Biologic effects of ionising radiation

Assessment of the health impact of ionising radiation requires an understanding of the interactions of radiation with living cells and the subsequent reactions that lead to injury. These subjects are surveyed in the following sections, with particular reference to the principal sources and levels of radiation in the environment and the different types of biologic effects that may be associated with them.

Historical background

Within weeks after Röntgen revealed the first X-ray photographs in January 1896, news of the discovery spread throughout the world. Soon afterward, the penetrating properties of the rays began to be exploited for medical purposes, with no inkling that such radiation might have deleterious effects.

The first reports of X-ray injury to human tissue came later in 1896. Elihu Thomson, an American electrical engineer, deliberately exposed one of his fingers to X rays and provided accurate observations on the burns produced. That same year, Thomas Alva Edison was engaged in developing a fluorescent X-ray lamp when he noticed that his assistant, Clarence Dally, was so "poisonously affected" by the new rays that his hair fell out and his scalp became inflamed and ulcerated. By 1904 Dally had developed severe ulcers on both hands and arms, which soon became cancerous and caused his early death.

During the next few decades, many investigators and physicians developed radiation burns and cancer, and more than 100 of them died as a result of their exposure to X rays. These unfortunate early experiences eventually led to an awareness of radiation hazards for professional workers and stimulated the development of a new branch of science--namely, radiobiology.

Radiations from radioactive materials were not immediately recognized as being related to X rays. In 1906 Henri Becquerel, the French physicist who discovered radioactivity, accidentally burned himself by carrying radioactive materials in his pocket. Noting that, Pierre Curie, the co-discoverer of radium, deliberately produced a similar burn on himself. Beginning about 1925, a number of women employed in applying luminescent paint that contained radium to clock and instrument dials became ill with anaemia and lesions of the jawbones and mouth; some of them subsequently developed bone cancer.

In 1933 Ernest O. Lawrence and his collaborators completed the first full-scale cyclotron at the University of California at Berkeley. This type of particle accelerator was a copious source of neutrons, which had recently been discovered by Sir James Chadwick in England. Lawrence and his associates exposed laboratory rats to fast neutrons produced with the cyclotron and found that such radiation was about two and a half times more effective in killing power for rats than were X rays.

Considerably more knowledge about the biologic effects of neutrons had been acquired by the time the first nuclear reactor was built in 1942 in Chicago. The nuclear reactor, which has become a prime source of energy for the world, produces an enormous amount of neutrons as well as other forms of radiation. The widespread use of nuclear reactors and the development of high-energy particle accelerators, another prolific source of ionising radiation, have given rise to health physics. This field of study deals with the hazards of radiation and protection against such hazards. Moreover, since the advent of space flight in the late 1950s, certain kinds of radiation from space and their effects on human health have attracted much attention. The protons in the Van Allen radiation belts (two doughnut-shaped zones of high-energy particles trapped in the Earth's magnetic field), the protons and heavier ions ejected in solar flares, and similar particles near the top of the atmosphere are particularly important.

By http://www.britannica.com/bcom/eb/article/idxref/2/0,5716,398966,00.htmlbreaking both strands of the DNA molecule, radiation also can break the chromosome fibre and interfere with the normal segregation of duplicate sets of chromosomes to daughter cells at the time of cell division, thereby altering the structure and number of chromosomes in the cell. Chromosomal changes of this kind may cause the affected cell to die when it attempts to divide, or they may alter its properties in various other ways.

Chromosome breaks often heal spontaneously, but a break that fails to heal may cause the loss of an essential part of the gene complement; this loss of genetic material is called gene deletion. A germ cell thus affected may be capable of taking part in the fertilization process, but the resulting zygote may be incapable of full development and may therefore die in an embryonic state.

When adjoining chromosome fibres in the same nucleus are broken, the broken ends may join together in such a way that the sequence of genes on the chromosomes is changed. For example, one of the broken ends of chromosome A may join onto a broken end of chromosome B, and vice versa in a process termed translocation. A germ cell carrying such a chromosome structural change may be capable of producing a zygote that can develop into an adult individual, but the germ cells produced by the resulting individual may include many that lack the normal chromosome complement and so yield zygotes that are incapable of full development; an individual affected in this way is termed semisterile. Because the number of his descendants is correspondingly lower than normal, such chromosome structural changes tend to die out in successive generations.

As would be expected from target theory considerations, X rays and gamma rays given at high doses and high dose rates induce more two-break chromosome aberrations per unit dose than are produced at low doses and low dose rates. With densely ionising radiation, by comparison, the yield of two-break aberrations for a given dose is higher than with sparsely ionising radiation and is proportional to the dose irrespective of the dose rate. From these comparative dose-response relationships, it is inferred that a single X-ray track rarely deposits enough energy at any one point to break two adjoining chromosomes simultaneously, whereas the two-break aberrations that are induced by high-LET irradiation result preponderantly from single particle tracks.

In irradiated human lymphocytes, the frequency of chromosome aberrations varies so predictably with the dose of radiation that it is used as a crude biologic dosimeter of exposure in radiation workers and other exposed persons. What effect, if any, an increase in the frequency of chromosome aberrations may have on the health of an affected individual is uncertain. Only a small percentage of all chromosome aberrations is attributable to natural background radiation; the majority result from other causes, including certain viruses, chemicals, and drugs.

Effects on the incidence of cancer

Atomic-bomb survivors, certain groups of patients exposed to radiation for medical purposes, and some groups of radiation workers have shown dose-dependent increases in the incidence of certain types of cancer. The induced cancers have not appeared until years after exposure, however, and they have shown no distinguishing features by which they can be identified individually as having resulted from radiation, as opposed to some other cause. With few exceptions, moreover, the incidence of cancer has not been increased detectably by doses of less than 0.01 Sv.

Because the carcinogenic effects of radiation have not been documented over a wide enough range of doses and dose rates to define the shape of the dose-incidence curve precisely, the risk of radiation-induced cancer at low levels of exposure can be estimated only by extrapolation from observations at higher dose levels, based on assumptions about the relation between cancer incidence and dose. For most types of cancer, information about the dose-incidence relationship is rather meagre. The most extensive data available are for leukaemia and cancer of the female breast.

Control of radiation risks

In view of the fact that radiation is now assumed to play a role in mutagenic or carcinogenic activity, any procedure involving radiation exposure is considered to entail some degree of risk. At the same time, however, the radiation-induced risks associated with many activities are negligibly small in comparison with other risks commonly encountered in daily life. Nevertheless, such risks are not necessarily acceptable if they can be easily avoided or if no measurable benefit is to be gained from the activities with which they are associated. Consequently, systematic efforts are made to avoid unnecessary exposure to ionising radiation in medicine, science, and industry. Toward this end, limits have been placed on the amounts of radioactivity (Tables 9 and 12) and on the radiation doses (Table 14) that the different tissues of the body are permitted to accumulate in radiation workers or members of the public at large.

Although most activities involving exposure to radiation for medical purposes are highly beneficial, the benefits cannot be assumed to outweigh the risks in situations where radiation is used to screen large segments of the population for the purpose of detecting an occasional person with an asymptomatic disease. Examples of such applications include the "annual" chest X-ray examination and routine mammography. Each use of radiation in medicine (and dentistry) is now evaluated for its merits on a case-by-case basis.

Other activities involving radiation also are assessed with care in order to assure that unnecessary exposure is avoided and that their presumed benefits outweigh their calculated risks. In operating nuclear power plants, for example, much care is taken to minimize the risk to surrounding populations. Because of such precautions, the total impact on health of generating a given amount of electricity from nuclear power is usually estimated to be smaller than that resulting from the use of coal for the same purpose, even after allowances for severe reactor accidents such as the one at Chernobyl.

Sample examination doses (Not from Britannica)

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