Dating from the fifteenth century, radiation is defined as energy sent out in the form of waves or particles. Although considered by physicists as incomplete, the particle-wave theory remains a useful model by which to understand the toxic aspects of radiation. Despite the strong nuclear force that holds the basic building blocks of atoms together, many isotopes are unstable. Various influences such as quantum fluctuations and the weak nuclear force can tip the balance toward instability to transform an isotope. This process may be intentional, as with the criticality events in a nuclear reactor or nuclear bomb, but mainly occurs spontaneously in nature as the process called radioactive decay.
In 1900, Marie Curie discovered that unstable nuclei decay or transform into more stable nuclei (daughters) via the emission of various particles or energy. Radioactive decay occurs mainly through five mechanisms: emission of γ-rays, α-particles, β-particles, positrons, or by capture of an electron. The emission of these various particles makes radioactive decay dangerous because these particles form ionizing radiation. Each radioisotope has a specific decay energy signature. That is, the emitted particles from a given radioisotope have known energies, which make identification of radiation sources possible.
The half-life (t1/2) is the period of time it takes for a radioisotope to lose half of its radioactivity. Every radioisotope has a characteristic half-life, some lasting millionths of a second while others last billions of years. In every case, the activities of radioactive isotopes diminish exponentially with time (Table 134–1).
Photons are elementary particles that mediate electromagnetic radiation. Depending on their energy the radiation has different names ranging from extremely long radio waves to high-energy γ-rays.
TABLE 134–1.Physical Properties of Radioisotopes ||Download (.pdf) TABLE 134–1. Physical Properties of Radioisotopes
|Isotope ||Half-Life ||Mode of Decay ||Decay Energy (MeV) |
|Radioisotopes of Medical Examinations |
|131I ||8 days ||β- ||0.97 |
|201Tl ||73 hours ||EC ||0.41 |
|99mTc ||6 hours ||IT ||0.14 |
|133Xe ||5.27 days ||β- ||0.43 |
|67Ga ||78 hours ||EC ||1.00 |
|18F ||109 months ||β-, EC ||1.65 |
|Military Radioisotopes |
|3H ||12.26 years ||β- ||0.02 |
|235U ||7.1 × 108 year ||α, SF ||4.68 |
|238U ||4.51 × 109 year ||SF ||4.27 |
|210Po ||138 day ||α ||5.307 |
|239Pu ||24,400 years ||1, SF ||5.24 |
|241Am ||470 years ||α, γ ||5.14/0.02 |
X-rays and γ-rays are high-energy photons and are only distinguishable by their source. γ-Radiation is emitted by unstable atomic nuclei via radioactive decay and will have a fixed wavelength depending on the energy that formed it. X-rays come from atomic processes outside the nucleus. For example, an x-ray machine generates x-rays by accelerating electrons through a large voltage and colliding them into a metal target. The rapid deceleration of electrons in the target generates x-rays and in general, the higher the voltage, the greater the energy of the x-rays. Because of their nature, high-energy γ- and x-rays can penetrate several feet of insulating concrete.
β-Particles are also called electrons. They are emitted during β-decay from an unstable radionuclide. Positrons are positively charged electrons and may also be emitted during decay processes. Electrons have less penetrating ability than γ-radiation but may still pass several centimeters into human skin. β-Particles may also cause health problems chiefly through incorporation, or internalization into living organisms.
α-Particles are helium nuclei (two protons and two neutrons) stripped of their electrons and are emitted during α-decay. These particles are the most easily shielded of the emitted particles mentioned and may be stopped by a piece of paper, skin, or clothing. Unlike β-particles, α-particles principally cause health effects only when they are incorporated.
Neutrons are primarily released from nuclear processes although high-energy photon beams used in radiotherapy may also produce them. The natural decay of radionuclides does not include emission of neutrons, which is mainly a health hazard for workers in a nuclear power facility or victims of a nuclear explosion. Unique among the particles of radioactivity, when neutrons are stopped or captured they can cause a previously stable atom to become radioactive in a process known as neutron activation.
Cosmic rays complete the group of various kinds of radiation to which an individual may be exposed. Cosmic rays are streams of electrons, protons, and α-particles thought to emanate from stars and supernovas. They rain down on the earth from all directions only to give up their energy as they strike the nuclei of oxygen and nitrogen in the upper reaches of the Earth’s atmosphere. By the time they reach the earth, the energy of cosmic radiation is reduced by several orders of magnitude. Traveling or living at altitude where the atmosphere shields relatively less cosmic radiation naturally means greater exposure to cosmic rays but in general is not considered a significant threat to humans.
Isotopes and nuclides are very closely related terms and most experts in the field use them interchangeably. An isotope is a set of nuclides with the same number of protons (eg, 123I, 125I, 127I, 131I). Nuclide is a more general term that may or may not be isotopes of a given element, such as fissile nuclides or primordial nuclides. Radioisotopes are isotopes that are radioactive, that is, they spontaneously decay and emit energy. Of the iodine nuclides listed previously, 123I, 125I, and 131I are radionuclides. The nuclide 127I is stable. Finally, radionuclides are simply nuclides that are radioactive.
Ionizing Radiation Versus Nonionizing Radiation
Ionizing radiation refers to any radiation with sufficient energy to disrupt an atom or molecule with which it impacts. In this interaction, an electron is removed or some other decay process occurs, leaving behind a changed atom. Depending on the specifics of the interaction, the chemical bonds become altered producing ions or highly reactive free radicals. Hydroxyl-free radicals, formed by ionizing water, are responsible for biochemical lesions that are the foundation of radiation toxicity.
The space between collisions of ionizing radiation and their target molecules varies with the particle type and its energy. A charged particle, such as an α particle, loses kinetic energy through a series of small energy transfers to other atomic electrons in the target medium, such as tissues. Most of the energy deposition occurs in the infratrack, a narrow region around the particle track extending about 10 atomic distances. The energy loss per unit length of particle track is called the linear energy transfer (LET), which is expressed in kiloelectron volts per micrometer (keV/μm). Heavy charged particles, such as α-particles, are referred to as high-LET radiation, whereas x-rays, γ-rays, and fast electrons are low-LET radiation.
Because of its large size, collisions along the path of an α particle are clustered together, limiting its ability to penetrate tissue. By comparison, collisions along the path of γ-rays are spread out, increasing their ability to penetrate tissue. It is this ability to penetrate tissue and transfer energy that accounts for the relative dangers of the forms of radiation and tissue susceptibility.
For a source of radiation to pose a threat to tissue, the ionizing particle must be placed in close proximity to vital components of tissue to inflict damage. High-energy photons penetrate deeply and so pose a similar risk whether they come from an external source or from an incorporated source. As noted previously, α-particles have much more limited tissue penetration. Thus, α-emitters, radionuclides that radiate these particles, must first be incorporated to pose a threat to tissue. β-particles similarly have limited tissue penetration and usually are incorporated before damage may occur, although a large external exposure to β-emitters may cause serious cutaneous injury that could be life threatening, as discussed next.
Nonionizing radiation spans a wide spectrum of electromagnetic radiation frequencies. Generally, nonionizing radiation consists of relatively low-energy photons and is used safely in cell phone and television signal transmission, radar, microwaves, and magnetic fields that emanate from high-voltage electricity and metal detectors. Although these are all considered radiation in that they are all energies released from a source, these photons lack the necessary energy required to cause ionization and cellular damage.
Radiation Units of Measure
The amount of radiation to which an object is exposed, that is, the amount emitted from a source that falls on an object, is given in units called roentgens (R), a term that is now considered obsolete. A roentgen is a unit for measuring the quantity of γ- or x-radiation by measuring the amount of ionization produced in air. As an example, an individual standing at a given distance from the x-ray–generating tube of a particular x-ray machine is exposed, on the skin, to a particular number of roentgens of x-rays (Fig. 134–1).
The definitions associated with radiation. Both curie and becquerel describe a quantity of radionuclide in terms of the number of disintegrations rather than mass. Roentgens describes the amount of air ionized by either γ- or x-rays, which indirectly quantifies the amount of radiation in the air around a source. Rad and gray (Gy) describe the fraction of radiation that actually interacts with cellular material and potentially causes injury. Roentgen equivalent man (rem) and sievert (Sv) calculate the effective dose taking into account the different particles. For example, a 100 keV α-particle causes more damage to cellular material than a 100 keV β-particle.
Not all radiation to which an individual is exposed poses a risk for cellular damage. Much of the radiation passes through the body and causes no harm. Only the fraction that is absorbed by the tissue has a chance of causing cellular damage. The International System (SI) unit that describes absorbed radiation is the gray (Gy), which has replaced the rad (radiation-absorbed dose). One Gy equals 100 rad.
To measure the risk of biologic damage regardless of the type of radiation, the effective dose is given in sievert (Sv), which has replaced the rem (roentgen equivalent man). One Sv equals 100 rem. This calculation, known as dosimetry, takes into account the type of exposure (external or internal, partial or total), the particle or particles involved (eg, α, β, γ), and the radiosensitivity of the organ or organs exposed. The effective dose is calculated according to the following equation:
where E is the dose in sieverts, D is the absorbed dose in gray, WR is the radiation weighting factor, also called Q, and WT is the tissue weighting factor indicating the radiosensitivity of each organ.
In 1910, the curie (Ci) became the unit describing the amount of radioactivity in a source. One curie equals 3.7 × 1010 disintegrations per second based on the decay of 1 g of radium. The curie was replaced by the SI unit the becquerel (Bq) where 1 Bq is equivalent to 1 disintegration per second. Thus, 1 Ci is equivalent to 3.7 × 1010 Bq. For example, following the Chernobyl incident, 1.2 × 1019 disintegrations per second of radioactive material was released into the atmosphere. By comparison, the radioactive source of 137Cs in Goiânia contained 50.9 × 1012 Bq (13.7 × 105 mCi) of cesium. A thallium stress test uses 111 × 106 Bq (3 mCi) of 201Tl, and the average indoor concentration of 222Rn in the United States is 55 Bq/m3 (14.8 × 10–6 mCi).
Protection From Radiation
Shielding refers to the process by which one may limit the amount of unwanted ionizing radiation in a given setting. Placing a specific material between a radiation source and a target will limit the amount of ionizing radiation that will interact with the target. When a particle of ionizing radiation is incident on a material, there exists some probability that it will interact with the material and be attenuated. What happens as a result of this interaction is dependent upon several factors, including the type of particle, its energy, and the atomic number of the target material. The photoelectric effect for photons, Bremsstrahlung for β-particles, and elastic scattering for neutrons are several examples of specific interactions. The shielding equation below allows calculation of the efficacy of shielding.
where I is the radiation intensity after shielding, I0 is the radiation intensity before shielding, μ is the linear attenuation coefficient, and x is the thickness material in centimeters. The linear attenuation coefficient is defined as the fraction of photons removed from the radiation field per centimeter of absorber through which it passes. Examples of shielding materials are Lucite, lead, and concrete.
Distance is an important safety factor in limiting radiation exposure. Because of their mass and electric charge, α- and β-particles have a high probability of interacting with matter, such as the atmosphere. The result is that these particles do not travel more than a few centimeters through air and, in general, moving a few feet from the source of this kind of radiation is enough protection by distance. However, x-rays and γ-rays are uncharged and have no rest mass, greatly reducing their probability of interacting with matter resulting in an unlimited range in space. Photon radiation that is emitted from a point source diverges from that source to cover an increasingly wider area. The intensity of this radiation follows the inverse square law:
where I1 is the initial intensity, I2 is the final intensity, r1 is the initial distance from the source, and r2 is the final distance from the source. For example, if the intensity of radiation 1 m from a source is 1 Gy, its intensity would be 0.11 Gy at 3 m from the source.
Time of exposure is another important safety factor in limiting radiation exposure. Obviously the longer a person is exposed to radiation the greater the exposure. The US federal and state regulations based on National Council on Radiation Protection and Measurement (NCRP) and US Food and Drug Administration (FDA) recommendations specify the limits of occupational exposure as well as exposure to patients designed to limit the potentially damaging effects of radiation.
Irradiation, Contamination, and Incorporation
An object is irradiated when it is exposed to ionizing radiation. One may be irradiated when handling radioactive isotopes, undergoing medical diagnostic imaging such as x-ray or CT, and rare exposures to criticality events. These sources of ionizing radiation generate particles that can penetrate tissue well and possibly cause tissue damage. Whole-body irradiation is one in which the entire body is exposed at once. However, more commonly shielding devices such as lead aprons, and collimation techniques used in radiotherapy limit the amount of exposed tissue to the intended target. The risk of tissue damage depends on the total amount of radiation and the tissue type because different tissue types have their own intrinsic resistance to radiation damage. An irradiated object does not become radioactive itself, unless exposed to neutrons and therefore irradiated individuals pose no risk to others.
The FDA approved irradiation of wheat and flour in 1963. They concluded from 40 years of study that irradiation is a safe and effective process for many foods to control bacteria such as Escherichia coli, Salmonella spp, and Campylobacter spp. Irradiation does not make food radioactive, compromise nutritional quality, or noticeably change the taste, texture, or appearance of food, as long as it is applied properly to a suitable product. Organizations that support irradiation of food include the American Medical Association (AMA), the Centers for Disease Control and Prevention (CDC), and the World Health Organization (WHO).
Contamination occurs when a radioactive substance covers an object completely or in part. Several examples include a laboratory or industrial worker who unintentionally spills a radionuclide on clothing or skin or a victim of a radiologic dispersing device, a “dirty bomb,” in which a radionuclide is packaged with a conventional explosive and the resultant explosion disperses the radionuclide. In these similar cases, the source of radiation is the nuclide undergoing its normal decay process, and the individual is exposed to particles such as those mentioned in Table 134–1. The risk for tissue damage from the radiation particles is usually quite low, assuming that the contamination is detected and appropriate measures for decontamination are instituted.
Incorporation occurs when a radionuclide is taken up by tissue via some route that permits the radionuclide to enter the body. This principle is used in many diagnostic and therapeutic procedures such as a thallium stress test, gallium scan, or thyroid ablation therapy. Depending on the dose and type of radionuclide, incorporation may lead to tissue damage, as was the situation for several people following the event at Goiânia.