Deterministic radiation exposure

Given the models for estimating external or internal radiation doses in specific organs or tissues, the following sections consider the responses resulting from a given dose by any route of exposure. As is the case with hazardous chemicals, both stochastic and deterministic radiation effects can occur. 

Two kinds of radiation exposure are distinguished, stochastic exposure (the effects are distributed statistically over a large population), and deterministic exposure (the effects are inevitable or intended, as in the case of deliberate irradiation, e.g. in radiotherapy). 

Exposure to radiation can cause detrimental health effects. At large doses, radiation effects such as nausea, reddening of the skin or, in severe cases, more acute syndromes are clinically expressed in exposed individuals within a short period of time after the exposure such effects are called deterministic because they are certain to occur if the dose exceeds a threshold level. Radiation exposure can also induce effects such as malignancies, which are expressed after a latency period and may be epidemiologically detectable in a population this induction is assumed to take place over the entire range of doses without a threshold level. Hereditary effects due to radiation exposure have been statistically detected in other mammalian populations and are presumed to occur in human populations also. These epidemiologically detectable effects—malignancies and hereditary effects—are termed stochastic effects because of their random nature. 

Deterministic effects due to radiation exposure are the result of different processes, mainly cell killing and delayed cell division, which can, if extensive enough, impair the function of the exposed tissue. The severity of a particular deterministic effect in an exposed individual increases with the dose above the threshold for the occurrence of the effect. 

The stochastic effects of radiation exposure (e.g. an increased risk of cancer) are not quantified in the derivation of the D values. However, given that the risk of stochastic effects increases with exposure, higher category sources will, in general, present a higher risk of stochastic effects. Furthermore, the deterministic effects resulting from an accident or malicious act are likely in the short term to overshadow any increased risk of stochastic effects. The... 

Based on these differences, the use of RfDs for hazardous chemicals that induce deterministic effects to define acceptable exposures of the public often may be considerably more conservative (provide a substantially larger margin of safety) than the dose limits for radiation induced deterministic effects. The likely degree of conservatism embodied in RfDs has important implications for establishing limits on allowable exposures to substances causing deterministic effects for the purpose of developing a risk-based waste classification system. Dose limits for deterministic effects for radiation should not be important in classifying waste (see Section 3.2.2.1). 

In contrast, risk management for substances that cause deterministic effects must consider unavoidable exposures to the background of naturally occurring substances that cause such effects. Based on the assumption of a threshold dose-response relationship, the risk from man-made sources is not independent of the risk from undisturbed natural sources, and the total dose from all sources must be considered in evaluating deterministic risks. In the case of ionizing radiation, thresholds for deterministic responses are well above average doses from natural background radiation (see Section 3.2.2.1)... 

In setting limits on exposure intended to prevent the occurrence of deterministic responses, the safety and uncertainty factors that are applied to the assumed thresholds for hazardous chemicals that cause deterministic effects usually are considerably larger (by at least a factor of 10) than the safety factor normally applied to the thresholds for deterministic responses from exposure to radiation. Furthermore, the assumed threshold usually is more conservative for hazardous chemicals than for radiation (i.e., a lower confidence limit of the threshold often is used for... 

For the purpose of developing a risk-based hazardous waste classification system, prevention of deterministic responses should be of concern only for hazardous chemicals, but not for radionuclides. Deterministic responses from exposure to radionuclides can be ignored because radiation dose limits for the public intended to limit the occurrence of stochastic responses are sufficiently low that the doses in any organ or tissue would be well below the thresholds for deterministic responses (see Section 3.2.2.1). 

If a person is exposed to a large amount of radiation (i.e., large radiation dose) delivered to the entire body, cells in tissues can be destroyed in large numbers. Because tissues have important functions, the destruction of significant numbers of cells can lead to impairment in one or more of these functions. The biological effects that arise when large numbers of cells are destroyed by radiation are called acute somatic effects if they occur in a relatively short period of time (e.g., within a few weeks) after brief exposure. Acute somatic effects are a subset of what is now formally called early and continuing deterministic effects (once called nonstochastic effects). 

Deterministic effects are those that increase in severity as the radiation dose increases and for which a threshold is presumed to exist. Besides acute somatic effects, deterministic effects also include radiation effects (other than cancer and genetic effects) that continue to occur after an extended period (e.g., years) of chronic exposure. Such chronic exposures can arise from long-lived radionuclides (e.g., isotopes of plutonium and cesium) ingested via contaminated food or inhaled via contaminated air... 

Radiation injury causes two types of effects on biologic symptoms, stochastic and deterministic. Stochastic effects are aU or nothing effects. At increasing doses, the probability of a stochastic effect increases, but once the stochastic effect occurs, further inaeases in exposure will not worsen the severity of the effect. A conunon stochastic effect is radiation-associated malignancy. In comparison, the severity of deterministic effects is proportional to the dose. Examples of deterministic effects include suppression of hematopoiesis, cataract formation and fertility impairment (4). 

Examples of nonstochastic (or deterministic) effects are erythema, nausea, loss of hair, cataracts, sterility, etc. Stochastic (or probabilistic) effects are cancer and genetic defects (birth defects) (Table 16.7). Genetic effects are abnormalities that may appear in the offspring of persons exposed to radiation, one or many generations after the exposure. 

A dangerous source is defined as a source that could, if not under control, give rise to exposure sufficient to cause severe deterministic effects. A deterministic effect is defined as a health effect of radiation for which generally a threshold level of dose exists above which the severity of the effect is greater for a higher dose. Such an effect is described as a severe deterministic effect if it is fatal or life threatening or results in a permanent injury that reduces the quality of life. 

The deterministic effects occur when above a certain threshold , an appropriately high dose (above 500-1,000 mSv) is absorbed in the tissues and organs to cause the death of a large number of cells and consequently to impair tissue or organ functions early after exposure. The severity of injury depending on the absorbed dose according to an s-shaped dose-response curve might be manifested in the various syndromes of radiation illness, that is, the bone... 

In accidents, there is a possibility for deterministic (non-stochastic) and stochastic health effects. The principles for planning intervention in the case of a radiation emergency are set out in ICRP Publication 60 and 63 (ICRP 1991a, b, respectively). Intervention is the term applied to steps taken, in an accident situation, to restrict the exposure of member of the public and to minimize the consequences of unavoidable exposure. The assessment of the radiological situation and the implementation of the individual protection measures make up an overall protection strategy that is recommended to be justified and optimized. In the process of optimization, reference levels are to be applied and particular attention should be given to the prevention of severe deterministic health effects. 

Sometimes irradiation will not kill the affected cells, but may only alter them. A viable but modified somatic cell may still retain its reproductive capacity and may give rise to a clone. If the clone is not eliminated by the body s defence mechanisms, after a prolonged and variable period of delay termed the latent period, it may result in the development of malignant conditions, usually termed cancers, which are the principal late somatic effects of exposure to radiation. In contrast to deterministic effects, it is assumed that there is no threshold of dose below which stochastic effects (e.g., cancer) cannot occur. These effects do not occur in every exposed individual the probability that an individual or one of his or her descendants may develop one of these effects increases with the dose received. Thus, even if the dose is very small, the person still has a chance, albeit a very small one, of incurring such an effect. 

The plume will travel downwind and the concentration of radioactive materials will tend to decrease as it travels further from the plant. As the concentration of radioactive materials in the plume decreases, the dose rate to the affected population will also decrease. Thus, those who are further away from the plant will generally be at less risk of deterministic (early) health effects. While the exposures further from the plant are small, they all add to the chance of getting cancer (stochastic effects). Since the total amount of human exposure is larger further from the plant (large number of people exposed to small amounts of radiation), this is where most cancers will occur. Following the Chernobyl release the vast majority of the excess thyroid cancers caused by the accident occurred between 50 and 350 km from the plant. 

This dose can be exceeded if justified but every effort shall be made to keep dose below this level and certainly below the thresholds for deterministic effects. The workers should be trained in radiation protection and understand the risk they face. They must be volunteers and be instmcted on the potential consequences of exposure. 

Accidents resulting in deterministic health effects will be very rare, and usually this will occur among employees or other professionals. However, in the case of a lost or stolen source, limited number of the general public may receive doses that can lead to deterministic health effects. Such a situation requires special medical care and supportive treatment for the early effects of acute radiation. In the event of internal exposure, especially by long-lived ra onuclides, decorporation might be considered, even if the dose is below the threshold for deterministic health effects. The decision about decorporation levels should be based on committed equivalent dose to the organs and the effective committed dose. 

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