Radiation Injury
INTRODUCTION
The widespread use of radiation and radioactive materials in medicine, research, and industry, the transport of radioactive materials throughout all parts of the country, and the unfortunate looming threat of radioactive materials used in acts of terrorism (so-called “dirty bombs”) mandate that every emergency department (ED) develops protocols for the treatment of victims of radiation injury. Emergency physicians should be familiar with recognition of exposure and treatment protocols. Telephone numbers for regional Department of Energy Assistance Offices by state are listed in Table 53-1.
There are two mechanisms of radiation injury that must be recognized and treated: actual contamination with radioactive, particulate matter, which may be external or internal, and exposure to particulate or electromagnetic radiation. This chapter discusses some of the physical aspects of radiation, the common nuclides involved, ED planning with respect to radiation injury, and the nature and initial treatment of radiation illness.
PHYSICAL PROPERTIES
Particulate Radiation
Particulate radiation includes alpha and beta particles, neutrons, protons, and positrons.
Alpha Particles
Alpha particles are composed of two protons and have an atomic mass of 4 and a +2 charge. The decay of certain heavy elements, such as uranium, radium, and plutonium, results in the emission of alpha particles, each with its own discrete energy. Alpha particles, because of their charge, have significant ionizing ability but virtually no penetrating ability; they are stopped by paper and keratin and cause injury by internal contamination only. Alpha particles cannot be measured by most radiation survey meters.
Beta Particles
Beta particles is composed of electrons, each carrying a -1 charge, and is often produced during the decay of lighter elements (such as tritium), usually resulting from the conversion of a neutron to a proton in the atom’s nucleus. Beta rays have a continuous spectrum of energy from 0 to a maximum value characteristic of each beta emitter. Although beta radiation may travel several meters in the air, it penetrates only a few millimeters of skin tissue and is stopped by clothing.
Table 53-1 United States Department of Energy Regional Coordinating Offices for Radiologic Assistancea | ||||||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||
Positrons
Positrons have the same mass and energy characteristics as the electron but have a charge of +1; they result from conversion of a proton to a neutron in the nucleus or by electron capture.
Neutrons
Neutrons, resulting from elements decaying by spontaneous fission, have a mass of 1 and are uncharged; free neutron particles are unstable and decay with a half-life of approximately 13 minutes into a proton, electron, and a neutrino. Neutron particles readily penetrate all human tissues and may cause widespread ionization by collision or neutron capture. Protons produced within tissues by neutron radiation are potent ionizers.
Protons
Protons are the same as hydrogen nuclei, having a mass of 1 and a charge of +1.
Electromagnetic Radiation
Electromagnetic radiation is composed of gamma and x-rays. Gamma radiation originates from the decay of unstable atomic nuclei, often accompanies the emission of alpha and beta particles, and has discrete energy levels related to the nuclide from which it is emitted. Gamma rays easily penetrate tissues and are detected by Geiger-Müller counters. X-rays originate outside the atomic nucleus and have a penetrating power related to the energy of the particular photon from which they are emitted. X-ray machines produce x-rays by applying a high positive voltage between the source of electrons and a collecting terminal in a vacuum tube. The electrons produced strike a target, such as tungsten, and their energy is converted into x-ray photons. X-rays and gamma radiation produce ionization in tissues by a variety of indirect mechanisms that involve the ejection of a high-speed electron.
DOSE UNITS
The curie (Ci) is the basic unit used to describe the quantity of radioactivity in a sample of material. One curie equals 3.7 × 1010 disintegrations per second (dps), the approximate rate of decay of 1 g of radium. The roentgen is the unit of exposure related to the amount of ionization caused in air by gamma or x-radiation. The rad is the unit of measure of radiation absorbed dose and represents the actual amount of radiation energy deposited in any material; 1 rad equals 0.01 J/kg in any medium. The rem (roentgen-equivalent-man) is equal to the absorbed dose (rad) multiplied by a specific quality factor (QF) derived for each source of radiation. The QF is based on the linear energy transfer of the radiation—the rate at which charged particles transfer energy to the medium. Generally, the higher the linear energy transfer, the greater the tissue injury for a given absorbed dose. The QF for electrons, positrons, and x-rays equals 1. Although the QF for alpha particles and neutrons is currently under discussion, the International Committee on Radiation Protection recommends the use of a QF of 10 to 20 for these particles.
The International Committee on Radiation Units and Measurements recommends that the aforementioned units, which continue to be used, be replaced with the International System of Units. These are as follows and are being increasingly used.
• 1 Becquerel (Bq) = 1 dps = 2.703 × 10-11 Ci
• 1 Gray (Gy) = 100 rad = 1 J/kg; 1 rad = 1 cGy
• 1 Sieverty (Sv) = 100 rem = 1 J/kg × QF; 1 rem = 1 cSv
• Roentgen is to be expressed as coulomb/kg (C/kg).
RADIATION SOURCES
Ambient sources of radiation include cosmic rays and naturally occurring nuclides (65-235 mrem/y); medical diagnostic and therapeutic radiation (77 mrem/y); medical radiopharmaceuticals (4 mrem/y); nuclear power reactors and weapons testing (4 mrem/y); and miscellaneous industrial, research, and consumer sources (4 mrem/y). An airplane trip from Boston to London will expose a traveler to 5 mrem, whereas a chest x-ray gives a skin dose of approximately 20 to 30 mrem. Generally, the yearly amount of radiation to which a person is exposed depends on occupation, geographic location, and the number of diagnostic medical or other tests using radioactive materials to which he or she is exposed.
MECHANISM OF INJURY
Ionizing radiation reacts with molecules in two ways:
Ionization, in which an orbital electron is ejected from the molecule resulting in the formation of an ion pair
Excitation, in which an orbital electron is raised to a higher energy level
In both cases, reactive free radicals are produced that may then react with biologically important molecules. These reactions have been shown to involve proteins, enzymes, nucleic acids, lipids, and carbohydrates. Cytopathologically, such reactions may result in cell death, temporary cellular injury, or genetic mutation.
SPECIFIC RADIONUCLIDES
Familiarity with the more common radionuclides used in medicine, research, and industry is useful when developing treatment protocols. Efforts should be made to determine which, if any, nuclides are being used in the hospital and local area; this information should be included with the radiation treatment protocol. Medical centers may use 131I, 99mTc, 67Ga, 32P, 133Xe, 201Tl, and 60Co. Laboratories may use 3H, 14C, 125I, 60Co, and 238U. Industrial processes may use 60Co, 137Cs, 192Ir, and enriched or depleted uranium. Nuclear power facilities produce 3H, 131I, 137Cs, 60Co, 90Sr, 144Ce, 239Pu, and numerous other radionuclides.
Radium
Radium is a metabolic analog of calcium and after exposure becomes deposited in bone. It is a source of alpha, beta, and gamma radiation with a physical half-life of 1,622 years. Inhalation or ingestion is associated with an increased incidence of bone sarcoma and carcinomas of the head and neck. Radium, a disintegration product of uranium, is used in medicine, industry, and research.
Strontium
Strontium-90 is a metabolic analog of calcium and is deposited in bone. Strontium is a beta emitter with a half-life of 28 years. It is abundant in the fission products of nuclear weapons and power plants. It is readily absorbed and produces an increased incidence of bone and bone-related sarcomas. Long-term ingestion is associated with a high incidence of myeloproliferative disease and overt leukemia.
Iodine
Iodine-131 is absorbed rapidly and is nonuniformly deposited in thyroid tissue. It is a low-energy beta and gamma emitter with a half-life of approximately 8 days.
It is widely used in medicine and found in nuclear weapons and power plants. An increased risk of thyroid carcinoma is associated with exposure to 131I. 125I has a halflife of 60 days and otherwise has similar properties to 131I.
It is widely used in medicine and found in nuclear weapons and power plants. An increased risk of thyroid carcinoma is associated with exposure to 131I. 125I has a halflife of 60 days and otherwise has similar properties to 131I.
Cesium
Cesium-137 is a metabolic analog of potassium that is rapidly absorbed and distributed throughout all tissues of the body. It is an energetic beta and gamma emitter with a half-life of 30 years. Symptoms of acute toxicity are similar to those of whole-body radiation exposure; death is caused by bone marrow failure. Chronic low-level contamination may be associated with a variety of neoplasia; 137Cs is an abundant product of nuclear detonation and power plants, is present in nuclear fallout, and is used in certain industrial applications.
