Acute Radiation Syndrome
Acute radiation syndrome which is also sometimes known as radiation sickness or radiation toxicity is an extremely serious illness that occurs when the entire body (or most of it) receives a high dose of penetrating radiation, a dose greater than 50 rads, in a very short period of time. Studies in animals and humans exposed to radiation have allowed researchers to describe acute radiation syndrome. The most replicative cells are the most sensitive to the acute effects of radiation, particularly spermatocytes, lymphohematopoeitic elements, and intestinal crypt cells. The inherent sensitivity of these cells results in a constellation of clinical syndromes that predominates within a predictable range of doses of whole-body or significant partial-body exposure. People exposed to radiation will get acute radiation syndrome only if the following conditions are satisfied: the radiation dose was high; the radiation was penetrating; the person’s entire body, or most of it, received the does; and the radiation was received in a short time, usually within minutes. Doses from medical procedures such as x-rays are too low to cause acute radiation syndrome; however, doses from radiation therapy to treat cancer may be high enough cause some symptoms. The radiation must come from an external source such as high energy gamma-rays and penetrate to internal organs. The person’s entire body, or most of it, most be exposed acutely, that is, in a matter of seconds or minutes. Examples of people who suffered from acute radiation syndrome are the survivors of the Hiroshima and Nagasaki atomic bombs (image to the left contains a Japanese girl recovering from the effects of radiation sickness), the firefighters the first responded after the Chernobyl Nuclear Power Plant 1986 event, and some unintentional exposures to sterilization irradiators. The main consequence of an acute radiation exposure is a decrease in life span of the exposed organism. The decrease in life span has a positive correlation with the dose of radiation exposure. Because there are high variations in survival times between different species and between animals within the same species, organism survival times are expressed as the average survival time which takes the many variations into consideration. A value known as the lethal dose is used to describe the relationship of survival of a whole population of the same species exposed to the same dose compared to the percentage of the population that will be killed from that same dose within a given period of time. To illustrate, the lethal dose required to kill fifty percent of a population in thirty days is expressed as LD 50/30. Table 4-1 lists the LD 50/30 doses in rads for several species. For humans, a LD 50/60 is more useful since humans usually survive over thirty days of the radiation exposure. The LD 50/60 for humans is approximately 250-300 rads.
The dose of radiation exposure has an inverse relationship with the survival time. A mammalian dose survival curve can be seen below; one can deduce the fact that as the radiation dose goes up, the survival time and number of survivors decreases.
In referring to the three regions of the mammalian dose survival curve above, there are three different systems that can result in death to the animal. Other organs and systems have suffered damage, but the principal cause of death is destruction to one specific system. The three defined acute radiation syndromes are named according to failure of that organ system which causes death: the bone marrow (hematologic) syndrome, the gastrointestinal syndrome, and the cardiovascular/central nervous system syndrome.
Depending on the absorbed dose, symptoms appear within hours to weeks, following a predictable clinical course. The signs and symptoms that develop in acute radiation syndrome occur through four distinct phases: prodromal phase, latent phase,...
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