Weighting Factor for Internally Deposited Radionuclides

For internally deposited radionuclides, potential effects depend not only on the absorbed dose, but also on the type or 'quality' of the radiation. This issue is addressed through consideration of the RBE of the various radiations relative to a reference radiation. The reference radiation is generally taken to be 250 kVp X-rays or gamma rays from 137Cs or Co. However, it is important to understand that the choice of reference radiation will affect the RBE. For example, it has been shown that at low doses, X-rays are about twice as effective as gamma rays in producing damage, and hence an RBE established by comparison to gamma rays will be about twice than if X-rays are used as the reference radiation.

The concept of RBE can be defined as

dose of the reference radiation needed to produce the same effect dose of the given radiation needed to produce a given biological effect

A number of authors have reported evaluations of published data on RBE for internally deposited alpha emitters and nominal values are reported in Table 2. In considering these values, it is important to note that the experimental RBEs are specific to the endpoint studied, the biological environment, exposure conditions (e.g., reference radiation, dose rate, dose, etc.), and other factors; therefore, there is some uncertainty associated with the selection ofan appropriate radiation weighting factor for use in an ERA.

A nominal alpha radiation weighting factor of 5 for population-relevant deterministic and stochastic endpoints has been suggested based on literature reviews for use in ERAs; however, to reflect the limitations in the experimental data, uncertainty ranges of 1-10 and 1-20 for population-relevant deterministic and stochastic endpoints, respectively, can be applied.

The European Community Framework for Assessment of Environmental Impact (FASSET) program suggests to modify the absorbed dose by a radiation weighting factor of 10 for alpha radiation, 3 for low energy beta radiation (E <10keV), and 1 for both beta radiation with energies >10 keV and gamma radiation in order to illustrate the effect of radiation quality of emissions from internally deposited radionuclides.

Although somewhat contentious, for present purposes, an RBE value of 10 is suggested for internally deposited alpha emitters. For gamma emitters and internally deposited beta emitters, noting that deterministic effects (e.g., reproductive capacity) are thought to be ofgreatest significance, a value of1 is suggested as a nominal RBE, although a factor of3 may be appropriate for tritium.

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