Effects of Radiation to Human

Biologic Effect of Radiation

As a radiographer, we all know that xray can ionize substance by removing electrons from their orbits. This process results in a free, negatively charged electron and leaves the remainder of the atom with positive charge. When human beings are irradiated, ionization may occur to any part of a living cell, such as the material that makes up it membrane, the water within the membrane, or the DNA that makes up the cell’s chromosomes and directs its activity. The initial ionization may produce a “domino effect”, causing ionization in the surrounding area. Exposure also creates free radicals (temporary molecules and parts of molecules with electrical charges). Free radicals may interact directly with the DNA or may produce toxic substance that are injurious to DNA.

Most effects of exposure are extremely short-lived because electrons find new homes in the orbits of other atoms and the balance of charges returns to normal. Free radicals combine to form more stable compounds. Occasionally, however, the damage is not instantly resolved. A cell may be so damaged that is cannot sustain itself and dies. Cell death is an insignificant injury unless a large number of cells is involved. Cells may sustain damage that requires several days for the body to make repairs. The body produces special enzymes that function to repair the DNA protein molecules. A cell may be damaged in such a way that is DNA “programming” is changed, and the cell no longer behaves normally. This type of injury may eventually result in the runaway production of new, abnormal cells, causing a tumor or malignant blood disease.

Law of Bergonie and Tribondeau

The relative sensitivy of different types of cells is summarized in the Law of Bergonie and Tribondeau, which state that cell sensitivity to radiation exposure depends on four characteristic of the cell:

• Age. Younger cells are more sensitive than older ones.
• Differentiation. Non-specialized cell are more sensitive than highly complex ones.
• Metabolic rate. Cells that use energy rapidly are more sensitive than those with a slower metabolism.
• Mitotic rate. Cells that divide and multiply rapidly are more sensitive than those that replicate slowly.

According to this laws, we see that blood cells and blood-producing cells are very sensitive. Cells in contact with the environment are quite simple, have relatively short lives, and are quite sensitive. These include the cells of the skin and the mucosal lining of the mouth, nose, and gastrointestinal tract. Some glandular tissue is also particularly sensitive, especially that of the thyroid gland and the female breast. The tissue of embryos, fetuses, infants, children, and adolescents tend to be more sensitive that adult tissue because of their younger age and their higher metabolic and mitotic rates. Nerve cells, which have a long life and are quite complex, are much less vulnerable to radiation injury. Cortical bone cells are relatively insensitive.

Classification of Radiation Effects

Radiation effects are classified in various ways. Short-term effects are those observed within 3 months of exposure. They are associated with relatively high radiation doses ( greater than 50 rad). Short term effects may be further categorized according to the body system affected:

• Central nervous System (CNS)
• Gatrointestinal (GI)
• Hematologic effects

Long-term effect, sometimes referred to as latend effects, may not be apparent for as many as 30 years. Somatic effects are those that effect the body of the irradiated individual directly, whereas genetic effects occur as a result of damage to the reproductive cells of the irradiated person and are observed as defects on the children or grandchildren of the irradiated individual.

Short-Term Somatic Effects

Short-term radiation effects are predictable, and the quantity of exposure required to produce them is well documented. These are termed nonstochastic effects. Nonstochastic effects occur only after a certain amount of exposure has been received, and the severity of the effect depends upon the dose. One observable short-term effect reddening of the skin called erythema. This phenomenon is sometimes called a "radiation burn." In the very early days of radiation use, the amount of radiation necessary to produce reddening of the skin was called the "erythema dose." It was the first unit used to measure radiation exposure.

Other short-term effects from doses in excess of 50 rad have been observed and studied in radiation therapy patients and in the victims of radiation accidents and atomic bomb blasts. This is vastly more exposure than is delivered by diagnostic x-ray machines. Extremely high doses produce CNS effects, seizures, and coma that can result in death in a short period of time. Lesser doses will result in "radiation sickness," a Gastro Intestinal effect in which the mucosal lining of the digestive tract is damaged, breaks down, and becomes infected by the bacteria that normally inhabit the bowel.

These victims also have a compromised immune system because of the death of white blood cells and are unable to fight the infection. Radiation sickness is usually fatal, and suffering may be prolonged. A lesser dose, affecting primarily the blood and blood-forming organs, results in hematologic effects, including anemia and compromise of the immune system. These victims are prone to infectious diseases that may or may not be fatal, depending on the radiation dose and the severity of the disease process. One way that scientists describe the risk of high-level radiation exposures is to calculate the whole-body radiation dose that is lethal to 50% of the irradiated population within 30 days, a calculation that is abbreviated as LD 50/30. The LD 50/30 for humans is approximately 300 rad (3 Gy). 

Long-Term Somatic Effects

“Long-term" here refers to the length of time between exposure and observation of the effect. The time required for long-term effects to manifest is generally considered to be 5 to 30 years, with the greatest percentage occurring between 10 and 15 years. In contrast to the predictable nature of short-term effects, longterm effects are apparently random, and there is no threshold amount of exposure that must be received in order for them to occur. These effects are termed stochastic. The likelihood of stochastic effects is greater when the dose is increased, but there is no correlation between the dose and the severity of the effects. They may occur as the result of repeated small doses, such as those used in radiography.

The percentage of observable effects from the radiation involved in typical x-ray examinations is extremely low and the risk to any single patient is minimal. Most of us take greater risks when we drive a car or cross a busy street. Nevertheless, there is a risk of long-term effects that has been demonstrated by studying large populations over long periods. The incidence of certain conditions is greater when results for irradiated groups are compared to those of nonirradiated control groups.

Long-term radiation effects are not easily identified as such because they occur years after the initial exposure and because these same effects also occur in the absence of radiation exposure. Only extensive research with large populations (epidemiologic studies) and computer analysis can demonstrate the role of radiation in causing these effects. In other words, radiation causes increased risk of these effects, but the effects cannot be predicted with respect to any one individual. While the individual risk may be extremely small, increasing exposure to the entire population poses public health risks that require the attention and concern of everyone involved in applying ionizing radiation to human beings.

The documented latent effects of low doses of ionizing radiation include the following:

• Cataractogenesis. The formation of cataracts, or clouding of the lens of the eye. This effect concerns radiologists and radiographers who work extensively in fluoroscopy and those who perform other work that involves repeated exposure to the eyes.
• Carcinogenesis. Increased risk of malignant disease; particularly cancer of the skin, thyroid, and breast; and leukemia, a malignant blood disease associated with radiation exposure.
• Life span shortening. A study of the life span of radiologists who died during a 3-year period before 1945 showed that they had shorter life spans than physicians who did not use radiation in their practices. This group included radiologists who had used radiation since the early days of x-ray science. More recent studies show that occupational exposure no longer has a measurable effect on the life span of radiologists. Nevertheless, because radiation exposure has been linked to life span shortening, it is a public health concern and another reason to practice a high level of radiation safety.
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Genetic Effects

Genetic effects in the form of changes or mutations to the genes may be caused when the ovaries or testes are exposed to ionizing radiation. In the female, all of the ova cells that an individual will ever produce are present in an immature state at birth. Because no new egg cells are produced as the individual ages, the effect of radiation exposure to the ovaries is cumulative. The genetic effects of radiation to the testes also have a longer- term effect than may at first be presumed, because damage to the stem cells that produce the sperm may result in continued production of sperm with the genetic mutation. The majority of genetic mutations are considered negative, or less well suited to survival of the individual than nonmutated cells.

Because reproductive cells have only half the number of chromosomes found in all other cells, each parent contributes one chromosome to each pair in the new individual, and nature makes the choice as to which gene of each pair will affect the characteristics of the offspring. Those genes that are expressed are said to be dominant, and those that are not expressed are called recessive. Mutated genes are usually recessive and therefore do not manifest their characteristics in the offspring. Both dominant and recessive genes, however, occur in the reproductive cells of the offspring and may be passed on to future generations.

As an increasing percentage of the population is exposed to radiation from natural, occupational, and

health care sources, the likelihood increases that individuals will be conceived with a mutation of both genes in a strategic pair, resulting in some type of deformity or maladaption. Public health officials and governments are very concerned about preserving the integrity of the population's gene pool by minimizing harmful, defect-causing radiation. This concern should motivate those who apply ionizing radiation to humans to minimize gonad doses in every way possible. Gonadal shielding would be the best option to minimize radiation exposure to the reproductive system.

Genetic effects from mutations caused by x-ray exposure have long been demonstrated in animal research. Interestingly, very little genetic effect has been confirmed by the continuing research of the Japanese populations affected by the atomic bombs dropped on Hiroshima and Nagasaki during World War I1 or in other studies of human populations.