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Wednesday, October 04, 2006

Biological effect of radiation
effect on living tissue of ionizing radiation. These effects can be due to natural radiation sources, medical exposure, occupational hazards or exposure to thermonuclear weapons. One must distinguish between acute and chronic effects of radiation. While the former are essentially due to exposure to thermonuclear blasts or catastrophic events such as the Chernobyl accident, the latter are the long-term results of acute or chronic exposures to radiation inculding radiation therapy.
Acute radiation syndrome
can occur following total body exposure exceeding 1 gray Gy and a dose of about 2.5 Gy will lead to the death of 50% of exposed persons within 60 days. Above these thresholds the effects observed vary according to dose, severity and duration of the exposure and are related to the radiation sensitivity of various tissue types. The most radiation-sensitive cells are: haematopoietic stem cells, oogonia, spermatogonia, intestinal crypt cells, epidermal cells and lymphocytes; and relatively sensitive cells are endothelial cells, precursors of the haematopoietic series, spermatocytes and oocytes. Other cells have a medium to low sensitivity to radiation. The syndromes observed are the haematopoietic, the gastrointestinal and neurological acute radiation syndromes.
The haematopoietic syndrome results from damage to the stem cells in the bone marrow and lymphatic organs and usually occurs above doses of 2 Gy. Death occurs within 3 weeks as a result of infections and haemorrhage due to granulo- and thrombocytopenia.
The gastrointestinal syndrome occurs when doses exceed 7 Gy because loss of intestinal stem cells leads to a breakdown of the intestinal/vascular barrier resulting in sepsis and death within 3 days.
The central nervous syndrome is characterized by immediate onset of severe neurological changes with convulsions and death within 2 days following massive exposures of over 10 Gy.
Chronic effects of radiation
are due to natural and artificial environmental exposures including medical applications. The effects induce tumours and genetic defects. The radiation-induced tumours observed are leukaemia, lung, breast, thyroid, stomach, liver, bone, bladder and other cancers. The latency period is over 10 years for tumours other than leukaemia, and large cumulative doses are needed to produce a significant increase in tumour incidence. Genetic effects were originally thought to be a major problem, but data acquired in the last 40 years demonstrate that genetic repair mechanisms are very effective in mammals. Thus genetic effects of radiation exposure appear to be small, and it is estimated that a doubling in mutations requires an acute exposure of 2 Gy and a chronic exposure of 4 Gy.
Exposure of fetuses
to radiation has very differing effects depending on the gestational age. Before implantation the fetus is highly susceptible and substantial radiation exposure leads to early spontaneous abortion. Irradiation during the first trimester, in weeks 2 to 10, is dangerous as this is the time of organogenesis and multiple malformations can result. It should be noted, however, that medical personnel who are occupationally exposed to ionizing radiation, need not fear the low levels of irradiation, and no additional safety measures are warranted except that once pregnancy has been confirmed they should be removed from a duty with recurrent radiation exposure. GvS

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