Sunday, December 27, 2009

Radiation health effects : Stochastic effects

2- Stochastic effects

If the dose is lower, or is delivered over a longer period of time, there is a greater opportunity for the body cells to repair, and there may be no early signs of injury. Even so, tissues may still have been damaged in such a way that the effects appear only later in life (perhaps decades later), or even in the descendants of the irradiated person.

These types of effect are called stochastic effects: they are not certain to occur, but the likelihood that they will occur increases as the dose increases, whereas the timing and severity of any effect does not depend on the dose. Because radiation is not the only known cause of most of these effects, it is normally impossible to determine clinically whether an individual case is the result of radiation exposure or not.

The most important of these stochastic effects is cancer, which is always serious and often fatal. Although the exact cause of most cancers remains unknown or poorly understood, exposure to agents such as tobacco smoke, asbestos and ultraviolet radiation, as well as ionizing radiation, are known to play a role in inducing certain types of cancer. The development of cancer is a complex, multistage process that usually takes many years. Radiation appears to act principally at the initiation stage, by introducing certain mutations in the DNA of normal cells in tissues. These mutations allow a cell to enter a pathway of abnormal growth that can sometimes lead to the development of a malignancy.

Given that we cannot distinguish between those cancer cases resulting from radiation exposure and those with other causes, how can we calculate the risk of cancer from radiation? In practice, we have to use epidemiology - the statistical study of the incidence (the number of cases and their distribution) of specific disorders in specific population groups. Suppose that we know the number of people in an irradiated group and the doses they have received. Then by observing the occurrence of cancer in the group and comparing with the doses and the number of cancers expected in an otherwise similar but unirradiated group, we can estimate the raised risk of cancer per unit dose. This is commonly called a risk factor. It is most important to include data for large groups of people in these calculations so as to minimize the statistical uncertainties in the estimates and take account of factors, such as age and gender, that affect the spontaneous development of cancer.

Not all cancers are fatal. Average mortality from radiation-induced thyroid cancer is about 10 per cent (although it is much lower -less than 1 per cent - for the cases caused in children and teenagers by the Chernobyl accident), from breast cancer about 50 per cent, and from skin cancer about 1 per cent. Overall, the total risk of inducing cancer by uniformly irradiating the whole body is about half as great again as the risk of inducing a fatal cancer. In radiological protection the risk of fatal cancer is of more concern because of its extreme significance. The use of fatal cancer risks also makes it easier to compare them with the other fatal risks encountered in life. In contrast, comparisons of non-fatal risks are fraught with difficulty.

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