Radiation Injuries

Background and Importance

Radiation is energy that travels through space in the form of a particle or wave. It is produced by radioactive decay of an unstable atom (radionuclide or radioisotope) or by the interaction of a particle with matter. Particle radiation consists of particles that have mass and energy and may carry an electric charge. Examples of particle radiation include alpha particles (helium nuclei), protons, beta particles (electrons ejected from the nucleus), and neutrons. Electromagnetic radiation consists of photons that have energy but no mass or charge. Radiation can be either ionizing or nonionizing depending on its energy and ability to penetrate matter. Electromagnetic radiation varies by frequency and wavelength as shown in Figure 134.1 .

Radioactive decay (radioactivity) is the process by which a nucleus of an unstable atom loses energy by emitting ionizing radiation in the form of high-energy particles or rays. Radioactive decay can emit particles (e.g., alpha and beta) or rays such as gamma or x-rays. Gamma rays and x-rays are high-energy photons that differ in their place of origin: gamma rays are emitted from the nucleus, whereas x-rays are produced as the result of changes in the positions of electrons orbiting the nucleus.

The type and rate of radioactive decay varies by radionuclide. The rate of decay is measured by the radioactive half-life (the time for half the radioactive nuclei in any sample to undergo radioactive decay) and varies from a few microseconds to billions of years. Radiation exposure can be external (e.g., exposure to x-rays) or internal, resulting from the inhalation, ingestion, or injection of radioisotopes.

Radiation Measurements

The four different but interrelated units for measuring radiation (radioactivity, exposure, absorbed dose, and dose equivalent) are shown in Table 134.1 . These units are also commonly expressed as fractions of whole units using the terms and abbreviations milli (m; 1/1000th) and micro (μ; 1/1,000,000th).

Radiation Sources

Ionizing radiation and radioactive substances are natural and permanent features of the environment. The average annual radiation dose per person in the United States is 6.2 mSv (620 mrem). Fifty percent of this average dose comes from background radiation and 48% from medical procedures. The major sources of background radiation are radon and thoron (37%), cosmic radiation (5%), naturally occurring internal radioisotopes (e.g., potassium-40 [5%]), and terrestrial background radiation (3%). The major medical sources include computed tomography (CT; 24%), nuclear medicine (12%), interventional fluoroscopy (7%), and conventional radiography and fluoroscopy (5%). The remainder of the average annual radiation dose comes from occupational and consumer sources.

Radon is a naturally occurring radioactive gas that is formed from the radioactive decay of uranium. Radon can accumulate in homes and is the second leading cause of lung cancer in the United States after tobacco exposure. Radon exposure is estimated by measuring radon levels in the air using inexpensive and readily available kits. If indoor levels of radon are 4 pCi/L or greater, then the US Environmental Protection Agency (EPA) recommends that the homeowner consult a certified radon mitigation specialist to reduce radon air levels in the home.

Although radiation-related incidents are rare, the consequences of exposure or significant internal contamination can be fatal. The Radiation Emergency Assistance Center at Oak Ridge National Laboratory maintains a worldwide registry of serious radiation incidents. Between 1944 and 2012, there have been 454 radiation incidents recorded worldwide. The greatest numbers of serious exposures have occurred with sealed sources, which include brachytherapy sources used in radiation oncology and industrial radiography devices (n = 214), followed by x-ray devices (n = 86). Radioisotopes used in medical diagnosis and therapy have caused approximately 10% of major radiation incidents.

Serious nuclear power incidents include the Fukushima Daiichi disaster (2011), the Chernobyl disaster (1986), Three Mile Island (1979), and the SL-1 accident (1961). The radioisotopes most commonly released from nuclear reactor accidents include iodine, cesium, and strontium. Chernobyl had the largest number of radiation-related injuries. About 150 individuals who received very high whole-body doses were treated for acute radiation sickness; 28 of these died within a relatively short time, and approximately 20 more have since died from radiation-related diseases. Radiation to the thyroid from radioisotopes of iodine released during the Chernobyl event has caused several thousand cases of thyroid cancer, with children being the most susceptible population.

The detonation of nuclear bombs has the greatest potential to produce mass casualties. The acute and long-term effects following the bomb blasts in Hiroshima and Nagasaki in 1945 (18 and 22 kilotons) have been well documented. Today’s nuclear weapons are orders of magnitude potentially more devastating. Another threat is the detonation of a low-yield nuclear bomb by terrorists. A 10-kiloton nuclear detonation within a city in the United States would result in a zone of destruction of more than 2 miles from ground zero and would expose hundreds of thousands of people to radiation. A more likely terrorist scenario is the explosion of a dirty bomb. A dirty bomb is the combination of a conventional explosive with a radioisotope. The radioisotopes most likely to be used in a dirty bomb are cesium-137, cobalt-60, or strontium-90. Although the acute radiation risks from a dirty bomb detonation are low, the localized residual radiation contamination would likely cause widespread panic.

Anatomy, Physiology, and Pathophysiology

The biologic effects of radiation exposure are determined by the type of radiation, the total dose, the dose rate, the volume of tissue or anatomic body part irradiated, and individual susceptibility factors. The amount of energy released in matter (linear energy transfer) varies by type of radiation. Different types of radiation are assigned a quality factor (QF) based on their ability to produce biologic damage in exposed tissue. The higher the QF, the more biologically damaging the radiation is. Gamma rays, x-rays, and beta particles have a QF of 1. Alpha particles (internal exposure only) have a QF of 20, whereas neutrons have a QF range of 3 to 20, depending on their energy.

Ionizing radiation includes particles and photons that have sufficient energy to detach electrons, thus causing ionization of the atoms that they encounter. Alpha particles, beta particles, and neutrons are examples of particle ionizing radiation. Only the high-frequency portion of the electromagnetic radiation spectrum (gamma rays, x-rays, and far-ultraviolet) has sufficient energy to produce ionization. Other frequencies and wavelengths (near-ultraviolet, infrared, microwaves, radio waves, and very or extremely low frequency radiation) are considered nonionizing . The health effects of exposure to nonionizing radiation depend on the frequency and wavelength. For example, ultraviolet light can produce sunburns, visible light (such as lasers), can produce corneal and retinal burns, and microwaves can produce heating of body tissues.

The effects of ionizing radiation on tissue can be direct or indirect . Direct effects include single- and double-strand DNA breaks. Indirect effects act through generation of free radicals that then attack other molecules in the cell. Cells vary in their sensitivity to radiation. In general, cells that are undifferentiated, divide quickly, and have high metabolic activity are most radiosensitive. Examples of these types of cells include bone marrow stem cells, lymphocytes, spermatogonia, ovarian cells, intestinal crypt cells, and epidermal basal cells. Less radiosensitive tissues and organs are made up of cells with little or no turnover such as connective tissue or the central nervous system. The effects of radiation can be deterministic or stochastic . Deterministic effects are those in which the severity of injury is a function of dose (e.g., bone marrow suppression). Stochastic or probabilistic effects are those in which the probability of an effect, rather than its severity, is a function of dose. An example of a stochastic effect is the development of radiation-induced cancer.

For external exposure, the site of the body that is irradiated (e.g., bone marrow vs. upper extremity) is an important determinant of the resulting effects. For internal exposure, the biodisposition of the radioisotope, its radiologic and biologic half-lives, as well as the types of radioisotopes produced during radioactive decay are important determinants of the effects. Biodisposition refers to the absorption, distribution, metabolism, and excretion of a radioisotope.

The effective half-life reflects both the radiologic and biologic half-life and can be calculated as 1/effective half-life = 1/biologic half-life + 1/physical half-life. For example, iodine-131 has an approximate biologic half-life of 57 days and a radiologic half-life of 8 days. The resulting effective half-life is approximately 7 days.

Radioisotopes will have their greatest effects at the sites in the body where they are concentrated. For example, radioiodine concentrates in the thyroid gland and the resulting effects, such as thyroiditis or thyroid cancer, occur at the site of concentration.

Acute Radiation Syndrome

Acute radiation syndrome (ARS) occurs after a patient is exposed to whole body radiation. ARS from external or internal exposure to radiation varies in nature and severity by dose, dose rate, dose distribution, and individual susceptibility. There are three phases to ARS: prodromal, latent, and manifest illness. The progression and patterns of symptoms and signs of the phases of ARS can overlap. The timing of the progression through the phases can be accelerated with increasing radiation doses.

In the prodromal phase, initial symptoms are typically nonspecific, and include anorexia, nausea, vomiting, and fatigue. This phase is useful to help predict the severity of the radiation injury. The presence, onset, and frequency of nausea and vomiting, although nonspecific, can serve as a prognostic factor. Early onset or persistent nausea and vomiting as well as the presence of diarrhea, indicate a more severe radiation injury.

The latent phase is a period of initial symptom improvement. Patients may even become symptom-free. Those victims with lethal radiation doses may not have a symptom-free period and progress from the prodromal phase directly to the manifest illness phase.

The manifest illness phase has three sub-syndromes that may occur and overlap depending on the radiation dose received ( Table 134.2 ). All organs are affected by radiation. However, the relative sensitivities of the organ systems exposed to radiation determines the clinical symptoms. Tissues with greater rates of cellular division, particularly the hematopoietic and gastrointestinal systems, are most radiosensitive.

The hematopoietic sub-syndrome is the first sub-syndrome seen. This sub-syndrome can appear at doses greater than 1 Gy and typically results in bone marrow suppression. At doses less than 1 Gy (100 rem), most cells survive but may be susceptible to radiation-induced cancer. Lymphocytes are the first cell line to decrease and with high doses of radiation this drop will occur sooner and with greater severity. The hematopoietic system in children has been estimated to be more than twice as radiosensitive as in adults.

The gastrointestinal sub-syndrome begins to occur at doses nearing 6 Gy, about 1 week after exposure. Patients will display nausea, vomiting, gastrointestinal bleeding, malabsorption, and fluid losses, potentially leading to hypovolemia and cardiovascular collapse. These symptoms are due to death of the intestinal epithelial precursor cells and resultant denuding of the intestinal epithelial surface. Thrombocytopenia and immunosuppression from the accompanying hematopoietic sub-syndrome also predispose patients to infection and bleeding.

The neurovascular sub-syndrome results from doses greater than 10 Gy and is typically lethal. Patients will develop irritability, altered mental status, seizures, prostration, ataxia, and hypotension. Coma and death usually occur within a few hours. Because of the high dose of radiation needed to produce these findings, patients often die without experiencing a latent phase.