Radiation dose equivalent

RBE is used to denote the experimentally determined ratio of the absorbed dose from one radiation type to the absorbed dose of a reference radiation required to produce an identical biologic effect under the same conditions. Gamma rays from cobalt-60 and 200-250 keV x-rays have been used as reference standards. The term RBE has been widely used in experimental radiobiology, and the term quality factor used in calculations of dose equivalents for radiation safety purposes (ICRP 1977 NCRP 1971 UNSCEAR 1982). RBE applies only to a specific biological end point, in a specific exposure, under specific conditions to a specific species. There are no generally accepted values of RBE. 

Contemporary radiation protection systems (ICRP, 1977a 1991 NCRP, 1987 1993) include dose limits expressed in such a quantity. To obtain the quantity, absorbed doses are first multiplied by a quality factor (ICRP, 1977a) or a radiation weighting factor (ICRP, 1991), selected for the type and energy of the radiation incident upon the body, yielding, respectively, the dose equivalent in the tissue (ICRP, 1977a) or equivalent dose in the tissue (ICRP, 1991). Therefore ... 

For low-LET radiation, the quality factor and radiation weighting factor have the value of one. Therefore, dose equivalent and equivalent dose have the same numerical value. 

Notice that the dose has a strict definition of energy per unit mass of the absorber and, in principle, can be measured for a given radiation at a certain energy in a specific material. The equivalent dose is a relative unit in that a radiation weighting factor is applied to a measured quantity. The dose can be measured from ionization in an electronic radiation detector the equivalent dose must take into account the type of radiation causing the ionization. 

For routine exposures of the public, ICRP recommends a total detriment per unit equivalent dose from uniform whole-body irradiation of 7.3 X 10 2 Sv 1, as shown in Table 3.2. Of this, the recommended probability coefficient for fatal cancers is 5.0 X 10 2 Sv-1, or about two-thirds of the total detriment, and the contributions from severe hereditary responses and weighted nonfatal cancers are 1.3 X 10 2 Sv-1 and 1.0 X 10 2 Sv, respectively. These probability coefficients are summarized in Table 3.3, and their use in radiation protection is discussed in the following section. As noted previously, the probability coefficient for weighted nonfatal cancers is not the same as the probability coefficient for incidence of nonfatal cancers. The probability coefficient for fatal cancers also gives the probability of a fatal cancer per unit effective dose. The effective dose was developed to describe nonuniform irradiations of the body and is discussed below. 

Radiation Dose Limits. For routine exposure of individual members of the public to all man-made sources of radiation combined (i.e., excluding exposures due to natural background, indoor radon, and deliberate medical practices), NCRP currently recommends that the annual effective dose should not exceed 1 mSv for continuous or frequent exposure or 5 mSv for infrequent exposure. The quantity effective dose is a weighted sum of equivalent doses to specified organs and tissues (ICRP, 1991), which is intended to be proportional to the probability of a stochastic response for any uniform or nonuniform irradiations of the body (see Section 3.2.2.3.3). 

Note that the radiation weighting factor for alpha particles is 20 times that of gamma rays and f3 particles. With the above information, we can calculate the tissue equivalent dose Hr using ... 

The units used in radiation dosimetry are summarized in Table 22.1. The energy dose and the ion dose are also used in radiation chemistiy, whereas the equivalent dose is only applied in radiation biology and in the field of radiation protection. 

The advantages of the equivalent dose are that the biological effectiveness is directly taken into account and that equivalent doses received from different radiation sources can be added. However, the dimension J/kg is only correct if wr = 1. 

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... 

Average equivalent dose rates received from natural radiation sources are listed in Table 22.8. The values vary appreciably with the environmental conditions. The influence of cosmic radiation increases markedly with the height above sea level, and terrestrial radiation depends strongly on the local and the living conditions. 

Average equivalent dose rates due to artificial radiation sources are listed in Table 22.9. These dose rates originate from application of X rays and radionuchdes for diagnostic and therapeutic purposes, from various radiation sources applied in daily life and from radioactive fall-out. 

In this equation, the risk of cancer is assumed to be 100% for an effective equivalent dose of 20 Sv. The equation is based on the probabilities listed in Table 22.10. In order to take into account that a dose dehvered with a relatively low dose rate over a longer period of time has an appreciably smaller effect than a single dose, a dose reduction factor of 2 is recommended for smaller dose rates. However, in the report of the United Nations Scientific Committee on the Effects of Atomic Radiation... 

Different types of radiation affect biological materials in different ways, so a different unit is needed to describe the dose necessary to produce an equivalent biological damage. Historically, this unit is the rem (roentgen-equivalent-man). The dose in rem is equal to the dose in rad multiplied by a quality factor, which varies with the type of radiation. For )3 -, y-, and X-ray radiation, the quality factor is 1 for neutrons, it is 2-11, depending upon the energy of the particle and for a-particles, the quality factor is 20. The SI unit for equivalent dose is the sievert (Sv), which is equivalent to 100 rem. 

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