sievert can be expressed as follows:

Measurements Used in Dental Radiography

In dental radiography, the gray and sievert are equal, and the roentgen, rad, and rem are considered approximately equal. Smaller multiples of these radiation units are typically used in dentistry because of the small quantities of radiation used during radiographic procedures. The prefix “milli,” meaning 1/1000 (e.g., millirad, or mrad), allows the dental radiographer to express small quantities of exposure, dose, and dose equivalent.

Sources of Radiation Exposure

To understand radiation risks, the dental radiographer must be familiar with the potential sources of radiation exposure. This knowledge can then be used to better understand the radiation risks associated with dentistry.

Humans are exposed daily to background radiation, a form of ionizing radiation that is ubiquitous in the environment. Naturally occurring background radiation includes cosmic radiation and terrestrial radiation. Cosmic radiation originates from the stars and the sun. A person’s exposure to cosmic radiation depends on altitude: the higher the altitude, the greater is the exposure to cosmic radiation. Terrestrial radiation also occurs naturally and is emitted from radioactive materials present in the earth and air. Examples of terrestrial radiation include potassium-40 and uranium.

In the United States, the average dose of background radiation received by an individual ranges from 150 to 300 mrads (0.0015–0.003 Gy) per year. This exposure may vary according to geographic location. For example, geographic areas that contain more radioactive materials are associated with increased amounts of terrestrial radiation, whereas areas at higher elevations (e.g., Denver) are associated with increased amounts of cosmic radiation.

In addition to naturally occurring background radiation, modern technology has created artificial, or man-made, sources of radiation. Consumer products (e.g., luminous wristwatches, televisions), fallout from atomic weapons, weapons production, and the nuclear fuel cycle are all sources of radiation exposure.

Medical radiation, another source of radiation exposure, is the greatest contributor to artificial radiation exposure. Medical radiation includes medical radiographic procedures, dental radiography, fluoroscopy, nuclear medicine, and radiation therapy. Medical radiation exposure equals the average yearly dose from all other exposures combined and typically accounts for half of the total exposure received.

Table 4-5 summarizes the sources of low-level radiation.

Risk and Risk Estimates

A risk can be defined as the likelihood of adverse effects or death resulting from exposure to a hazard. In dental radiography, risk is the likelihood of an adverse effect, specifically cancer induction, occurring from exposure to ionizing radiation.

The potential risk of dental radiography inducing a fatal cancer in an individual has been estimated to be approximately 3 in 1 million. The risk of a person developing cancer spontaneously is much higher, or 3300 in 1 million. To keep the concept of risk in perspective, the risk of incurring a fatal cancer from dental radiographic procedures should be compared with commonplace risks. For example, a 1-in-1-million risk of a fatal outcome is associated with each of the following activities: riding 10 miles on a bike, 300 miles in an auto, or 1000 miles in an airplane; or smoking 1.4 cigarettes per day. These risk estimates suggest that death is more likely to occur from common activities than from dental radiographic procedures and that cancer is much more likely to be unrelated to radiation exposure. In other words, the risks from dental radiography are not significantly greater than the risks of other everyday activities in modern life.

Dental Radiation and Exposure Risks

To calculate the risk from dental radiographic procedures, doses to critical organs must be measured. As previously defined, a critical organ, if damaged, diminishes the quality of an individual’s life. With dental radiographic procedures, the critical organs at risk include the thyroid gland and active bone marrow. The skin and eyes may also be considered critical organs.

Patient Exposure and Dose

Dental patients must be protected from excess exposure to radiation. (Chapter 5 discusses patient protection in detail.) How much radiation exposure results from dental radiography? The amount of exposure varies, depending on the following:

Surface exposure, or the measure of the intensity of radiation at the patient’s skin surface in coulombs per kilogram or roentgens, is typically used when referring to patient exposure. A single intraoral radiograph (D-speed film, 70 kVp, long PID) results in a mean surface exposure of 250 milliroentgens (mR). With F-speed film, a single intraoral radiograph results in a mean surface exposure of 100 mR.

The concept of absorbed dose may also be used when referring to patient exposure and dose. The absorbed dose from a 20-film series of dental radiographs (round collimation, F-speed film, long PID) is estimated to be 41 mrads (0.00041 Gy). If rectangular collimation is used, the absorbed dose decreases to approximately 13 mrads (0.00013 Gy) (Table 4-6).

Risk Versus Benefit of Dental Radiographs

X-radiation is harmful to living tissues. Because biologic damage results from x-ray exposure, dental radiographs should be prescribed for a patient only when the benefit of disease detection outweighs the risk of biologic damage. When dental radiographs are properly prescribed and exposed, the benefit of disease detection far outweighs the risk of damage from x-radiation (see Chapter 5).

• All ionizing radiation is harmful and produces biologic changes in living tissue.

• Radiation injury results from ionization or free radical formation.

• A dose–response curve is used to demonstrate the response (damage) of tissues to the dose (amount) of radiation received.

• A threshold dose for damage does not exist, and the response of tissues is directly proportional to the dose received.

• Radiation injury follows a sequence of events: latent period, period of injury, and period of recovery.

• Radiation injury is affected by total dose, dose rate, amount of tissue irradiated, cell sensitivity, and patient’s age.

• Short-term radiation effects occur when large amounts of radiation are absorbed in a short period; long-term radiation effects occur when small amounts of radiation are absorbed over a long time.

• Radiation effects are classified as somatic (seen in the irradiated person) or genetic (passed on to future generations).

• Cellular response to radiation depends on mitotic activity, cell differentiation, and cell metabolism.

• Radiosensitive cells include blood cells, immature reproductive cells, young bone cells, and epithelial cells. Radioresistant cells include the cells of bones, muscle, and nerve.

• Exposure is the measurement of ionization in air produced by x-rays; the units for exposure are the roentgen (R) and coulombs per kilogram (C/kg).

• Dose is the amount of energy absorbed by a tissue; the units for dose are the radiation absorbed dose (rad) and the gray (Gy).

• Dose equivalent measurement is used to compare the biologic effects of different types of radiation; the units for dose equivalent are the roentgen equivalent (in) man (rem) and the sievert (Sv).

• The risks of radiation exposure involved in dental radiography are not significantly greater than other everyday risks in life.

• The amount of exposure a patient receives from dental radiographs depends on the film speed, collimation, technique, and exposure factors used.

• Dental radiographs should be prescribed only for a patient when the benefit of disease detection outweighs the risk of damage from x-radiation.

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