injuries and poisoning
Radiation Injury
Effects of Radiation
The damaging effects of radiation (that is, the severity of the tissue reaction) depend on several factors: The amount (dose) How rapidly the dose is received How much of the body is exposed The sensitivity of particular tissues to radiation The presence of genetic abnormalities that impair normal DNA repair The person's age at time of exposure The person's general state of health prior to the exposure A single, rapid dose of radiation to the entire body can be fatal, but the same total dose given over a period of weeks or months may have much less effect. The effects of radiation also depend on how much of the body is exposed. For example, more than 6 Gy can be fatal when the radiation dose is to the entire body. However, when limited to a small area and spread out over a period of weeks or months, as in radiation therapy for cancer, 10 or more times this amount can be given without serious harm. Some parts of the body are more sensitive to radiation. Organs and tissues in which cells multiply quickly, such as the intestine and bone marrow, are harmed more easily by radiation than those in which cells multiply more slowly, such as muscles and brain cells. The thyroid gland is susceptible to cancer after being exposed to radioactive iodine because radioactive iodine concentrates in the thyroid gland. Other factors can increase the sensitivity to radiation injury. People who have two Ataxia-Telangiectasia genes (one from each parent) are much more sensitive to radiation injury. Disorders, such as In children, some organs and tissues such as the brain, lens of the eye, and thyroid gland, are more sensitive to radiation than in adults. However, some tissues in children are no more sensitive to radiation than in adults, and some, such as the ovaries, are actually less sensitive. The reasons for the differences are complicated and not completely understood, but doctors think that the higher sensitivity of some tissues in children is due, at least in part, to the fact that children's cells grow and mature more quickly and will undergo many more cell divisions than those in adults. The fetus is sensitive to damage from radiation because fetal cells are dividing very quickly and also differentiating from immature into mature cells. In the fetus, exposure in excess of 300 mGy during 8 to 25 weeks after conception may cause reduced intelligence and poor school performance. Birth defects may occur due to exposure in the womb to high doses of radiation. However, at doses less than 100 mGy, particularly at the low doses used in imaging tests that a pregnant woman might typically undergo, there is no apparent increase beyond the normal risk of having a child born with a birth defect. Radiation is not as potent a cause of cancer or birth defects as people might think. A large radiation exposure increases the risk of cancer because of damage to the genetic material (DNA) in cells that survive the radiation. However, radiation is a weaker cause of cancer than people might think. Even a whole-body dose of 500 mGy (over 150 times more than the average annual background radiation dose) increases a typical person’s lifetime risk of dying from cancer from 22% to about 24.5%, only a 2.5% absolute risk increase. In a fetus or child, the risk of radiation-induced cancer is several times higher than in an adult. Children may be more susceptible because their cells divide more often and because they have a longer lifespan during which cancer may develop. The lifetime risk of dying as a result of cancer for a 1-year-old who has a computed tomographic (CT) scan of the abdomen may be increased by about 0.1%. Recently, concerns about the possible risks due to CT scans have led to controversy about whether CT scans are being used too often. Because of these concerns, CT scan techniques are being optimized to reduce the radiation dose. Doctors also try to do a CT scan only when it is more accurate than other tests that use less or no radiation. When CT is clearly the most accurate test, the risk resulting from not making the correct diagnosis because a less accurate test was used is much greater than the risk of the CT scan. In animals, high doses of radiation of the ovaries or testes have been shown to lead to defective offspring (hereditary effects). However, no increase in the percentage of birth defects was observed in the children of survivors of the nuclear bomb explosions in Japan. It may be that the radiation exposure was not high enough to cause a measurable increase. No increase in the risk of birth defects is found in children conceived after their parents received radiation therapy for cancer where the average dose to the ovaries was about 0.5 Gy and to the testes was about 1.2 Gy (typical exposure to tissues next to but not directly in the treatment area during radiation therapy). 