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Radiotherapy => Radiation => Radiation Effects
Radiation Effects
INTRODUCTION
Radiation Effects, Biological, effects observed when ionizing radiation strikes living tissue and damages the molecules of cellular matter. Cellular function may be temporarily or permanently impaired from the radiation, or the cell may be destroyed. The severity of the injury depends on the type of radiation, the absorbed dose, the rate at which the dose was absorbed, and the radiosensitivity of the tissues involved. The effects are the same, whether from a radiation source outside the body or from material within.
The biological effects of a large dose of radiation delivered rapidly differ greatly from those of the same dose delivered slowly. The effects of rapid delivery are due to cell death, and they become apparent within hours, days, or weeks. Protracted exposure is better tolerated because some of the damage is repaired while the exposure continues, even if the total dose is relatively high. If the dose is sufficient to cause acute clinical effects, however, repair is less likely and may be slow even if it does occur. Exposure to doses of radiation too low to destroy cells can induce cellular changes that may be detectable clinically only after some years.
ACUTE EFFECTS
High whole-body doses of radiation produce a characteristic pattern of injury. Doses are measured in grays or rads, 1 gray being equal to the dose absorbed when one kilogram of matter absorbs one joule of ionizing radiation, and 100 rads being equal to 1 gray. Doses of more than 40 grays severely damage the human vascular system, causing cerebral edema, which leads to profound shock and neurological disturbances; death occurs within 48 hours. Whole-body doses of 10 to 40 grays cause less severe vascular damage, but they lead to a loss of fluids and electrolytes into the intercellular spaces and the gastrointestinal tract; death occurs within ten days as a result of fluid and electrolyte imbalance, severe bone-marrow damage, and terminal infection. Absorbed doses of 1.5 to 10 grays cause destruction of human bone marrow, leading to infection and hemorrhage; death, if it occurs, can be expected about four to five weeks after exposure. Currently only the effects of these lower doses can be treated effectively; but if untreated, half the persons receiving as little as 3 to 3.25 grays to the bone marrow will die.
Exposure of small areas of the body-the most frequent kind of radiation accident-leads to localized tissue damage. Damage to the blood vessels in exposed areas causes disturbed organ function and, at higher doses, necrosis (localized tissue death) and gangrene.
Injury from internally deposited radiation sources is not likely to cause acute effects but, rather, delayed phenomena, depending on the target organ and on the half-life, radiation characteristics, and biochemical behavior of the radiation source. Consequences may include degeneration or destruction of the irradiated tissue and the initiation of cancer.
LATE EFFECTS
Nonmalignant delayed effects of ionizing radiation are manifested in many organs-particularly bone marrow, kidneys, lungs, and the lens of the eye-by degenerative changes and impaired function; these are largely secondary to radiation-induced damage to blood vessels.
The most important late effect of radiation exposure, however, is an increased incidence of leukemia and other cancers. Statistically significant increases in leukemia and of cancers of the thyroid, the lung, and the female breast have been demonstrated in populations exposed to relatively high doses (greater than 1 gray).
NONIONIZING RADIATION
The radio-frequency radiation, or electromagnetic fields (EMFs), from sources such as power lines, radar, communications networks, cellular phones, and microwave ovens is nonionizing, and for many years only high doses of such radiation were known to be harmful, causing burns, cataracts, temporary sterility, and other effects. In the 1980s and early 1990s, however, with the proliferation of such devices, the possible effects of long-term exposure to low levels of nonionizing radiation began to be a matter of scientific concern and controversy. Subtle biological effects were reported in some studies, while other studies failed to find these effects. In 1996 the National Academy of Sciences reviewed more than 500 scientific papers addressing the health effects of nonionizing radiation and concluded that they do not pose a hazard to human health. Similarly, a major study by the National Cancer Institute, published in 1997, found no evidence that residential levels of non-ionizing radiation increased the risk of childhood leukemia.
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