What would be an external source of ionizing radiation

14.10.1 Introduction

Ionizing radiation is generally considered to be any form of radiation exposure that will produce a subatomic ionization event. Ionizing radiation may be in the form of either electromagnetic waves or particles; however, it must have sufficient energy to cause ionization in the target molecule. For electromagnetic radiation (EMR), the wavelength is usually shorter and therefore the energy is higher than for nonionizing radiation; thus, ionizing radiation is more likely to produce a biological effect. Ultraviolet (UV) EMR is intermediate in wavelength and energy and is typically considered as nonionizing; however it is clearly mutagenic, can produce ionization, and should be considered as a carcinogen. (Nonionization radiation is dealt in Chapter 14.09.) Although perhaps recognized later than its chemical counterparts, ionizing radiation is now regarded as a carcinogen and can act independently or synergistically with other carcinogens to produce neoplasia in living systems due to its unique mechanisms of mutation and biological effect. This chapter discusses the carcinogenic nature of ionizing radiation.

This chapter will first explain the physical interaction between the different forms of ionizing radiation and cells and subcellular components, as well as the factors that are more likely to produce an elevated risk of neoplasia. Next, the biological and molecular effects of radiation within living systems will be examined, followed by a presentation of the epidemiological evidence for radiation as a carcinogen in animals and humans. Some risk models for carcinogenesis following an exposure to ionizing radiation, as well as some strategies for protection against radiation-induced biological damage, are included. Important new avenues of research and some of the controversial issues surrounding radiation carcinogenesis are brought forth. Finally, the significance of ionizing radiation carcinogenesis in the context of understanding and managing the difficult problem of human cancer etiology will be addressed.

Ionizing Radiation

IR is high-energy radiation that is powerful enough to cause displacement of electrons from atoms and breaks in chemical bonds. It is capable of introducing DNA strand breaks, introducing mutations, and causing cell death. The major types of IR exposures are from background, external, internal, and medical sources, and include nuclear radiation, X-rays, and radon. Geographic variability can result in different levels of background IR. External and internal sources of IR exposures can occur as a result of man-made sources and pollutants. Medical exposures occur through X-rays, CT scans, and cancer radiotherapy.

IR is primarily genotoxic and is a well-described carcinogen. Varying levels of IR exposures are associated with thyroid cancer, hematopoiesis, and other malignancies. The magnitude of the genotoxic effect depends on the degree of exposure and the concentration of ions induced by the absorption of the energy emitted by the IR source. Increased rates of thyroid cancer have been identified in patients with prior head and neck radiotherapy. IR as a result of the 1986 Chernobyl nuclear power plant accident resulted in increased childhood thyroid cancer and adult breast cancer. Increased rates of childhood leukemia were identified 5–6 years after the atomic bombings of Hiroshima and Nagasaki. Ongoing studies suggest that there may be increases in solid cancer rates in children either exposed in utero or at less than 6 years of age at the time of the bombings. A recent German study showed a significant excess of leukemias in young children living close to nuclear power plants, reviving the debate about hazards of nuclear power plants, but radiation doses measured in the vicinity of the nuclear installations are thought to be too low to increase the risk of cancer.

Results on low doses of IR are controversial. Studies before the 1980s on childhood cancer after prenatal X-ray exposures yielded associations with some pediatric cancers, especially leukemia. Recent studies did not confirm this, and radiation doses from X-ray examinations become considerably lower over time.

No clear overall picture emerges from studies on radon and childhood leukemia, although a recent Danish study shows a moderate association between domestic radon and ALL but not other cancers.

Better study designs are needed to investigate possibly small risks related to low doses of IR, and an international study to follow up children with CT examinations has been suggested.

Ionizing Radiation

Ionizing radiation (IR) is high-energy radiation that is powerful enough to cause displacement of electrons from atoms and breaks in chemical bonds. It is capable of introducing DNA strand breaks, introducing mutations, and causing cell death. IR is primarily genotoxic and is a well-described carcinogen. Although some uncertainty remains at low dose levels, there is accumulating evidence from studies in adults that the relationship between IR and cancer is best described with a linear nonthreshold model, that is, monotonic increase in risk with increasing exposure starting from lowest doses. Major type of IR exposures are natural, namely terrestrial or cosmic or naturally occurring radioactive nuclides in the environment. Geographic variability can result in different levels of background IR, but some background IR is ubiquitous and zero exposure does not exist. Other external and internal sources of IR exposures can occur as a result of man-made sources and pollutants. Medical exposures occur through X-rays, CT scans and cancer radiotherapy.

Higher doses of IR is an established cause of childhood cancer. Increased risks of second hematological malignancies but also some solid cancers are observed in children having had radiation treatment of their primary cancer. Increased rates of thyroid cancer have been identified in patients with prior head and neck radiotherapy. After the Hiroshima and Nagasaki atomic bomb detonation increased rates of childhood leukemia were identified 5–6 years later. Recent studies suggest that there may be increases in solid cancer rates in children exposed either in utero or < 6 years of age at the time of detonation. IR (here radio-iodine specifically) as a result of the 1986 Chernobyl nuclear power plant accident resulted in increased rates of childhood thyroid cancer.

Findings from studies of low doses of IR are more controversial. Studies before the 1980s on childhood cancer after prenatal X-ray exposures yielded associations with some pediatric cancers, especially leukemia. Recent studies did not confirm this and radiation doses from X-ray examinations became considerably lower over time. Computed tomography (CT) examinations however are increasingly used in most HICs and studies from the United Kingdom and Australia suggest increased risks of leukemia and CNS tumors with increasing dose from CT exposures; although reverse causation is a concern, it appears that after taking this into account some attenuated risk increase remains. A multicenter study in nine European countries is currently under way. While there is no doubt CT is an important diagnostic instrument, results may urge better dose adjustment when planned examinations with children are conducted. No clear overall picture emerges from studies on radon and childhood leukemia. A German study showed a significant excess of leukemias in young children living close to nuclear power plants, reviving the debate about hazards of nuclear power plants, but radiation doses measured in the vicinity of the nuclear installations are thought to be too low to increase the risk of any cancer. Notably, thyroid cancer remains the only cancer for which an excess has been established following the Chernobyl nuclear accident. For the recent Fukushima Daiichi nuclear accident, thyroid doses are manifold lower and an increase in thyroid cancer has so far been attributed to over-diagnosis from organized thyroid ultrasound examinations carried out in large childhood populations living in the Fukushima prefecture.

A recent pooled study of nine cohorts examined the risk of childhood leukemia with IR from various sources, and detected a threefold risk for acute myeloid leukemia and almost sixfold for ALL with each increase of 100 mSv; an increase was seen at exposures < 50 mSv. This is so far the most convincing evidence of a low-dose IR effect at least in leukemias.

Abstract

Ionizing radiation is a part of the natural environment humans are exposed to on Earth. For cells, ionizing radiation is only one of many sources of stress that will be experienced during the course of a lifetime. The primary cellular target of ionizing radiation is DNA. Damage to DNA caused by ionizing radiation leads to mutations in somatic cells, often resulting in cancer; mutations of germ cells lead to hereditary mutations and, theoretically, genetic diseases. In addition, the presence of damaged nuclear DNA in cells can lead to genomic instability – a cellular state that can result in the accumulation of new mutations even several cell divisions after the DNA damage event. Types of mutations created in each case are slightly different and accumulate in nuclear, genomic DNA as well as mitochondrial genomes. The situation is further complicated by dissimilar effects of radiation of differing qualities, doses, and dose rates. Moreover, identical radiation treatments can have on different effects on specific cell types under specific environmental and physiological conditions, in organisms of different species, or different gender. Despite extensive efforts, there is much that is still not understood about radiation exposure, and the coming years will offer more opportunities to explore these areas.

5.14.2.5.3.4 Ionizing Radiation

Ionizing radiation inactivation is a method of low temperature sterilization by using gamma ray, X-ray or electron radiation to penetrate the products and kill the microorganisms. The use of ionizing radiation for inactivation of viruses in foods and materials derived from animals in the manufacture has certain advantages, without causing denaturation by thermal process, and without harmful substance residues. Among the different forms of ionizing radiation, gamma irradiation and electron beam are most commonly employed.55

The disinfection effect of radiation depends on the dosage, but a higher dosage may cause damage to the material components. Disinfection of materials is not based on the successful preservation of their activities, but minimzes the impact on physical and biological properties of the material through disinfection and sterilization. Therefore, appropriate disinfection methods and dosage should be selected according to the characteristics of biomaterials and the contamination of pathogenic microorganisms, so as to achieve the best clinical application effect.

39.3.2.6 Ionizing radiation

Ionizing radiation can target rapidly dividing cells in multiple organs, including the testes and ovaries, as well as the developing embryo and fetus (Cockerham et al., 2008; Foster and Gray, 2013; Rogers, 2013). Exposure of males to ionizing radiation can result in diminished spermatogenesis and testosterone production by the testes, with increased secretion of LH and FSH by the anterior pituitary (Cockerham et al., 2008). Consistent with these effects, Ukrainian workers involved in the cleanup of radioactive materials after the Chernobyl nuclear accident had increased ultramorphological sperm abnormalities (Cockerham et al., 2008; Fischbein et al., 1997). Similar to the radiation-induced endocrine effects observed in the testes, ovarian steroid production is reduced by exposure to ionizing radiation (Cockerham et al., 2008). Depending on the timing and dose of the radiation exposure, ionizing radiation can cause pubertal failure, ovarian failure, or premature menopause in women (Cockerham et al., 2008).

Clusters of Down syndrome in Belarus 9 months following the explosion at the Chernobyl nuclear power plant suggest a radiosensitive phase of oogenesis in mammals around the time of ovulation and conception (Zatsepin et al., 2007). High radiation exposure in late-gestational women or pregnant animals has the potential to cause abortion or preterm births associated with maternal and/or fetal radiation sickness and stress. Depending on the stage of development and the dose of radiation, exposure of the conceptus, embryo, or fetus to ionizing radiation can result in lethality or morphologic abnormalities (Cockerham et al., 2008), and observations in humans and animals after the Chernobyl incident are consistent with these developmental effects (Østerås et al., 2007; Peterka et al., 2007). In addition, anxiety associated with exposures of pregnant women to ionizing radiation from the Chernobyl nuclear accident reportedly led to increased incidences of induced abortions in several European countries, even in instances where the exposure was minimal (Cordero, 1993).

29.10 Gamma and High-Energy Irradiation

Ionizing radiation generated by radioactive materials such as cesium 127 or cobalt 60 and high-energy electron beams can inactivate microorganisms either directly or indirectly by production of free radicals. Nucleic acids are the main targets of ionizing radiation. Ionizing radiation has been studied in great detail for preservation of foods and for wastewater and sewage sludge treatment. Factors that influence the effectiveness of ionizing radiation include the type of organism (generally, the smaller the organism the more resistant); composition of the suspending medium (organic material offers protection); presence of oxygen (greater resistance in the absence of oxygen); and moisture (greater resistance of dried cells and radiolysis of water). The unit of dose is the rad, which is equivalent to the absorption of 100 ergs per gram of matter. A kilorad (krad) is equal to 1000 rads. Typical doses to produce a D value of 90% inactivation are shown in Table 29.8. Viruses are the most resistant to ionizing irradiation in water and sludge.

Table 29.8. Sludge Irradiation: D Values for Selected Pathogens and Parasites

Modified from Ahlstrom and Lessel (1986).

Sludge irradiators have been built in Europe and experimental electron beam irradiators in the United States. The electron beams are generated by a 750-kV electron accelerator. The unit treats a thin layer (≈ 2 mm) of liquid sludge spread on a rotating drum. Such systems are costly for waste treatment and require thick concrete shielding.

Introduction

Ionizing radiation is ubiquitous and all living things are, and always have been, exposed to naturally occurring radiation and radioactivity. However, people’s activities have added to the levels of radiation and radioactivity globally through fallout from aboveground nuclear (weapons) testing and locally through activities such as mining, production of phosphate fertilizers, oil and gas off-shore platforms, and nuclear fuel cycle activities among others.

Until quite recently, the prevailing view has been that, if humans were adequately protected, then “other living things are also likely to be sufficiently protected” (International Commission on Radiological Protection, 1977) or “other species are not put at risk” (International Commission on Radiological Protection, 1991). Consistent with this view, a great deal of effort has been made over the past 50 years or so to study the behavior of radioactivity in the environment, particularly as it relates to potential pathways of exposure to humans. However, by the 1990s, attempts were made to look in general at the effects of radiation on plants and animals at levels implied by the radiation protection standards for humans. Events such as the United Nations Conference on Environment and Development in Rio de Janeiro, Brazil, in 1992 and the Convention on Biodiversity gave impetus to environmental protection in general and stimulated international and national agencies to become more active in radioecotoxicology.

The 1996 report of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) was of seminal importance as it was the first time that the UNSCEAR Committee had examined the effects of ionizing radiation on nonhuman biota. Until that time, UNSCEAR, along with other international and national organizations, had considered living organisms primarily as part of the human food chain.

Since 1996, there has been increasing attention given to the potential effects of ionizing radiation on the environment; this increasing interest since 1996 is well illustrated by the numerous activities of national and international organizations such as the Canadian Nuclear Safety Commission, the European Community, the US Department of Energy, the United Kingdom Environmental Agency, and the International Atomic Energy Agency, which have undertaken initiatives to investigate the effects of ionizing radiation on the environment.

The following sections build on the dialog that has taken place over the last 11 years and provides an overview of the current generic approach to assessing risks to nonhuman biota from ionizing radiation and radioactivity in the environment, including discussion of selected issues unique to ionizing radiation.

Ionizing and Nonionizing Radiation

Ionizing radiation is energy in the form of waves or particles that dislodge electrons from atoms, and is an important component of the universe. Stars are heated and illuminated by nuclear-fusion reactions that produce ionizing radiation (e.g., neutrons and gamma rays). Radiation-related nuclear synthesis within stars and supernovas produced the life-supporting hydrogen, oxygen, nitrogen, and other elements in our bodies. All life on earth therefore is due, at least in part, to previous radiation reactions.

There are two general types of ionizing radiation: particulate and electromagnetic. Particulate ionizing radiations include alpha (α) particles, beta (β−; negative electrons) particles, positrons (β+; positive electrons), neutrons (neutral), protons (positively charged), and heavy ions (positively charged heavy nuclei). Ionizing electromagnetic radiation includes X-rays and gamma (γ) rays.

Charged particulate radiations from the sun and stars interact with the earth’s atmosphere and magnetic field, producing a continuous shower of secondary ionizing radiation (including β and γ radiation). Radioisotopes from terrestrial sources are contained in our bodies from birth, and additional natural radioactivity is added each day from breathing air, eating foods, and ingesting water and other liquids. All humans are therefore radioactive.

Radiation hits to the body of humans from natural radiation sources have not been demonstrated to cause harm. By the age of 2 years, each person will have received more than 60 trillion harmless natural radiation hits to the body. By the age of 20 years, the harmless hits exceed 630 trillion.

Electromagnetic radiation with insufficient energy causing ionizations is called nonionizing radiation. Examples are ultraviolet (UV) radiation, radiofrequency radiation (which includes microwaves), and extremely low frequency (ELF) radiation (associated with electric power lines).

The field of radiation toxicology has mainly focused on ionizing radiation. The sections that follow relate only to ionizing radiations.

I.A Origins of Ionizing Radiation

Ionizing radiation is invariably the consequence of physical reactions, involving subatomic particles, at the atomic or nuclear level. The possible radiation-producing reactions are many, and usually, although not always, involve altering the configuration of neutrons and protons in an atomic nucleus or the rearrangement of atomic electrons about a nucleus. These reactions can be divided into two categories:

Radioactive Decay. In the first type of radiation producing reaction, the nucleus of an atom spontaneously changes its internal arrangement of neutrons and protons to achieve a more stable configuration. In such spontaneous radioactive transmutations, ionizing radiation is almost always emitted. The number of known different atoms, each with a distinct combination of Z and A exceeds 2900 nuclides. Of these, 266 are stable and are found in nature. There are also 65 long-lived radioisotopes found in nature. The remaining nuclides have been made by humans and are radioactive with lifetimes much shorter than the age of the solar system. Both naturally occurring and manmade radionuclides are the mostly commonly encountered sources of ionizing radiation.

Binary Reactions. The second category of radiation-producing interactions involves two impinging atomic or subatomic particles that react to form one or more reaction products. Examples include neutrons interacting with nuclei of atoms, or photons interacting with nuclei or atomic electrons. Many binary reactions, in which an incident subatomic particle x strikes an atom or nucleus X, produce only two reaction products, typically a residual atom or nucleus Y and some subatomic particle y. These binary two-product reactions are often written as X(x, y)Y.