Dosimetric Considerations

Greig et aL (1970) considered the 2-fold dose increase to the hormonogenic parts (i.e., the colloid-cell interface) from therapeutic 1 to be the basis for the decrease in subsequent myxedema, without loss of cellular reproductive capabilities. Gavron and Feige (1972) questioned whether the cell-colloid interface rather than the follicular cell nuclei can be regrurded as the critical site of radiation damage they, too, considered the dose to nuclei from to be about half that at the cell-colloid interface. 

Bryant (1970) derived working limits associated with continuous atmospheric release rates of Her calculations indicate that dose equivalent rates to the infant thyroid gland would be about 0.6 mrem y pCi maintained in the gland. For the adult, the dose equivalent rate would be about 0.05 mrem y pCi . Soldat et al. (1973) determined similar adult values, 0.06 mrem y for each pCi maintained in the thyroid. Colard et al. (1965) estimated the average thyroidal dose equivalent from an initial thyroidal burden of 1 pCi to be 0.013 mrem. The difference between the infant and adult dose equivalent rates is primarily due to the difference in thyroid gland size. Colard s calculation is based on an initial 1 pCi burden while the others are based on maintaining a 1 pCi burden of I. 

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Children s Susceptibility. No studies were located in which comparisons were made between the sensitivity of children and adults to the toxicity of americium. Animal studies indicate that juvenile dogs are less susceptible than adults to americium-induced bone cancer (Lloyd et al. 1999). No direct evidence was located to indicate that the pharmacokinetics of americium in children may be different from that in adults. Based on dosimetric considerations related to differences in the parameters of available models, as well as studies in animals, it seems likely that children may be more susceptible to americium toxicity than are adults by virtue of age-related differences in pharmacokinetics. Absorption of ingested americium may be as much as 200 times greater in neonatal animals than in adults. (Bomford and Harrison 1986 David and Harrison 1984 Sullivan et al. 1985). 

Maletskos DJ, Keane AT, Telles NC, et al. 1969. Retention and absorption of Ra-224 and Th-234 and some dosimetric considerations of Ra-224 in human beings. In May C, ed. Delayed effects of bone- seeking radionuclides. Salt Lake City, LIT University of Utah Press, 29-49. 

Specificity, dosimetric considerations as well as various pharmacological and toxicological problems, have so far limited the application of immunotoxins and radioimmunotherapy to life-threatening diseases where other treatment regimes have failed [105]. 

The Aj and A2 values tabulated in the 1973 edition of the Regulations were subject to an upper cut-off hmit of 1000 Ci in order to protect against possible effects of bremsstrahlung. Within the Q system this cut-off was retained at 40 TBq. It was recognized as an arbitrary cut-off and is not specifically associated with bremsstrahlung radiation or any other dosimetric consideration. It remains unchanged. 

Decay information for 41 Dietary contribution to thyroid I, 19 Dosimetric consideration for 29... 

Accumulation of in thyroid, 18 Age dependent parameters, 19 Dosimetric considerations, 29 Environmental concentrations, 22... 

It is also noted that there is overlap in the individual UFs and that the application of five UFs of ten for the chronic reference value (yielding a total UF of 100,000) is inappropriate. In fact, in cases where maximum uncertainty exists in all five areas, it is unlikely that the database is sufficient to derive a reference value. Uncertainty in four areas may also indicate that the database is insufficient to derive a reference value. In the case of the RfC, the maximum UF would be 3,000, whereas the maximum would be 10,000 for the RfD. This is because the derivation of RfCs and RfDs has evolved somewhat differently. The RfC methodology (US-EPA 1994) recommends dividing the interspecies UF in half, one-half (10° ) each for toxicokinetic and toxicodynamic considerations, and it includes a Dosimetric Adjustment Factor (D AF, represents a multiplicative factor used to adjust an observed exposure concentration in a particular laboratory species to an exposure concentration for humans that would be associated with the same delivered dose) to account for toxicokinetic differences in calculating the Human Equivalent Concentration (HEC), thus reducing the interspecies UF to 3 for toxicodynamic issues. RfDs, however, do not incorporate a DAF for deriving a Human Equivalent Dose (HED), and the interspecies UF of 10 is typically applied, see also Section 5.3.4. It is recommended to limit the total UF applied for any particular chemical to no more than 3000, for both RfDs and RfCs, and avoiding the derivation of a reference value that involves application of the full 10-fold UF in four or more areas of extrapolation. 

An increased awareness of the complexity of plant response also developed as considerations of the response of populations of plants exposed to a toxicant were initiated. Detailed descriptions of acute responses and even chronic responses were the result of chamber studies of individual plants held under relatively constant experimental conditions with only dosimetric variables. As investigators became more adept at recognizing symptomatology, whole fields of damaged plants were investigated, and the population aspects of the response became evident. 

In this section the dosimetric models and assumptions underlying the derivation of five principal Q values are described in detail. The specific radiation pathways considered are outlined, and the considerations affecting the methods of derivation used are discussed. 

The dosimetric models described in the previous section apply to the vast majority of radionuclides of interest and may be used to determine their Q values and associated Aj and values. However, in a limited number of cases the models are inappropriate or require modification. The special considerations applying in such... 

As noted earlier, the derivation of Qg applies to noble gases which are not incorporated into the body and whose progeny are either a stable nuclide or another noble gas. In a few cases this condition is not fulfilled and dosimetric routes other than external exposure due to submersion in a radioactive cloud must be considered [1.30]. The only case of practical importance within the context of the Regulations is that of Rn-222, where the lung dose associated with inhalation of the short lived radon progeny has received special consideration by the ICRP [1.31]. 

Following the 1985 edition of the Regulations, the application of the Q system as described here treats the derivation of each Q value, and hence each potential exposure pathway, separately. In general this will result in compliance with the dosimetric criteria defined earlier, provided that the doses incurred by persons exposed near a damaged package are dominated by one pathway. However, if two or more Q values closely approach each other this will not necessarily be the case. For example, in the case of a radionuclide transported as a special form radioactive material for which = Qg, the effective dose and skin dose to an exposed person could approach 50 mSv and 0.5 Sv, respectively, on the basis of the Q system models. Examination of Table 1.2 shows that this consideration applies only to a relatively small number of radionuclides, and for this reason the independent treatment of exposure pathways is retained within the Q system. 

The purpose of the dose assessment for the public Uving in conditions of chronic (prolonged) radiation exposure is usually the justification of remedial actions that involve considerable expense associated with them. The doses of critical population groups should therefore be estimated on the basis of realistic, not screening, dosimetric models. To the extent possible, available data from environmental measurements and selective data from individual measurements, such as data from whole body counting for internal dosimetry and individual doses for external dosimetry, should be used to validate these models. 

CONSEQUENCES OF IODINE FALL OUT DOSIMETRIC AND RADIOBIOLOGICAL CONSIDERATIONS... 

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