Tissue weighting factors

Table 3.4—Tissue weighting factors used in calculating effective dose. ...

This option does not appear to be advantageous for either radionuclides or chemicals that cause stochastic responses. In radiation protection, total detriment is used mainly to develop the tissue weighting factors in the effective dose (see Section 3.2.2.3.3), but ICRP and NCRP have continued to emphasize fatal responses as the primary health effect of concern in radiation protection and radiation risk assessments. Since total detriment is based on an assumption that fatalities are the primary health effect of concern, the same difficulties described in the previous section would occur if this measure of response were used for chemicals that induce stochastic responses. Other disadvantages of using total detriment include that detriment is not a health-effect endpoint experienced by an exposed individual and the approach to weighting nonfatal responses in relation to fatalities is somewhat arbitrary. Furthermore, total detriment is not as simple and straightforward to understand as either incidence or fatalities. 

Tissue weighting factors are used with the tissue dose equivalent Hr to calculate the effective dose equivalent HE ... 

It is usefiil to calculate doses using the tissue weighting factor in protracted exposures (exposures over significant periods of time). Most nonexperimental companion or domestic animals do not receive protracted exposures. A major exception to this is the animal receiving repeated fractionated doses as radiation therapy, or the relatively rare animals in which radiation has been implanted so that it acts as an internal emitter of radioactivity (Harley, 2001). 

In order to take into account the radiation sensitivity of different tissues, tissue weighting factors wt are introduced, and the effective equivalent dose received by the tissue E is defined by... 

A summation of the tissue equivalent doses, each multiplied by the appropriate tissue-weighting factor ... 

A factor by which the equivalent dose to an organ or tissue is multiplied in order to account for the different sensitivities of different organs and tissues to the induction of stochastic effects of radiation. The tissue weighting factors used for radiation protection purposes are shown in Table 6.2. 

The tissue weighting factor for radiation protection purposes... 

Table 2 Radiation and tissue weighting factors used in radiation protection guidelines ...

W, is the radiation weighting factor, and l/K is the tissue weighting factor. 

Effective dose equivalent (He) is the sum of the products of the tissue weighting factors (Wr) applicable to each of the body organs or tissues that are irradiated and the committed dose equivalent to the corresponding organ... 

Table 9.2. Tissue weighting factors, Wj, of different tissues...

Tissue weighting factor (Wt)- A factor related to the radiosensitivity of different tissues in living systems. 

Organ or tissue" Weighting factor Dose to human organ or tissue Radiation absorbed dose Weighted (mRad or mrem) dose (mrem)... 

Organ or Tissue Weighting Factor W Risk Coefficient (per rem) Probability (per rem)... 

It is necessary to take into account the different sensitivities of various tissues and organs to the induction of deleterious health effects to the whole organism. The effective dose (E) is defined as a summation of the tissue equivalent doses multiplied by the appropriate tissue weighting factor W[. According to this definition. 

Mainly due to the additional experimental data, the concept of tissue weighting factors is to be extended. Based on new data, the improved values of the factors are issued regularly. In general, the Wr-values recommended by the ICRP were based on the risk of fatal cancer and of serious heritable disease. The last concept and the values of Wr are based on the incidence of radiation-induced cancer, rather than on mortality, as well as on the risk of heritable disease over the first two generations. 

According to the definition of the tissue weighting factor, the summation of the factor over all the tissues results in the value 1, that is... 

In addition, the risk of harm is not the same for various tissues in the body. For example, it is lower for the bone surfaces than for the breast. This can be dealt with by taking the equivalent dose in each of the major organs and tissues of the body and weighting it by a factor related to the risk associated with that organ or tissue (tissue-weighting factor). The sum of the weighted equivalent doses is called the effective dose. Thus, the effective dose broadly represents the risk to health from any exposure to ionizing radiation. 

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