Threshold radiation dose

Threshold hypothesis A radiation-dose-consequence hypothesis that holds that biological radiation effects will occur only above some minimum dose. 

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). 

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... 

Table 4 Threshold gamma or X radiation doses (lower, central, and upper estimates) for specific deterministic effects ...

An important difference between stochastic and nonstochastic effects is that nonstochastic effects have a threshold stochastic effects do not. The threshold is a minimum radiation dose that has to be received in a relatively short time period for the effect to appear (Fig. 16.5). A dose below the threshold will not... 

Information on the effects of large "whole-body internal" radiation doses comes mainly from laboratory experiments on animals. Figure 18.7 shows the excess tumor frequency for mice irradiated with doses up to 120 Gy in a conq>arison of the effects of different radiation sources A, Sr-induced osteosarcomas in female CBA mice (Nilsson 1975) B, bone tumors in humans from incorporated Ra (Rowland 1971) C, kidney tumors in rats by X-radiation (Maldague 1969) D, skin tumors in rats by electrons (Bums 1968). Figure 18.7 seems to indicate a threshold dose at < 5 Gy below which no effect is observed. 

Figure 18.10 shows a number of hypothetical dose-effect relations The "unmeasurable range" is indicated widiin the circle. The dashed-dotted line outside the circle indicates the uncertainty in the "measurable range". Line a is based on the ICRP recommendations and the message is clear the risk is zero only at zero radiation dose. Curve b indicates a threshold around SO mSv, below which their is no increase in cancer (or other radiation induced diseases) many radiologists support this hypothesis. Curve d assumes that there is a constant risk at the lowest doses. Curve c illustrates the "quadratic-linear" model, which presently seems to be favored by several radiation protection agencies (incl. ICRP), who assume that the slope near zero is one half of the slope at higher doses and dose rates. As this slope is unknown, it could as well be less. 

In the light of previous discussion, one may state that the ICRP rules are "extra-safe . However, it is unlikely that the risk is underestimated, vdiich is an important safety aspect. The ICRP does not recognize a threshold value, below which there is no harm at all. The ICRP notes (see e.g. BEIR V or UNSCEAR 1993) that the straight line a in Figure 18.10 may overestimate the true risks at low radiation doses and low dose rates, but should nevertheless be adhered to for safety reasons. However, it should not be used for prediction of cancer induction in large populations. 

In Russia, radiation safety of the personnel, the population, and the environment is stipulated by appropriate legislation that specifies the basic radiation dose threshold values for personnel and population exposure as well as the conditions under which radiation energy is to be utilized. 

Edematous and inflammatory reactions of the vagina that persist for up to about 6 months after radiotherapy are associated with an increased signal intensity of the vaginal wall on T2-weighted images. The observed increase in signal intensity is directly proportion to the total radiation dose administered and always occurs above a threshold dose of 45 Gy. The mode of admin-... 

As with chemical toxicity, there is no question that high doses of radiation cause biological damage. However, as discussed in Section 5.3.8, there are two models for the effects of radiation at low doses and we don t know with certainty if there is a low dose (or threshold) below which no damage is done. Therefore, it becomes difficult to set acceptable radiation doses, particularly in the context of a natural background of radiation to which we are all necessarily exposed. In medicine, we have accepted the notion that the use of radioisotopes in the body and the use of X rays are warranted when the overall known benefits outweigh probable or certain negative side effects. 

It is well recognized that the probability to permanently control tumors increases as a sigmoid function with increasing radiation dose. Below a threshold, the dose is not sufficient to inactivate all cancer stem cells in a tumor, i.e. all tumors recur. After this threshold, tumor control increases with increasing radiation dose, approaching 100% at high doses. 

A cancer stem cell is defined as a cell within a tumor that possesses the capacity to self renew and to generate the heterogeneous lineages of cancer cells that comprise the tumor (Clarke et al. 2006). In the context of radiotherapy a cancer stem cell is defined as a cell that, if not killed by radiation, forms a tumor recurrence (Baumann et al. 2008, in press). Curative radiotherapy therefore aims at inactivation of all cancer stem cells in the primary tumor and locoregional lymph nodes. It is well recognized that the probabiUty to permanently control tumors (tumor control probability, TCP) increases as a sigmoid function with increasing radiation dose. Below a threshold, the dose is never sufficient to inactivate aU cancer stem cells in a tumor, i.e. all tumors recur. After... 

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. 

It is assumed that within the range of exposure conditions usually encountered in radiation work, the risks of cancer and hereditary damage increase in direct proportion to the radiation dose. It is also assumed that there is no exposure level that is entirely without risk. Thus, for example, the mortality risk factor for all cancers from uniform radiation of the whole body is now estimated to be 1 in 25 per sievert (see below for definition) for a working population, aged 20 to 64 years, averaged over both sexes. In scientific notation, this is given as 4 X 10 per sievert. Effects of radiation, primarily cancer induction, for which there is probably no threshold and the risk is proportional to dose are known as stochastic, meaning of a random or statistical nature. ... 

Within the range of exposure conditions usually encountered in radiation work, there is a linear relationship, without threshold, between dose and probability of stochastic health effects (such as latent cancer and genetic effects) occurring ... 

The first assumption, the linear nonthreshold dose-effect relationship, implies that the potential health risk is proportional to the dose received and that there is an incremental health risk associated with even very small doses, even radiation doses much smaller than doses received from naturally occurring radiation sources. These health risks, such as cancer, are termed stochastic because they are statistical in nature i.e., for a given level of dose, not every person exposed would exhibit the effect. The second assumption means that when a stochastic effect is induced, the severity of the effect is not related to the radiation dose received. The third assumption implies that there are effects, termed nonstochastic effects, for which there is an apparent threshold i.e., a dose level below which the effect is unlikely to occur. An example of a nonstochastic effect is the formation of radiation-induced cataracts of the eyes. 

In setting radiation limits, it is customary to assume that, for exposure of the whole body to radiation, the effect produced is proportional to the absorbed dose. Since this assumption ignores the possibility of damaged tissue repairing itself during long-term exposure to low radiation doses, it is believed to represent a conservative approach to the estimation of possible damage. Somatic effects where the probability of injury (e.g., the initiation of cancer) is proportional to the dose are known as stochastic effects. In contrast to these, an effect such as cataract of the eye is nonstochastic in this case, a threshold dose exists such that no cataract will be induced if the absorbed dose is below this amount. 

Radiation doses of under 80 to 100 rad will probably have no noticeable overt effect. Some changes in the blood count will probably occur, but this is the short-term extent of exposures. At about 100 rad, the threshold for... 

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