To define the task of calculating internal doses, we must define the quantities we wish to estimate. The principal quantity of interest in internal dosimetry is the absorbed dose, or the dose equivalent. Our ultimate goal in internal dose assessment calculations is the protection of people. Our experience is that radiation effects are fairly well correlated with the absorbed dose (as will be defined momentarily, basically the energy absorbed per unit mass of material, in our application, human tissue). This trend is generally follwoed for many situations, particularly involving high doses. Absorbed dose (D) is defined (ICRU 1980) as:
D = de / dm (1)
where de is the mean energy imparted by ionizing radiation to matter of mass dm. The units of absorbed dose are typically erg/g or J/kg. The special units are rad (100 erg/g) or the gray (Gy) (1 J/kg = 100 rad = 1E+04 erg/g).
However, there are situations in which certain types of radiations seem to have more effectiveness in leading to certain radiobiological endpoints, when the same amount of absorbed dose was received. Thus, researchers have developed a term called the Relative Biological Effeciveness (RBE), which is basically a fudge factor (i.e an admission of ignorance of the exact reasons for observed differences in radiation effects) that accounts for this observed difference. The RBE, however, is defined in a very complex way, and may depend on the effect observed, dose rate, the biological system, etc. Operationally, a term called the 'quality factor' was derived to roughly summarize current information about the RBE for different types of radiation. The dose equivalent (H) is the absorbed dose multiplied by this 'quality factor' (Q):
H = DQ (2)
Because the quality factor is in principle dimensionless, the pure units of this quantity are the same as absorbed dose (i.e. erg/g or J/kg). However, the special units have unique names, specifically, the rem and sievert (Sv), thus the quality factor can be thought of as having units of rem/rad or Sv/Gy, but in a mass-length-time dimensional system, this is not defined. Values for the quality factor have changed as new information about radiation effectiveness has become available. Current values, recommended by the International Commission on Radiological Protection (ICRP 1991), are given in Table 1.
Table 1. Quality Factors Recommended in ICRP 60
Alpha particles 20 Beta particles +/- 1 Gamma rays 1 X-rays 1 The quantity dose equivalent was originally derived for use in radiation protection programs. The development of the effective dose equivalent (EDE) (to be defined later) by the ICRP in 1979, and the effective dose (ED), in 1991, however, as we will learn later, took dose calculations one step further and allowed nonuniform internal doses to be expressed as a single value, representing an equivalent whole body dose.
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